Phycology - Kostikov I.Yu. - 2009-2013

Chapter 23. Red Algae – Rhodophyta

Red Algae, or red seaweeds – Rhodophyta – comprise about 5,000 species of predominantly multicellular macroscopic algae. A small fraction of species is represented by microscopic forms. Almost all red algae inhabit benthic and periphytic marine communities, with only a few exceptions found in freshwater habitats and soils.

Fossil remains of Rhodophyta are known from the Early Paleozoic (Cambrian, 600 million years ago). Based on paleoalgological data, heterotrichous forms are believed to have emerged within the division during the Ordovician (500–400 million years ago). In the Middle and Late Paleozoic (400–230 million years ago), Rhodophyta played a dominant role in marine environments, but experienced a catastrophic extinction event at the beginning of the Mesozoic, which wiped out almost all species of the Carboniferous flora. During the Cretaceous period (around 100 million years ago), the division entered a new phase of biological progress and continues to flourish to the present day.

Red algae belong to lamellate-cristate plants possessing Plastids of primary symbiotic origin. The sister divisions to Rhodophyta are Glaucocystophyta and Chlorophyta.

Class="center">Taxonomic CHARACTERISTICS OF THE Division

Pigments and Reserve Nutrients

The Cells of red algae are predominantly red in color; much less commonly, the coloration may be grey or blue-green (typically in species inhabiting freshwater bodies or soils). This coloration is primarily determined by the presence of chlorophyll a and phycobilin pigments – phycocyanin, allophycocyanin, and phycoerythrin. Typically, phycoerythrin predominates over the other two phycobilins. Carotenoids are represented by α- and β-carotenes and two groups of xanthophylls: firstly, specific xanthophylls (cryptoxanthin, taraxanthin), and secondly, lutein-group xanthophylls (zeaxanthin and lutein, supplemented by antheraxanthin and violaxanthin in certain species).

The primary assimilation product is the polysaccharide floridean starch, which is an α-1,4 glucan closely related to amylopectin and Glycogen. Floridean starch stains brownish-yellow with Lugol's iodine solution, subsequently turning red. In primitive bangiophyte algae, the assimilation product is true glycogen. Some Rhodophyta feature specialized secretory gland cells filled with iodine and bromine compounds, as well as cells containing iridescing protein bodies.

Additional reserve nutrients may include galactoside, floridoside, polyhydric alcohols (mannitol), and minor amounts of oil. Interestingly, the carbohydrate floridoside has also been detected in cyanobacteria and cryptophyte algae. The primary function of floridoside is the Regulation of Osmotic pressure.

All assimilation products in red algae are deposited outside the METABOLISM/14.html">Chloroplasts.

Cytological Characteristics

Cell walls are represented by pectin and Cellulose-pectin walls, which are typically trilaminar. The outer layer, or cuticle, is generally formed of condensed, polymerized mannose; the middle layer consists of phycocolloids (Agar-agar, agaroid, carrageenan, which are sulfated Heteropolysaccharides) and pectic substances. The inner layer is composed of cellulose (Fig. 23.1). Primitive bangiophyte algae lack the cellulosic layer.

Fig. 23.1. Diagram of The Cell wall Structure in red algae.

Growth of the middle wall layer occurs through The activity of the Golgi apparatus, while the inner layer grows via the synthesis of microfibrils by linear enzyme complexes located in the Plasmalemma. In many species, the walls are impregnated with lime – calcite, aragonite, calcium, magnesium, and iron carbonates.

Intercellular Communication occurs via plasmodesmata passing through Primary and secondary pits (Fig. 23.2). Primary pits are formed As a result of the incomplete closure of the edges of the transverse septum upon completion of Cell Division. Primary pits are sealed by specialized pit plugs.

Fig. 23.2. Formation of primary (1–3) and secondary (4–7) pits in Rhodophyta.

Cisternae of The Endoplasmic reticulum take direct part in The formation of pit plugs, helping to shape the proteinaceous core of the plug and occasionally its surface membrane. Additionally, plugs may possess extra inner and outer polysaccharide layers (Fig. 23.3). In some representatives, the outer polysaccharide layer is very thick, making such plugs readily visible under a Light Microscope.

Fig. 23.3. Diagram of pit plug formation with a surface membrane and additional polysaccharide layers: a – endoplasmic reticulum cisternae are located within the pit; b – a layer of Polysaccharides is deposited around the cisternae, polysaccharides of the future inner layer accumulate in the terminal Zones of the cisternae, and structural Proteins are deposited between the cisternae, forming the plug core; c – terminal Regions of the cisternae pinch off vesicles containing polysaccharides; d – polysaccharide vesicles fuse with one another to form the surface membrane; the proteinaceous core of the plug along with the inner and outer polysaccharide layers become condensed.

Consequently, seven types of pit plugs are distinguished in red algae (Fig. 23.4). Plugs of the first, second, and third types lack a surface membrane and differ from one another in the presence of additional polysaccharide layers. Specifically, plugs of the first type lack such layers, the second type possesses a single thin additional layer, and plugs of the third type feature two additional polysaccharide layers. Plugs without a surface membrane are characteristic of members of the class Bangiophycidae, as well as the most primitive Representatives of the class Florideophycidae.

Fig. 23.4. Types of pit plugs in red algae. The top row shows plugs without a surface membrane (types one, two, and three); the bottom row shows plugs with a surface membrane (types four, five, six, and seven).

Surface membranes are present in pit plugs of the fourth to seventh types. Variations within these types are also associated with additional polysaccharide layers: the fourth type lacks them, the fifth type has one additional polysaccharide layer located between the membrane and the protein core, and the sixth and seventh types feature two additional polysaccharide layers separated by a surface membrane, with the outer layer in the seventh type being very thick and clearly visible under a light microscope. Pit plugs with surface membranes are characteristic of those members of the class Florideophyceae that are considered evolutionarily more advanced.

Overall, the type of pit plug is a diagnostic feature that strongly correlates with molecular data regarding the phylogenetic relationships of various Rhodophyta orders, and thus serves as an important phenotypic marker at the class and order levels.

Secondary pits result from a specialized cell division: mitosis occurs within The Cell to form a daughter nucleus, which then migrates into a lateral bulge of the cell wall. This bulge fuses with the adjacent cell, the wall breaks down at the point of contact, and a secondary pit is formed. The daughter nucleus subsequently degenerates.

The nuclear apparatus is typically eukaryotic. However, mitosis exhibits several significant deviations from the classical scheme: mitosis is closed, meaning the nuclear envelope does not break down during prophase but instead becomes perforated. In late prophase, a so-called perinuclear endoplasmic reticulum forms around The Nucleus from a portion of the endoplasmic reticulum. Centrioles are absent in red algae, their function being performed by specialized protein polar rings.

Cytokinesis follows the completion of nuclear division. During this process, the plasmalemma and the inner layer of the cell wall grow inward, forming an annular furrow. The edges of the furrow typically do not meet, and a pore remains at the center of the new transverse partition. Subsequently, protein material is deposited in the pore region to form a pit plug.

Photosynthetic apparatus. The chloroplasts of red algae are enclosed in a double-membrane envelope, and thylakoids occur singly. Phycobilisomes are distributed On the surface of the thylakoids, and their structure is similar to that of the phycobilisomes found in cyanobacteria.

The chloroplasts of Rhodophyta can conventionally be divided into two types based on their cellular position, number, presence of pyrenoids, and girdle thylakoid: a) single, central, stellate chloroplasts with naked pyrenoids; b) numerous, small, parietal, disc-shaped chloroplasts lacking pyrenoids. Chloroplasts of the first type are predominantly characteristic of bangiophyte algae, whereas those of the second type are found in members of the class Florideophyceae (Fig. 23.5).

Fig. 23.5. Schematic diagrams of Cell Structure in red algae of the classes Bangiophyceae (a) and Florideophyceae (b): 1 - cell wall; 2 - plasmalemma; 3 - nuclear envelope; 4 - nucleolus; 5 - mitochondrion; 6 - plastid envelope; 7 - thylakoid with phycobilisomes; 8 - Golgi apparatus; 9 - pyrenoid; 10 - pit plug; 11 - endoplasmic reticulum; 12 - Reserve Polysaccharides; 13 - vacuole; 14 - polar rings.

According to molecular data, the chloroplast DNA of red algae shows the closest affinity, on the one hand, to the nucleoid DNA of cyanobacteria, and on the other hand, to the chloroplast DNA of green and glaucophyte algae. This is in strong agreement with cytological evidence supporting the primary endosymbiotic origin of Rhodophyta plastids.

Interestingly, the Gene encoding RuBisCo (the rbcL gene) was acquired by red algae from alpha-proteobacteria via Horizontal Gene Transfer, whereas green algal plastids inherited this gene from cyanobacteria. It is hypothesized that the divergence of the hypothetical primary endosymbiotic plastid into a chloroplast (green algal plastid) and a rhodoplast (red algal plastid) may have been driven precisely by the different origins of the rbcL gene, which encodes the key enzyme for the Initial Stages of the dark reactions of Photosynthesis.

Flagellated stages are completely absent in red algae.

Thallus Organization Patterns

Coccoid, filamentous, and heterotrichous structural types are represented within the division. Coccoid forms are few and occur among the most primitive bangiophyte algae. Coccoid red algae typically form gelatinous colonies (Fig. 23.6).

Fig. 23.6. Types of morphological structures and thallus organization patterns in Rhodophyta: 1 - coccoid; 2 - uniseriate filamentous; 3, 4 - multiseriate filamentous; 5 - foliose; 6, 7 - uniaxonal heterotrichous; 8, 9 - multiaxonal heterotrichous.

Algae with a filamentous structure type are found in taxa of varying degrees of evolutionary advancement. Filamentous thalli may be thread-like or foliose, and uniseriate or multiseriate. Some authors suggest that multiseriate foliose thalli should be regarded as parenchymatous or tissue-like (e.g., thalli of Porphyra) (see Fig. 23.6).

The most widespread representatives within the division exhibit a heterotrichous morphological body plan. Growth in these algae occurs primarily through the division of an apical cell. Lateral assimilatory filaments frequently coalesce tightly with one another, forming pseudoparenchymatous structures. Depending on the number of supporting axes, heterotrichous thalli are subdivided into uniaxonal and multiaxonal (or fountain-type) forms (see Fig. 23.6).

All filamentous and heterotrichous Rhodophyta are sessile and attached to a substrate. Attachment is accomplished via prostrate filaments, primary rhizoids, and holdfasts.

Reproduction and Life Cycles

Red algae reproduce both asexually and sexually. Asexual reproduction occurs vegetatively (by binary cell division, fragmentation of multicellular thalli, and adventitious "shoots" that may sprout from prostrate filaments or holdfasts) as well as by means of spores.

In the latter case, reproduction is carried out predominantly by monospores and tetraspores (bispores and polyspores occur rarely). The spores of red algae lack cell walls and are capable of amoeboid movement. During spore formation within sporangia in Rhodophyta, meiotic (reductional) division typically occurs, rendering the spores haploid. Spores are released through an apical pore of the sporangium. Monospores are produced singly from superficial (cortical) cells by the Separation of a portion of the protoplast of the sporangial cell via an oblique septum; this type of sporogenesis is characteristic of all bangiophyte algae and the most primitive florideophytes. Tetraspores are produced in groups of four; depending on the orientation of the division planes, tetrasporangia are classified as cruciate, zonal, or tetrahedral (Fig. 23.7).

Fig. 23.7. Sporangia of red algae: a - monosporangium; b–d - tetrasporangia (b - cruciate, c - zonal, d - tetrahedral).

Tetrasporangia are initiated either among the Cells of the cortical layer or on The surface of the thallus surrounded by short protective filaments—paraphyses—forming cushion-like structures known as nemathecia, or within specialized semi-enclosed cavities called conceptacles. Tetraspores always germinate into haploid gametophytes. Enclosed aplanospores are also known in some bangiophyte algae.

The Sexual process in red algae is exclusively oogamous and occurs in almost all representatives, with the exception of coccoid Bangiophycidae and some Florideophycidae. In the male sex Organs, known as spermatangia, a single non-flagellated spermatium is formed, whereas in the female sex organs, or carpogonia, a single egg cell is produced.

A carpogonium typically consists of an expanded basal portion, the venter, and a tubular outgrowth called the trichogyne, which captures spermatia. In the simplest case, carpogonia develop from any cell of the thallus, whereas in more complex cases, they arise from the apical cell of specialized carpogonial filaments. The cell from which the carpogonial branch diverges is called the supporting (or initial) cell. In many red algae of the class Florideophyceae, this cell subsequently transforms into a specialized auxiliary cell, with which the carpogonium fuses following Fertilization.

During sexual reproduction, spermatia are released from spermatangia, passively transported by Water currents, and upon reaching a carpogonium, fertilize the egg cell. Immediately after fertilization, the zygote develops without a resting period into a distinct generation found exclusively in the Rhodophyta—the carposporophyte. The carposporophyte produces carpospores, which subsequently give rise to a new sporophyte (Fig. 23.8).

Fig. 23.8. Life cycle diagram of Rhodophyta. S - sporophyte, G - gametophyte, KS - carposporophyte, Z - zygote, msp - monospores, tsp - tetraspores, ksp - carpospores, gm - Gametes, (2n) and (n) - diploid and haploid chromosome sets, x - gamete copulation.

The carposporophyte performs a specific, specialized function: it amplifies the outcome of a successful sexual event. It should be noted that due to the absence of flagellated stages, the probability of fertilization by a male gamete passively transported in water is extremely low. Consequently, the number of fertilized egg cells capable of giving rise to a new recombinant generation is very small, and the mortality rate during early ontogenetic stages is invariably elevated. Ensuring a sufficient level of recombinational diversity for the stable persistence of the population is the primary function of the carposporophyte. The carposporophyte develops from the fertilized carpogonium and is nourished by assimilates synthesized by the gametophyte. The cells of the carposporophyte contain diploid zygotic nuclei that undergo mitotic division. Eventually, numerous diploid carpospores are formed from the carposporophyte cells, which then germinate into sporophytes.

The formation of carpospores can occur through three main pathways, which determine the three principal structural plans of the carposporophyte:

In the simplest case, the zygote immediately begins to divide, producing carpospores directly within the carpogonium. In this variant, the carposporophyte is very simple, represented solely by the carpogonium containing carpospores.

In a more complex variant, diploid filaments with zygotic nuclei—gonimoblasts—grow out from the carpogonium. Subsequently, the cells of these gonimoblasts differentiate into carpospores. Here, the carposporophyte generation comprises the carpogonium, gonimoblasts, and carpospores.

The third variant is the most complex: the carpogonium with the fertilized egg cell fuses with an auxiliary cell, forming a heterokaryotic (multinucleate) fusion cell. The latter contains one or more of its own haploid "gametophytic" nuclei, alongside the diploid zygotic nucleus derived from the fertilized carpogonium. The diploid nucleus undergoes several mitotic divisions, and the daughter nuclei migrate into peripheral outgrowths of the fusion cell. Each outgrowth is delimited by a septum and develops into a gonimoblast, whose cells transform into carpospores. In this variant, the carposporophyte consists of the carpogonium, the fusion cell, auxiliary cells, gonimoblasts, and carpospores.

Thus, The life cycle in Rhodophyta is predominantly haplodiplontic, featuring sporic Meiosis and the alternation of three generations: the gametophyte, carposporophyte, and sporophyte.

Systematics of Rhodophyta

Based on The structure of the carpogonium, chloroplasts, modes of asexual reproduction, assimilation products, and the presence of secondary pits, the phylum is divided into two classes: Bangiophyceae and Florideophyceae. The class Bangiophyceae unites algae in which the carpogonium lacks a trichogyne, carpospores are formed directly from the zygote, and chloroplasts are stellate or ribbon-shaped, typically possessing a pyrenoid; the assimilation product may include glycogen In addition to floridean starch; asexual reproduction occurs primarily via monospores; primary pit connections either lack caps or possess simple caps of the first or second type; and secondary pits are absent. The evolutionarily more advanced class Florideophyceae includes algae in which the carpogonium bears a trichogyne, carpospores are formed via various pathways across different orders, chloroplasts are numerous and disc-shaped without pyrenoids, and the sole assimilation product is floridean starch (glycogen is absent); primary pits are plugged by caps of the third to seventh types (in the order Ahnfeltiales, the plugs are simple, of the first type), and cells are interconnected by both primary and secondary pits.

Class Bangiophyceae

Based on morphological types and reproduction, the class is divided into three orders: Porphyridiales, Compsogonales, and Bangiales (Table 23.1). The Classification of bangiophycean algae into these orders is further supported by molecular dendrograms constructed from sequencing data of the rbcL and SSU rDNA genes, which encode the large subunit of RuBisCo and the small subunit of cytoplasmic ribosomal RNA, respectively.

Table 23.1. Selected phenotypic features of representatives of the Bangiophyceae orders

Porphyridiales comprises coccoid algae that typically form mucilaginous colonies. They reproduce by binary cell division, monospores, or aplanospores. The latter are formed in groups of four within the sporangium, but unlike tetraspores, they possess a distinct cell wall. Special cytological studies of one representative of this order (Cyanidium) have shown that cytokinesis is accomplished via the formation of a Cleavage furrow, the growth of which is driven by the contraction of a peripheral contractile ring composed of the protein Actin.

Porphyridialean algae inhabit primarily freshwater bodies and soils. The most widespread genus is Porphyridium (Fig. 23.9). Species of this genus are intensively studied in numerous laboratories, as they are considered promising phycotechnological candidates for industrial cultivation aimed at The production of phycocolloids.

Fig. 23.9. Mucilaginous colony (a) and individual cells (b) of Porphyridium purpureum.

The thalli of representatives of the order Compsogonales are heterotrichous and usually appear as branched tufts. They reproduce via monospores, while the sexual process remains unknown. Compsogonalean algae predominantly inhabit freshwater bodies in tropical regions. These algae were introduced into Ukraine along with tropical aquarium plants. In particular, Compsogon is frequently encountered in aquaria, where it is known among aquarists as "blue beard." The alga develops as an epiphyte, mainly on cryptocoryne plants (Fig. 23.10).

Fig. 23.10. Compsogon: a - general appearance of the thallus; b - basal part of the thallus with rhizoidal filaments embedded in the Tissues of the cryptocoryne plant; c - upright portion of the filament; d - thallus fragment (showing cells containing ribbon-like chloroplasts, with isolated triangular-rounded monosporangia visible in the multiseriate upright region).

Bangiales. This order includes algae possessing filamentous, typically multiseriate unbranched or blade-like thalli. Bangialean algae reproduce by monospores (polyspores in some species) and accessory "shoots." The sexual process is known and characterized by zygotic meiosis, which occurs immediately after egg fertilization. Representatives of the order live predominantly in marine environments. The most common genera are Bangia and Porphyra. Bangia possesses a multiseriate filamentous thallus, whereas Porphyra has a blade-like thallus. Porphyra (the so-called "red sea lettuce") is an edible alga and has been introduced into industrial aquaculture (cultivated, for instance, in the Black Sea) (Fig. 23.11).

Fig. 23.11. Bangiophycean algae: a-c - Bangia (a - basal part of the thallus with rhizoids, b - middle multiseriate part of the thallus, c - cross-section of the thallus in the monosporangial development zone); d-g - Porphyra (d - general view of the thallus, e-g - cross-section of the blade in the zone of carpogons (e), spermatangia (f), and carposporangia (g) (adapted from Vinogradova, 1977).

Class Florideophyceae

The leading class within The system of Rhodophyta, uniting about 80% of known species of the division. The class includes algae exclusively with a heterotrichous type of organization. Molecular data indicate that within the class There are five molecularly distinct groups, which may correspond to taxa of the rank of orders or groups of orders. The molecular distinctness of each group strongly correlates with a complex of phenotypic traits related to the types of primary pit plugs, the presence and features of auxiliary cells, the presence of carpogonial branches, the type of Morphology/12.html">ALTERNATION OF GENERATIONS, tetraspore morphology, and ecological features. Different authors distinguish from six to fourteen orders within the class, eight of which are considered below: Hildenbrandiales, Nemaliales, Corallinales, Ahnfeltiales, Gelidiales, Gigartinales, Rhodymeniales, Ceramiales.

Hildenbrandiales is considered the most primitive order of floridean algae. Representatives of the order inhabit both marine and freshwater environments. The thalli of these algae develop mainly on rocky substrates, forming crust-like pseudoparenchymatous structures consisting of several cell layers on the surface. Primary pits are closed by type 3 plugs. These plugs lack a surface membrane; beneath the protein core, There are two additional polysaccharide layers, with the outer layer being very thick and convex. The sexual process is absent, and the life cycle represents a cyclomorphosis. Reproduction is solely vegetative: via thallus fragmentation, groups of vegetative cells (gemmae), and creeping filaments known as stolons. A typical representative is the genus Hildenbrandia. It is believed that two distinct ecological groups of florideans originated from Hildenbrandiales. The first branch, the order Nemaliales, includes algae that exhibit a transition from a marine to a freshwater lifestyle. The second branch is represented almost exclusively by marine species of all other orders of this class.

For Nemaliales, characteristic features include the presence of type 6 or 7 plugs: possessing a surface membrane and two polysaccharide layers, where the outer layer can be either thin or thick. The thalli in nemalian algae are predominantly heterotrichous or pseudoparenchymatous, both uniaxial and multiaxial. Asexual reproduction is carried out by tetraspores formed in cruciate tetrasporangia. Auxiliary cells are absent. Carpogons can be formed either on short carpogonial branches or from ordinary vegetative cells. Life cycles are haplo-diplontic, with sporic reduction and an alternation of isomorphic or heteromorphic generations. Characteristic representatives are the genera Palmaria, Nemalion, and Batrachospermum.

Palmaria inhabits marine environments exclusively. Sporophytes and male gametophytes in this alga are morphologically identical, appearing as dissected blades of pseudoparenchymatous multiaxial structure. Female gametophytes are small, crust-like, and consist of several cell layers. Carpogons develop from cells of the surface layer. Following fertilization, the zygote immediately grows into a sporophyte without forming gonimoblasts. The sporophyte remains attached to the female gametophyte. Cruciate tetrasporangia with haploid tetraspores develop on the mature sporophyte, which subsequently give rise to male or female gametophytes. Thus, in Palmaria, the life cycle is haplo-diplontic, with sporic meiosis, heteromorphic alternation of generations, Sexual Dimorphism, and lacking a carposporophyte stage. A fairly common representative, Palmaria palmata, is an edible marine alga belonging to the so-called red dulse.

Most species of the genus Nemalion also inhabit the sea, although some representatives are found in brackish and even freshwater bodies. The gametophytes of Nemalion are macroscopic, appearing as small tufts. The gametophyte thalli are multiaxial, consisting of a group of supporting axial filaments from which numerous lateral assimilatory filaments branch off. From the apical cells of the assimilatory filaments develop either numerous unicellular antheridia (spermatangia) that produce spermatia, or three- to five-celled carpogonial branches at the apex of which carpogons with trichogynes are initiated. After fertilization, the zygote divides without a resting period to form a dense group of few-celled gonimoblasts, in which the apical cells transform into carpospores.

Carpospores germinate into uniaxially branched bushy sporophytes. In autumn and winter, meiosis occurs in the apical cells of the lateral branches, forming cruciate tetrasporangia. The tetraspores subsequently germinate into new multiaxial gametophytes.

In Batrachospermum, a haploid macroscopic gametophyte develops on a diploid microscopic sporophyte (Fig. 23.12). The sporophyte in this alga is represented by uniseriate branched filaments, in the apical cells of which meiosis can occur. Following meiosis, such cells do not transform into tetrasporangia, but instead begin to germinate directly into sporophytes. The sporophyte thalli are surrounded by a thick layer of mucus, differentiated into a uniseriate supporting filament and whorled lateral assimilatory branches. Spermatangia are formed from the apical cells of the assimilatory filaments, while carpogons develop on short carpogonial branches, which in different species are located on various PARTS OF THE gametophyte thallus. After fertilization, a carposporophyte develops from the carpogon, resembling the carposporophyte of Nemalion. All species of the genus Batrachospermum are freshwater algae inhabiting mainly cold mountain streams or large lakes. The thalli of these algae can exhibit various shades of both red and blue-green coloration.

Fig. 23.12. Batrachospermum: a - diploid sporophyte; sporangial cells and cell tetrads homologous to tetrasporangia, marking the beginning of gametophyte development, are located at the apex; b - general view of the gametophyte; c - fragment of the gametophyte with spherical carposporophytes.

In the systems of some authors, Palmaria, Nemalion, and Batrachospermum are placed in separate, independent orders (Palmariales, Nemaliales, and Batrachospermatales, respectively) due to differences in life cycles, reproductive details, thallus structure, and ecology. However, molecular and cytological studies indicate close Phylogenetic relationships among these representatives despite their morphological differences.

The most striking feature of Corallinales is the calcification of cell walls. The thalli are multiaxial, usually lime-impregnated, and appear as red stones, small tufts, or resemble corals. Primary pit plugs belong to type 3—they lack a surface membrane and possess two additional polysaccharide layers, with the outer layer being very thick. The alternation of generations in representatives of this order is isomorphic. Both Selection/8.html">Asexual and sexual reproductive organs (tetrasporangia, carpogons, and spermatangia) develop within special depressions of the thallus called conceptacles. Tetrasporangia are cruciate or zonate.

The carpogon is initiated at the apex of a two-celled carpogonial filament. The cell located beneath the carpogon is called the ooblastema cell. The carpogonial filament, formed by the carpogon and the ooblastema cell, originates from the supporting (initial) cell, from which another short yet sterile filament also develops. The initial cell subsequently transforms into an auxiliary cell (Fig. 23.13).

Fig. 23.13. Structure of a conceptacle with carpogons (a) and successive stages of carposporophyte development in Corallinales representatives (b-g): 1 - carpogon with trichogyne; 2 - ooblastema cell; 3 - auxiliary cell; 4 - heterokaryotic fusion cell formed as a result of plasmogamy of auxiliary cells; 5 - gonimoblast; 6 - carpospore. Diploid zygotic nuclei are black, while haploid nuclei of vegetative cells inside are light.

Upon fertilization, the carpogon fuses with the ooblastema cell, which, in turn, fuses with the auxiliary cell. Through the ooblastema cell, the diploid zygote nucleus migrates into the auxiliary cell. Next, all auxiliary cells of a single conceptacle fuse with one another, forming a single large multinucleate heterokaryotic fusion cell in the conceptacle, which contains diploid zygotic nuclei and haploid vegetative nuclei. Diploid nuclei migrate into lateral outgrowths of the fusion cell and are separated from it by cell walls. Subsequently, these diploid cells divide, transforming into gonimoblasts, from the apical cells of which carpospores develop. Mature carpospores exit the conceptacle into the external environment through an opening and germinate into sporophytes.

The most common genera of the order are Lithothamnion and Corallina. In Lithothamnion, thalli are crustose or appear as more or less branched red calcareous stones (Fig. 23.14).

Fig. 23.14. General appearance of Lithothamnion thalli.

Corallina is characterized by thalli in the form of pinnately and dichotomously branched tufts. Each branch of the thallus consists of calcareous segments-joints formed by multiseriate supporting filaments and lateral assimilatory filaments (Fig. 23.15). Areas between adjacent segments lack assimilators, and the cells of the supporting filaments are quite elongated. Both genera occur in various seas, notably the Black Sea.

Fig. 23.15. Corallina. Thallus fragment (left) and the region connecting two joint-segments (right).

The order Ahnfeltiales includes only a single genus, Ahnfeltia (Fig. 23.16). In ahnfeltian algae, thalli are not impregnated with lime, and primary pit plugs belong to type 1—they lack both a surface membrane and polysaccharide layers. Thalli appear as dichotomously branched tufts of pseudoparenchymatous structure. The sexual process has not been observed, and the life cycle is represented by cyclomorphosis. Unlike other florideans, reproduction is carried out via monospores, the formation of which is not preceded by meiosis. Monospores develop in nemathecia, which are superficial thallus thickenings formed by short vertical filaments extending from cortical cells. Ahnfeltia is widespread in northern seas and serves as the primary source for obtaining White Sea agar-agar.

Fig. 23.16. General appearance of the Ahnfeltia thallus.

The next four orders—Gelidiales, Gigartinales, Rhodymeniales, and Ceramiales—share many common features: the pit plugs of primary pores possess a superficial membrane and either lack polysaccharide layers altogether or contain only a single thin polysaccharide layer (the fourth and fifth types of pit plugs); carpogons develop from the apical cell of three-celled carpogonial filaments; fertilized carpogons fuse directly with the auxiliary cell, without the participation of ooblastema cells. Alternation of generations is isomorphic, and only in some representatives of Gigartinales is it heteromorphic. The differences between these evolutionarily close orders lie in the thallus construction plans, the types of tetrasporangia, the presence of a polysaccharide layer in the pit plug, and features related to The Development of auxiliary cells (specifically, their site of formation and timing: before or after carpogonial fertilization).

Representatives of Gelidiales are characterized by uniaxial thalli of pseudoparenchymatous structure, in which pores between cells are closed by fifth-type pit plugs; tetrasporangia are cruciate, and auxiliary cells are formed from initial cells prior to carpogonial fertilization. Carpogonial branches are located deep within the thalli, and only the trichogynes of the carpogons protrude to the exterior. Upon fertilization, the carpogon fuses with the auxiliary cell, forming a heterokaryotic fusion cell in which the diploid nucleus undergoes several mitotic divisions (Fig. 23.17). Subsequently, diploid nuclei migrate into the lateral outgrowths of the fusion cell, these outgrowths become separated by septa, and they develop into gonimoblasts. At this stage, numerous small trophic cells proliferate around the carposporophyte in a continuous layer, resulting in the formation of a cystocarp, which appears as a Swelling or a small spherical tumor on the gametophyte thallus (Fig. 23.18).

Fig. 23.17. Successive stages of carposporophyte development in Gelidiales: a — fertilization; b — Formation of the fusion cell; c — formation of gonimoblasts and trophic cells; d — cystocarp. 1 — carpogon with trichogyne; 2 — auxiliary cell; 3 — basal cell of the carpogonial filament; 4 — heterokaryotic fusion cell; 5 — initial stage of gonimoblast development; 6 — trophic cells; 7 — mature gonimoblast cells; 8 — carpospore; 9 — layer of cystocarp trophic cells. Diploid zygotic nuclei are black, and haploid nuclei of vegetative cells inside are light.

Fig. 23.18. Gelidium: a — general appearance of the thallus; b — thallus fragment with cystocarps; c, d — sections through the apical (c) and medial (d) parts of the thallus.

A typical representative of the order, Gelidium latifolium, has relatively large (up to 10 cm high) flattened, pinnate, pseudoparenchymatous thalli of uniaxial structure, which can often be found on rocks in the littoral zone of temperate seas (including the Black Sea).

In algae of the order Gigartinales, the thalli are also pseudoparenchymatous, uniaxial or multiaxial; pit plugs belong to the fourth type, tetrasporangia are cruciate or zonal, developing mainly in nemathecia. Similar to the previous order, the carpogon develops from the apical cell of the carpogonial filament, and the auxiliary cell develops from the initial cell. Following fertilization, the carpogon fuses with the auxiliary cell, forming a heterokaryotic fusion cell. Gonimoblasts form from the lateral outgrowths of this cell, and carpospores subsequently develop from the gonimoblasts (Fig. 23.19). Gigartinalean algae are particularly interesting because they comprise the majority of species from which phycocolloids are commercially harvested.

In particular, species of the genus Phyllophora (Fig. 23.20) yield agaroid, commonly referred to as "Black Sea agar." Along the northwestern coast of the Black Sea, between Odessa and Ochakiv, there is an area at a depth of 5–60 m where Phyllophora nervosa forms continuous growths. This is the world's largest phyllophoran phytocenosis, known as Zernov's phyllophora field, named after its discoverer—the prominent marine hydrobiologist Academician S.A. Zernov. Thanks to Zernov's phyllophora field, Ukraine is the country with the world's largest reserves of industrial raw material for agaroid production.

Fig. 23.19. Successive stages of carposporophyte development in Gigartinales (using Chondrus as an example). 1 — carpogon with trichogyne; 2 — initial cell of the carpogonial branch; 3 — vegetative cell of the carpogonial branch; 4 — heterokaryotic fusion cell; 5 — gonimoblast; 6 — carpospore. Diploid zygotic nuclei are black, and haploid nuclei of vegetative cells inside are light.

The thalli of Phyllophora are bushy, represented by creeping "shoots" from which vertical stemlets arise. The upper part of each stemlet is flattened, expands along the margins, and forms a blade with a thickened central part—the midrib. The blade branches extensively and frequently bears daughter, younger blades along the margins and on the midrib. Tetrasporangia and sex organs develop in nemathecia located on the surface of the blades along the midrib.

Fig. 23.20. Phyllophora nervosa, the main source of agaroid production in Ukraine.

The primary raw material source for carrageenan production consists of two very similar algae inhabiting the northern Atlantic Ocean and the seas of the Arctic Ocean: Chondrus (Irish moss) and Gigartina. Both algae possess flattened, profusely dichotomously branched thalli (Fig. 23.21). However, the thallus surface in Chondrus is smooth, whereas in Gigartina it is covered with numerous additional small outgrowths.

Fig. 23.21. Gigartinalean algae: a–c — Chondrus (a — general appearance of the thallus, b — section through the sporophyte with tetrasporangia; c — gametophyte with cystocarps); d — general appearance of the Gigartina thallus.

Pseudoparenchymatous thalli, predominantly laminar in shape and hollow inside, are characteristic of representatives of Rhodymeniales. The thallus structure is exclusively multiaxial, intercellular pores are closed by fourth-type pit plugs, and tetrasporangia are cruciate and tetrahedral. The initial cell from which the development of female sex organs begins gives rise to two filaments: a three-celled carpogonial filament and a two-celled auxiliary filament, with the auxiliary cell at the apex (Fig. 23.22). This system comprising the initial cell along with the carpogonial and auxiliary filaments is termed a procarp. During the development of gonimoblasts, vegetative cells adjacent to the procarp proliferate to form the assimilatory and trophic envelope of the cystocarp. As carpospores mature, cystocarps increase in size and form spherical swellings on the gametophyte thalli.

Fig. 23.22. Successive stages of carposporophyte development in Rhodymenia: a — procarp; b — plasmogamy between cells of the carpogonial branch; c — formation of the fusion cell; d — cystocarp. 1 — carpogon with trichogyne; 2 — basal cell of the carpogonial branch; 3 — initial cell; 4 — auxiliary cell; 5 — heterokaryotic fusion cell; 6 — cells of the assimilatory layer

of the cystocarp; 7 — cells of the trophic layer of the cystocarp; 8 — gonimoblast; 9 — carpospore. Diploid zygotic nuclei are black, and haploid nuclei of vegetative cells inside are light.

A typical representative of the order, Rhodymenia, is widely distributed in the littoral zone of northern seas (Fig. 23.23). Like Porphyra, Rhodymenia is an edible red alga and even shares the same vernacular name—dulse (red sea lettuce).

Fig. 23.23. General view of the thalli of Rhodymenia stenogona (left) and R. pertusa (right). (After Vinogradova, 1977).

Algae of the order Ceramiales feature filamentous uniaxial thalli attached to the substrate by a holdfast formed by the basal cell; creeping thallus filaments are reduced. Ceramiales possess type IV pit connections, tetrahedral tetrasporangia, procarps, and cystocarps structurally similar to those of Rhodymeniales. The Development of the carposporophyte also resembles that of the previous order, but auxiliary cells are formed only after the fertilization of the egg cell (Fig. 23.24). This order is the most species-rich within the division, comprising between 1,000 and 1,500 species according to various authors' classification systems.

Fig. 23.24. Successive stages of carposporophyte development in Polysiphonia: a - procarp; b - g - successive stages of heterokaryotic fusion Cell Formation; d - initial stage of gonimoblast and cystocarp envelope formation; e - cystocarp. 1 - carpogonium with trichogyne; 2 - basal cell of the carpogonial branch; 3 - auxiliary cell; 4 - initial cell; 5 - assimilatory and trophic layers of the cystocarp; 6 - heterokaryotic fusion cell; 7 - gonimoblasts; 8 - carpospore. Diploid zygotic nuclei are black, while haploid nuclei of vegetative cells inside are light.

Characteristic genera of littoral and sublittoral algal communities across various seas include Callithamnion, Ceramium, Polysiphonia, and Odonthalia.

Callithamnion features bushy thalli formed by single-ranked, profusely branched filaments. Tetrasporangia, spermatangia, and carpogonia are formed in the axils of apical branchlets; cystocarps are predominantly paired (Fig. 23.25). Representatives of this genus are characteristic of temperate seas and occur abundantly in the Black and Azov Seas, where they mainly develop within benthic and periphyton communities at depths of up to 3–5 m.

Species of the genus Ceramium also appear as small bushes. The filaments branch dichotomously, with fork-like, incurved apices. The thallus is formed by a single-ranked axial filament, which is covered by a cortex in the contact zones between adjacent cells. In some species, the cortex covers the entire thallus, but more frequently, the median part of the axial cells remains uncovered, giving the plant a jointed appearance—transparent exposed areas of axial cells alternate with intensely red-colored corticated zones. The cortex consists of short assimilatory filaments that are tightly pressed against the cell walls of the axial filaments. Some assimilatory filaments are modified into hairs or spines. Spermatangia, carpogonial filaments, and tetrasporangia also develop from cortical cells (Fig. 23.25).

Fig. 23.25. Ceramialean algae: Callithamnion (a, b) and Ceramium (c-e). Thallus fragment with a cystocarp (a); apical branch with tetrasporangia (b); thallus fragment with corticated bands (c); band with tetrasporangia and hairs (d); band zone with spines (d, e). 1 - cystocarp; 2, 4 - tetrasporangia; 3 - band; 5 - Hair; 6 - spine.

Ceramium is found in all latitudes, but develops most abundantly in warm seas, particularly the Black and Azov Seas.

Another highly widespread representative of the order is Polysiphonia. The thalli of this alga are also bushy and have a jointed structure, albeit of a different type than in Ceramium. The filament is formed by Two Types of cells: central cells, which perform a supportive function, and pericentral cells, which carry out photosynthesis and form reproductive organs—tetrasporangia, spermatangia, and carpogonia. Pericentral cells are arranged in regular tiers, with a single-ranked ring of pericentral cells surrounding one central cell. The pericentral and central cells of a single tier are referred to as peripheral and central siphons, respectively.

Thalli with a polysiphonous structure are also characteristic of the genus Odonthalia, which is widespread in cold northern seas. However, unlike in Polysiphonia, two groups of peripheral siphons, located opposite each other on Two Sides of the central siphon, undergo intensive longitudinal division. As a result, the opposite sides of the filament expand and gradually acquire a lamellar shape. Occasionally, the thallus thickens around the central part, forming a midrib.

Fig. 23.26. Polysiphonia: a - general view of the thallus; b - thallus fragment with short lateral branches; c - transverse section of the thallus; d - cystocarp; e - branch with spermatangia; f - branch with tetrasporangia. 1 - pericentral cells (siphons); 2 - central cell; 3 - spermatangium; 4 - tetrasporangia.

Evolutionary Trends in the Division

The evolution of red algae is primarily associated with the Evolution of the carposporophyte. Since the sexual process occurs via the fertilization of an egg cell by a passive, non-motile spermatatium incapable of taxis, its efficiency would be low due to the limited number of successful fertilization events combined with significant resource expenditure required to increase the probability of such events (increasing the number of carpogonia and spermatangia). In Florideophyceae, this problem is solved in an original way: sexual reproduction efficiency is increased by boosting carpospore production in each successful mating event. In primitive forms (Bangiales), the zygote directly forms carpospores, but the zygote's production capacity is limited by its size (in particular, A large number of daughter nuclei would disrupt the nucleocytoplasmic ratio in carpospores). The nucleocytoplasmic ratio problem is resolved by the appearance of gonimoblasts, which form carpospores (Nemaliales).

However, restrictions are now imposed by the shortage of nutrients within the zygote from which the gonimoblasts develop. The next step forward is providing the gonimoblasts with nutrients accumulated by specialized trophic systems—auxiliary cells. In more primitive orders (Corallinales), the zygotic nucleus enters the auxiliary cell via an ooblastema cell after the latter fuses with the carpogonium and the auxiliary cell. In more evolutionarily advanced orders, the carpogonium fuses directly with the auxiliary cell, bypassing intermediary cells (Gelidiales, Gigartinales, Rhodymeniales).

Additional nourishment for the gonimoblasts is also provided by surrounding vegetative cells of the carposporophyte, which are modified into trophic cells of the cystocarp walls. The maximum efficiency in utilizing nutrient resources for carposporophyte development is achieved when the auxiliary cell is formed only after fertilization (Ceramiales). In this case, nutrients are not conserved in auxiliary cells near unfertilized carpogonia, but are directed exclusively to where a successful fertilization event has taken place.

Distribution

Red algae are predominantly distributed in benthic and periphyton marine communities. Rhodophyta are most diverse and abundant in the tropics and subtropics, with species richness decreasing toward the poles. In warm seas, the represented algae are predominantly small in size. In cold seas, most Rhodophyta possess large, macroscopic thalli. A similar trend can be observed in the depth distribution of red algae—representatives with large thalli are found at greater depths. Among Rhodophyta, There are many deep-water species (with a lower depth limit of 200 m). It is believed that deep-water representatives utilize the energy of penetrating short waves in the violet and ultraviolet parts of the spectrum and therefore reflect long-wave red rays.

A quite significant number of Bangiophycideae species and some primitive Florideophyceae inhabit freshwater bodies (Compsopogon, Batrachospermum, Chantransia). Unlike marine forms, freshwater red algae are colored blue or steel-gray. Some Porphyridiales inhabit non-aquatic environments, notably soils (Porphyridium).

Quite a few marine Rhodophyta parasitize other red algae. Interestingly, about 90% of parasitic red algae have hosts that are systematically closely related to the parasite. This type of parasitism is termed adelphoparasitism.

In this type of relationship, the parasite begins to develop on the host's surface as an epiphyte; subsequently, one of its cells forms a projection that penetrates the host's Cytoplasm. At the contact zone, the cell walls of both the projection and the host dissolve, allowing the parasite's nucleus, Mitochondria, and Ribosomes to enter the host's cytoplasm. The parasite's nucleus divides faster than the host's nucleus. The infected cell forms new projections through which the adelphoparasite's nuclei, mitochondria, and ribosomes spread into other cells. Thus, soon the cytoplasm of the major part of the host cells is replaced by the parasite's cytoplasm.

A minor number of red algae parasitize hosts that are evolutionarily distant from the parasite, although they also belong to Rhodophyta. This type of parasitism is called alloparasitism.

Significance in nature

In marine phytocoenoses, red algae are generally subdominants, yielding in biomass and production to brown seaweeds. However, in some cases, Rhodophyta serve as the leading group of producers over fairly large areas (for instance, in the region of Zernov's phyllophora field).

Typically, Rhodophyta form the trophic base for numerous aquatic organisms. To protect themselves against excessive grazing, certain red algae (such as Laurencia) produce toxic substances. Animals resistant to these toxins (in particular, sea hares of the genus Aplysia) sequester them as a defense mechanism against predators. In the tropics, cryptonemiales algae "bind" coral branches with their calcareous thalli, thereby stabilizing coral reefs. Species of the genus Lithothamnion form lithothamnion limestones across various seas.

Practical Application

In human practice, red algae are utilized primarily in three main directions: as a source of phycocolloids, as food, and for pharmaceutical production.

Phycocolloids (agar, agaroid, carrageenan) are the most valuable products derived from red algae. These phycocolloids are sulfated and pyruvated galactans; 1–2% solutions of phycocolloids solidify at room Temperature to form firm gels. The source of agar for Northern Eurasian countries is Ahnfeltia plicata, which inhabits northern seas. In subtropical and tropical regions, agar is obtained mainly from Gracilaria and Gelidium. The raw material for agaroid production is Phyllophora (in Ukraine, it is used to produce the so-called Black Sea agar). The largest supplier of carrageenan on the global market is the Philippines, where seaweeds of the genus Euchema have been introduced into aquaculture. In northern countries, carrageenan is extracted predominantly from Chondrus.

Agar is used for the preparation of solid nutrient media and as a gelling agent in the confectionery industry. Agarose, the primary gel for Electrophoresis and Chromatography, is derived from agaroid. Carrageenan finds application as an emulsifier in Pharmaceuticals and dairy production, as well as a binding agent in the textile and leather industries.

Edible red algae are primarily represented by red sea lettuces such as Porphyra and Rhodymenia: the former is cultivated on specialized marine farms (including in the Black Sea), while the latter is harvested as a by-product during kelp fisheries. Porphyra is considered a delicacy, its flavor determined by free Amino Acids and isofloridosides.

The principal pharmaceutical products manufactured from red algae include treatments for hyperacidity (derived from cryptonemialean algae, predominantly Corallina officinalis and Lithothamnion), anthelmintics (Digenia), and hemagglutinins specific to human Blood group B (Ptilota plumosa).

Additionally, red algae are utilized as organic fertilizers and as bioindicators for assessing water quality.