Phycology - Kostikov I.Yu. - 2009-2013

Chapter 5. Cytological Diversity of Algae

The phylogenetic heterogeneity of Algae results in a considerable variety in the Organization of cellular Organelles and structures. Algal Cells, much like those of other organisms, consist of a protoplast surrounded by a phospholipid membrane, the Plasmalemma. The plasmalemma delimits The Cell from its external environment. Cells covered solely by a plasmalemma are termed naked. However, the protoplast is more frequently enclosed by a specific type of cell covering In addition to the plasmalemma.

Class="center">Types of Cell Coverings

Algal cell coverings perform diverse Functions, the primary ones being the maintenance of cell shape constancy, Protection of the protoplast from adverse mechanical and Chemical factors, osmoregulation, chemoreception, and participation in METABOLISM between the cell and the external environment. Furthermore, in some algae, coverings are associated with motility and phagotrophic Nutrition.

Algal cell coverings can be either internal or external.

Internal coverings form a cellular Cytoskeleton located beneath the plasmalemma; in some cases, additional covering elements may be present outside the plasmalemma, superficial to this framework. The framework may consist of protein strips or plates, a flattened Endoplasmic reticulum cisterna, or flattened membranous vesicles. Some types of framework elements are also underlain by cytoskeletal microtubules.

External coverings are located exclusively outside the plasmalemma, beyond the bounds of the protoplast. Based on their chemical composition, these coverings are divided into organic (cell walls¹) and mineral (tests and skeletons).

Cells lacking coverings—so-called naked cells—are found across all algal divisions except Cyanophyta. Coverings are most commonly absent in reproductive cells, such as Gametes and zoospores; naked vegetative cells occur less frequently. In many cases, naked cells are capable of changing shape during movement and occasionally forming pseudopodia. In certain algal divisions, naked cells bear submicroscopic scales composed of organic or mineral compounds On the surface of the plasmalemma. The divisions in which no representatives with cell coverings have been discovered are Chlorarachniophyta and Raphidophyta.

Internal coverings are characteristic of the vegetative cells of euglenophytes, dinoflagellates, cryptophytes, haptophytes, and glaucophytes. In euglenophytes, the plasmalemma is underlain on its inner side by proteinaceous strips, forming a pellicle. The covering of cryptophytes—the periplast—consists of rectangular protein plates located on the inner side of the plasmalemma and attached to it by special protein pins; in many cryptophyte species, external protein plates or external submicroscopic scales are also present outside the plasmalemma, superficial to the internal plates.

A distinctive type of internal covering is inherent to haptophytes: the inner side of the plasmalemma is underlain by a giant flattened endoplasmic reticulum membrane, while cellulosic scales or granules are deposited on the plasmalemma surface, which in some species become calcified and transform into so-called coccoliths.

In dinoflagellates, the plasmalemma is underlain by flattened membranous vesicles, inside of which a thin organic plate may reside. This type of covering is called an amphiesma. The coverings of glaucophytes are very similar to the amphiesma—here, the plasmalemma is likewise underlain by membranous vesicles containing an internal plate, but unlike dinoflagellates, cytoskeletal Microtubules are also positioned beneath the vesicles. The coverings of glaucophytes are termed an amphiesma-like pellicle.

Ejective structures can frequently be observed beneath the plasmalemma in naked cells and cells with internal coverings. Ejective structures are single-membrane capsules containing tightly coiled protein ribbons or mucus. Based on the packing pattern of the ribbons and the shape of the capsule, ejective structures are categorized into trichocysts (flask-shaped capsules with a protein ribbon folded in a hexagonal pattern), discobolocysts (capsules filled with fibrillary substance, above which a ring-coiled protein ribbon is located), and ejectosomes (elongated capsules in which the protein ribbon is packed in a rod-like fashion). Muciferous bodies, which are capsules filled with mucus, are also classified as ejective structures. Ejective structures perform a protective function: upon irritation of the plasmalemma in the vicinity of the capsule, the protein ribbons straighten and SHOOT outward, repelling the cell from obstacles or predators, or the capsule (in the case of muciferous bodies) expels mucus that forms a protective layer around the cell.

External coverings are subdivided into walls, tests, and external skeletons. The framework portion of cell walls is formed predominantly by Polysaccharides polymerized by enzyme complexes on The surface of the plasmalemma. Pectin is found in substantial quantities in all types of walls, frequently alongside Cellulose or hemicellulose, while specific framework substances—murein, alginic acid and its salts, phycocolloids, sporopollenin, Lignin, and Chitin—occur in distinct divisions.

Pectins, alginates, and phycocolloids function as a glue binding other Cell wall substances together; additionally, pectins readily hydrolyze, thereby forming external mucilage. Cellulose and hemicellulose form microfibrils, which primarily perform skeletal functions. Chemically stable murein additionally protects the protoplast from adverse chemical agents. Sporopollenin and lignin occur as accessory components in the cell walls of green algae and higher plants, enhancing mechanical resistance. Chitin is capable of forming various external outgrowths of cell walls, such as spines, spinules, bristles, and horns. Such elements are often present in unicellular planktonic algae and may play a role in regulating cell buoyancy.

According to the Chemical Nature of the primary skeletal substance, cell walls are classified into murein walls (in blue-green algae), pectin walls (in golden, eustigmatophyte, and yellow-green algae), cellulosic and cellulose-pectin walls (green algae and certain dinoflagellates), cellulose-pectin walls with alginates (brown algae), and cellulose-pectin walls with phycocolloids (red algae).

Mineral external coverings based on silica are characteristic of diatoms and Dictyochophytes. In diatoms, silica forms a frustule structured like a bivalved box. In Dictyochophytes, silica forms a system of external tubes interconnected in the shape of a basket. The siliceous basket is covered by a layer of mucus and is termed a siliceous Skeleton.

Algal cell coverings perform diverse functions, the primary ones being the maintenance of cell shape constancy, protection of the protoplast from adverse mechanical and chemical factors, osmoregulation, chemoreception, and participation in metabolism between the cell and the external environment. Furthermore, in some algae, coverings are associated with motility and phagotrophic nutrition.

Structure OF THE Protoplast

Based on protoplast structure, algae are clearly divided into two groups corresponding to the cellular architectural plans of the superkingdom Prokaryota and the superkingdom Eukaryota. What PROKARYOTES AND EUKARYOTES share is the presence of analogous functional systems, which are conventionally categorized into the genetic apparatus, protein-synthesizing apparatus, Respiratory system, and photosynthetic system. Structurally, however, these systems exhibit substantial DIFFERENCES BETWEEN PROKARYOTES and eukaryotes, reflecting the distinct evolutionary histories of these organismal groups.

Protoplast of Prokaryotic Algae

The genetic apparatus in prokaryotic algae, much like in other prokaryotes, is represented by a single circular DNA molecule attached to the plasmalemma. This DNA is devoid of histone Proteins, is termed the nucleoid, and is the analogue of a single chromosome.

The protein-synthesizing apparatus of prokaryotes is generally similar to the eukaryotic one, being represented by Ribosomes organized into protein-synthesizing complexes that comprise a single Messenger RNA molecule and several sequentially arranged ribosomes (so-called Polysomes).

Cells of prokaryotic algae are not compartmentalized. Therefore, both the respiratory and Photosynthetic Systems are integrated into the nucleocytoplasmic system of prokaryotic algae. The respiratory system is represented by a complex of Enzymes and electron carriers of the Electron Transport Chain; these substances are located on Regions of the plasmalemma that functionally replace Mitochondria.

The photosynthetic apparatus in prokaryotic algae is represented by flattened membranous structures—thylakoids—which originate as invaginations of the plasmalemma. Chlorophyll molecules, enzymes, and electron carriers participating in the light phase of Photosynthesis are embedded in the thylakoid membranes. The enzyme responsible for the initial stage of the Dark Phase of photosynthesis, ribulose-1,5-bisphosphate carboxylase-oxygenase (RuBisCo), is localized in the Cytoplasm in the form of so-called polyhedral bodies.

The complex of nucleocytoplasmic structures in prokaryotic algae also includes various inclusions, primarily Glycogen-like polysaccharide granules, cyanophycin, polyphosphate bodies, and gas vacuoles.

Protoplast of eukaryotic algae

In eukaryotic algae, the protoplast is divided into compartments that form the complex of core cell organelles. Genetic control over these organelles, with the exception of Mitochondria and Chloroplasts, is exercised by nuclear DNA. The functioning of mitochondria and chloroplasts is partially controlled by Mitochondrial and Chloroplast DNA, but the interaction between the nuclear, mitochondrial, and chloroplast genomes is regulated by The Nucleus. Therefore, in eukaryotes, the GENETIC APPARATUS OF the cell is represented precisely by the nuclear DNA.

Genetic apparatus. The nuclear genome is represented by DNA molecules with a chromosomal organization: DNA is bound to histone proteins, forming Chromatin microfibrils. Depending on the stage of the Cell Cycle, chromatin can exist in a despiralized, spiralized, or hyperspiralized state. Despiralized chromatin—isochromatin—is functionally active, as Replication and Transcription processes take place precisely within isochromatin regions. Spiralized chromatin—heterochromatin—is functionally inactive; however, through despiralization, it is capable of transitioning into the isochromatin state and acquiring functional activity. Chromatin is in a hyperspiralized state predominantly during mitosis or Meiosis, at which point it becomes visible under a Light Microscope as Chromosomes.

GENES OF THE nuclear DNA are not grouped into operons, since each Gene has its own promoter and replicates and transcribes autonomously, independently of neighboring genes. Structurally, each gene of the nuclear genome includes a nucleotide sequence comprising exons and introns. After transcription into messenger RNA synthesized on The basis of a specific gene, introns are excised, and splicing (joining) of exons takes place within the messenger RNA. Following splicing, the messenger RNA exits the nucleus into the cytoplasm, where Protein Biosynthesis occurs based upon it.

The nuclear genome of eukaryotes is significantly larger compared to prokaryotes: nuclear DNA contains from several tens of millions to several hundreds of millions of nucleotide Base Pairs, whereas the DNA length of prokaryotic algae is measured in only a few million base pairs.

A number of deviations from the typical scheme of nuclear genome organization have been found in eukaryotic algae. In particular, in the divisions Euglenophyta, Dinophyta, and Cryptophyta, DNA remains in a hyperspiralized state throughout the entire cell cycle; therefore, following appropriate cytochemical staining, chromosomes can be observed in representatives of these divisions even during interphase.

Differences from the typical chromosomal organization of nuclear DNA have been discovered in dinoflagellate algae. Specifically, in most representatives of this division, the histone content is very low, and the "nuclear DNA : Histones" ratio averages 1 : 0.04, compared to the typical eukaryotic ratio of 1 : 1.

Nucleocytoplasm of eukaryotic algae. The nucleocytoplasmic structures in eukaryotes include the nuclear apparatus, single-membrane organelles (endoplasmic reticulum, Golgi apparatus, digestive vacuoles, Lysosomes, Peroxisomes, cell sap vacuoles, contractile vacuoles), inclusions, and a complex of organelles and structures built from tubulin microtubules, as well as a number of additional structures formed As a result of secondary endosymbiosis (in particular, the chloroplast endoplasmic reticulum).

The nuclear apparatus includes the nuclear envelope, chromosomes, and the nucleolus. The nuclear envelope is formed by two phospholipid membranes and separates the nuclear DNA from the cytoplasm. The thickness of the nuclear envelope membranes is 7-8 nm. The space between the nuclear membranes is called the perinuclear space. The nuclear membrane features nuclear pores with a diameter of 30-100 nm. Each nuclear pore contains eight peripheral globules and one large central globule. The structure of the pore apparatus in the nuclear envelope differs from that observed in the membranes of Mitochondria and Plastids.

The outer membrane of the nuclear envelope can form outpocketings toward the cytoplasm that are morphologically similar to the channels of The endoplasmic reticulum. Such outpocketings are called the perinuclear endoplasmic reticulum. This reticulum is highly developed in haptophytes and red algae. Sometimes the channels of the perinuclear reticulum transition into the channels of the endoplasmic reticulum, which is considered one of the proofs of the common Water/144.html">Origin of the Endoplasmic reticulum and the nuclear envelope. In some algae, single-membrane vesicles (nuclear vesicles) can bud off from the outer membrane of the nuclear envelope; these can subsequently move through the cytoplasm and fuse with the endoplasmic reticulum, the Golgi apparatus, or the plasmalemma.

The nuclear envelope has a special structure in most algae with secondarily symbiotic plastids, which are covered by four membranes. Here, the outer membrane of the nuclear envelope transitions into the outer, fourth membrane of the chloroplast envelope—the chloroplast endoplasmic reticulum. In this case, the nucleus and the plastid form a single structural entity. Chlorarachniophytes, Raphidophytes, Chrysophytes, Xanthophytes, Eustigmatophytes, Brown algae, Diatoms, Dictyochophytes, Haptophytes, and Cryptophytes possess a connection between the nuclear envelope and the plastid.

One manifestation of the cytological Diversity of the nuclear apparatus is The behavior of the nuclear envelope during nuclear division. In most eukaryotes, the nuclear envelope disintegrates during prophase, and the chromosomes are positioned directly in the cytoplasm during division. Mitosis with this type of nuclear envelope behavior is called open. However, in some algae, the behavior of the nuclear envelope may differ. In particular, the envelope may persist (remain intact) during nuclear division. In this case, mitosis is called closed. In some algal taxa, the envelope breaks down only at the poles of the nucleus during division, and the microtubules of the spindle apparatus pass into the nucleus through these openings. Mitosis with partial breakdown of the nuclear envelope is called semi-open.

In all eukaryotic algae, one or several nucleoli are present in the nucleus. Nucleoli are formed by accumulations of ribosomal RNA synthesized in the nucleus and complexes of this RNA with ribosomal proteins—the so-called pre-ribosomes. Nucleoli are usually observed in the interphase nucleus and disappear before the onset of mitosis or meiosis. However, in euglenoid algae, the nucleolus does not disappear before division; instead, it divides, and the daughter nucleoli subsequently become the organizing centers for the spindle microtubules. The nucleoli in euglenoids are called endosomes. The ability of the nucleolus to divide has also been discovered in some green algae belonging to the order Zygnematales.

The endoplasmic reticulum (ER) in algae of various divisions has no fundamental differences. Certain special modifications of this structure have been noted in haptophytes, in which the peripheral channels of the endoplasmic reticulum form a large flattened cisterna lining the plasmalemma, thus serving as a component of the cell covering and a special flagellum-like structure—the haptonema. The endoplasmic reticulum is capable of transitioning into the channels of the perinuclear reticulum and also of structurally combining with the Golgi apparatus. The Functions of the endoplasmic reticulum do not differ from those known in other eukaryotes: in particular, cytoplasmic 80S ribosomes are located on the endoplasmic reticulum, forming a complex known as the rough endoplasmic reticulum; the smooth endoplasmic reticulum performs transport functions, and Lipids and polysaccharides are also synthesized within its channels, vesicles, and cisternae.

One or more Golgi apparatuses are present in the cells of all eukaryotic algae. It is usually located near the nucleus. In raphidophytes, the Golgi apparatus is very large, closely adjoined to the nuclear envelope, can be visible under a light microscope, and is termed the supranuclear apparatus. In addition to the functions inherent to other eukaryotes, in many algae the Golgi apparatus plays an important role in The formation of cell coverings, since the synthesis of submicroscopic surface scales takes place within its cisternae and vesicles. In some algae, vesicles that pinch off from the cisternae of the Golgi apparatus at the final stages of mitosis can participate in the formation of new nuclear envelopes.

In algae capable of phagotrophic nutrition, digestive vacuoles may be observed in the cytoplasm. Digestive vacuoles are particularly characteristic of primarily heterotrophic algae from the divisions Euglenophyta, Dinophyta, and Cryptophyta, as well as of chlorarachniophytes and some raphidophytes and chrysophytes capable of mixotrophic nutrition.

Single-membrane nucleocytoplasmic organelles also include lysosomes. Similar to digestive vacuoles, lysosomes contain a complex of hydrolytic enzymes and participate in intracellular Digestion processes.

Representatives containing single-membrane vesicles with electron-dense contents, termed Microbodies or peroxisomes, have been found in almost all algal divisions. It has been established that peroxisomes contain the enzyme peroxidase, which decomposes hydrogen peroxide into oxygen and water and thus plays an important role in the light phase of photosynthesis.

In algae covered by cell walls, cell sap vacuoles can frequently be observed in the cytoplasm. These vacuoles are separated from the cytoplasm by an ordinary phospholipid membrane called the tonoplast. Cell sap vacuoles contain water, organic acids, Amino Acids, and Mineral Substances; they play an important role in regulating cellular osmotic pressure and accumulate a number of cell metabolism products. Cell sap vacuoles are particularly characteristic of multicellular algae from the divisions Phaeophyta, Rhodophyta, and Chlorophyta, as well as of certain unicellular algae, notably diatoms.

In most freshwater flagellated algae, as well as in some non-motile representatives, the osmoregulatory function is performed by contractile (pulsating) vacuoles, which expel excess water that constantly enters the cell. Contractile vacuoles in most algae are paired, located at the anterior end of the cell, and expel excess water directly into the surrounding environment. In euglenoids and raphidophytes, contractile vacuoles are numerous and located at the bottom of the reservoir (gullet); as they fill with water, small vacuoles fuse into a single large one that discharges its contents into the gullet. The water is then expelled to the outside through the reservoir canal.

In dinoflagellate algae, the functions of contractile vacuoles are performed by systems of large branched invaginations of the plasmalemma called pusules. Pusules are incapable of pulsation and open to the outside via a narrow canal. Water entering the pusules from the cell is passively expelled outward through the canal.

The system of microtubular organelles and structures is controlled by the nuclear genome. The basis of all structures in this system consists of microtubules built from tubulin protein subunits. Microtubules have The ability to elongate by attaching alpha- and beta-tubulin molecules, or to depolymerize by cleaving off these monomers. As a consequence, microtubular systems within the cell are predominantly highly dynamic formations. Microtubules are the main structural element of the flagellar apparatus and its derivatives—centrioles; spindle fibers are formed from microtubules, and microtubules constitute the skeletal framework of the cytoskeleton.

Flagellar apparatus. In algae, as in other eukaryotes, the flagellar apparatus includes flagella, their basal bodies, and a system of flagellar roots. A flagellum is formed by an outer outpocketing of the plasmalemma enclosing nine peripheral pairs of microtubules and two single central microtubules. This arrangement of microtubules is described by the formula "9+2", and the microtubular part of the flagellum itself is called the axoneme. The structure of the flagellar axoneme following the "9+2" pattern is considered universal for all eukaryotes. However, certain modifications of this structural plan are known in algae. In particular, in the non-motile male gametes of diatoms, the central pair of microtubules is absent (the "9+0" formula), and in many dinoflagellates, the peripheral part of the axoneme contains triplets of microtubules instead of doublets.

The surface of the flagellum can be smooth or bear submicroscopic hairs—mastigonemes, or submicroscopic scales. If mastigonemes are located across the entire surface of the flagellum, such a flagellum is called tinsel; if they are located unilaterally, it is called plumose. Mastigonemes in algae are quite diverse and are generally divided into simple, bipartite, and tripartite. Simple and bipartite mastigonemes are synthesized primarily on the flagellar plasmalemma and/or in vesicles of the Golgi apparatus. Tripartite mastigonemes—mastigonemes of the retoneme type—are formed as a result of a multi-step process of synthesis and assembly of individual parts, in which the nuclear envelope, chloroplast endoplasmic reticulum, Golgi apparatus, and plasmalemma participate.

At the Base of the flagellum lies a transition zone, followed by the flagellar basal body. In the transition zone, the central microtubules disappear, and various additional structures may appear depending on the taxonomic group: spirals (in Chrysophyta, Eustigmatophyta, Xanthophyta, Dictyochophyta), discs (Dinophyta, Cryptophyta), transition rings (Dinophyta, Haptophyta), or a stellate structure (Chlorophyta). The structure of the flagellar transition zone is considered an important taxonomic feature at the phylum (division) level.

Basal bodies of flagella are formed by the peripheral microtubules of the axoneme, where each doublet is augmented by an additional microtubule (formula "3(9) + 0"). Deviations from this structural plan are found only in the basal bodies of male gametes in diatoms, in which the basal bodies consist of nine doublets of microtubules rather than nine triplets.

Attached to the basal bodies are flagellar roots, which anchor the flagella within the cell and connect the flagellar apparatus to other organelles. The flagellar ROOT system includes microtubular (or microtubular-type) and microfibrillar roots. Microfibrillar roots are composed primarily either of the protein Actin (such roots appear smooth under an Electron microscope) or of the contractile protein centrin (appearing as striated structures). Centrin-based striated roots often connect the basal bodies of the flagella to the nucleus, in which case they are called rhizoplasts.

The types of flagellar root systems in algae are highly diverse. The differences among these systems lie in the number and proportions of microtubular and microfibrillar roots, the number of microtubules comprising the microtubular roots, the presence of additional structures on the flagellar roots (such as the multilayered structure), and the spatial arrangement of the roots (in particular, the organelles associated with the proximal end of each root).

In some algae, photoreceptor systems are associated with the flagellar apparatus. Specifically, in euglenids, golden algae, eustigmatophytes, yellow-green algae, and brown algae, the photoreceptor function is performed by a Swelling at the base of the flagellum known as the parabasal body, which contains photoreceptor flavin-like pigments. In cryptomonads and green algae, the photoreceptor function is carried out by a region of one of the plastid membranes; in some dinoflagellates and haptophytes, it is a condensed region of the cytoplasm; and in certain dinoflagellates, it is a complex cytoplasmic ocelloid (eyespot-like structure).

The photoreceptor may be coordinated with a stigma (eyespot), which acts as a screen enabling the cell to determine the direction of incident light falling on the photoreceptor. The stigma is red-pigmented and may be located either in the cytoplasm (Euglenophyta, Eustigmatophyta, Dinophyta) or within the plastid (Chrysophyta, Xanthophyta, some Dinophyta, Haptophyta, Cryptophyta, and Chlorophyta).

In flagellated cells, microtubular roots form the basis of the internal cell skeleton, or cytoskeleton. In addition to microtubular roots and individual Cytoplasmic microtubules, the cytoskeleton includes thin actin filaments (microfilaments) and short connecting protein strands (Intermediate filaments). The complex of microtubular roots, cytoplasmic microtubules, actin microfilaments, and intermediate filaments forms an intricate and dynamic internal network that enables the cell and all its organelles to function as a unified structural whole.

Centrioles are organelles derived from flagellar basal bodies. They have been found in members of Chrysophyta, Xanthophyta, Phaeophyta, and Chlorophyta. Centrioles serve as the microtubule-organizing centers (MTOCs) for the mitotic spindle. Their structure mirrors that of flagellar basal bodies, consisting of nine triplets of microtubules arranged in a circle. The affinity between centrioles and flagellar basal bodies is supported by the fact that in many algae lacking centrioles yet possessing flagella, the basal bodies themselves function as the microtubule-organizing center for the spindle (e.g., in Euglenophyta, Raphidophyta, Eustigmatophyta, Glaucocystophyta, and some Chlorophyta).

In a number of algal phyla, the microtubule-organizing centers of the spindle may be highly specific structures distinct from basal bodies and centrioles, in which microtubules are entirely absent: polar plates (diatoms), polar rings (red algae), the kinoplasmic sphere—a condensed cytoplasm surrounded by dictyosomes and the endoplasmic reticulum (dinoflagellates)—and cisternae of the endoplasmic reticulum along with rhizoplasts (haptophytes, cryptomonads, and certain green algae).

The mitochondrial apparatus in eukaryotic algae is relatively uniform: mitochondria are delimited from the cytoplasm by a double-membrane envelope. The inner membrane forms invaginations called cristae, which harbor enzymes and electron carriers that drive cellular respiration, ultimately resulting in the Synthesis of the bulk of ATP.

Mitochondria possess their own Mitochondrial DNA organized similarly to a prokaryotic nucleoid: it is circular, devoid of histones, and its genes are organized into operons. However, unlike the prokaryotic genome, the Mitochondrial Genome is heavily reduced, containing an average of 15–80 kb of base pairs, compared to 1.5–4 Mb in prokaryotes. Mitochondria feature their own protein-synthesizing machinery, represented by prokaryotic-type 70S ribosomes. Much like prokaryotes, mitochondria multiply by binary fission.

The Diversity of the mitochondrial apparatus is primarily manifested in the Morphology of the mitochondrial cristae. For instance, euglenids possess Three types of cristae within their mitochondria: lamellar, tubular, and discoid. In glaucophytes, green, red, and cryptophyte algae, only lamellar cristae are present, whereas tubular cristae predominate in the mitochondria of other phyla.

The photosynthetic apparatus in eukaryotic algae is represented by chlorophyll-containing plastids known as chloroplasts. Like mitochondria, chloroplasts are endosymbiotic organelles with their own Chloroplast Genome and protein-synthesizing apparatus. A chloroplast is bounded from the cytoplasm by a double-, triple-, or quadruple-membrane envelope. The number of envelope membranes reflects the evolutionary origin of the plastid (primary or secondary endosymbiosis).

Primary endosymbiotic plastids possess a double-membrane envelope. The envelope membranes contain prokaryotic-type submicroscopic pores formed by specialized Membrane Proteins called porins. On the outer membrane surface, porins form three channels that merge on the inner side into a single central pore channel. Such pores are also found in the mitochondrial envelope and the prokaryotic plasmalemma.

Flattened invaginations of the inner membrane, called thylakoids, lie beneath the chloroplast envelope. Embedded in the thylakoid membranes are molecules of chlorophylls as well as accessory Photosynthetic Pigments: carotenes, xanthophylls, and, in some algae, phycobilins. Consequently, thylakoids are considered the elementary structural photosynthetic unit of the plastid. Thylakoids may occur singly or aggregate into groups known as lamellas (lamellae). When a lamella comprises numerous closely appressed thylakoids stacked one upon another, and these stacks are interconnected by single unstacked thylakoids, a granum is formed. Thylakoids can be solitary (e.g., in red algae), form lamellae of two or three thylakoids (in the vast majority of phyla), or group into grana (in some green algae). The type of thylakoid organization serves as a taxonomic marker at the phylum level.

In the chloroplasts of many algae, a specialized structure called a pyrenoid can be observed. A pyrenoid is a proteinaceous body composed of the enzyme ribulose-1,5-bisphosphate carboxylase-oxygenase (RUBISCO). The activity of this enzyme regulates the Initial Stages of the dark reactions of photosynthesis. The pyrenoid may be surrounded by a starch sheath, making it easily visible under a light microscope; however, in most algae, the pyrenoid is "naked," requiring specialized cytochemical staining or Electron Microscopy for visualization.

Similar to mitochondria, the chloroplast contains its own genome represented by chloroplast DNA organized in a prokaryotic fashion—specifically, genes are grouped into operons, and the DNA is histone-free. The chloroplast DNA nucleoid may appear compact or exist as multiple fragments scattered throughout the plastid matrix. Chloroplast DNA is larger than mitochondrial DNA but smaller than the prokaryotic nucleoid DNA, with its size ranging between 100 and 300 kb of base pairs. The chloroplast matrix houses its own protein-synthesizing machinery, represented by prokaryotic 70S ribosomes.

Primary endosymbiotic plastids are found in three algal phyla: Glaucocystophyta, Rhodophyta, and Chlorophyta. A shared feature of primary endosymbiotic plastids is the presence of only two membranes in the chloroplast envelope. However, in glaucophyte algae, a layer of murein—representing a remnant of The cell wall of the prokaryotic endosymbiotic alga—persists between the outer and inner membranes. In red and green algae, murein is absent from the plastid envelopes. The phylum Rhodophyta is characterized by plastids with solitary, unstacked thylakoids that lack lamellae and bear phycobilisomes on their surface, composed of phycobilin pigments. Chloroplasts of green algae lack phycobilisomes; their thylakoids are assembled into lamellae, which in many taxa form grana morphologically similar to those of higher plants.

Secondary endosymbiotic plastids are characteristic of all other eukaryotic algal phyla. These plastids arose through the endosymbiosis of a eukaryotic heterotrophic host cell with a eukaryotic alga from Rhodophyta or Chlorophyta that already possessed a primary endosymbiotic plastid. Accordingly, secondary endosymbiotic plastids are subdivided by origin into secondary endosymbiotic rhodoplasts and secondary endosymbiotic chloroplasts.

All currently known secondary endosymbiotic rhodoplasts are surrounded by four membranes, of which the two innermost are the envelope membranes of the red algal endosymbiont's chloroplast. The third membrane represents a modified plasmalemma of the endosymbiont, whereas the fourth is a remnant of the food vacuole membrane of the host cell that engulfed the red alga without digesting it. The space between the second and third membranes is called the periplastidal compartment and represents the remnants of the red algal cytoplasm. Eukaryotic 80S ribosomes are frequently found within the periplastidal compartment, and in some phyla, assimilation products may be stored there. The third and fourth membranes are referred to as the chloroplast endoplasmic reticulum. As a rule, the outer membrane of the chloroplast endoplasmic reticulum is continuous with the outer nuclear membrane.

In cryptophytes, a reduced Nucleus of the red algal endosymbiont, termed the nucleomorph, persists within the periplastidal compartment. The nucleomorph is bounded by a double membrane, with the outer membrane bearing pores morphologically similar to nuclear pores. DNA has been detected in the nucleomorph, which, according to molecular data, comprises about ten genes related to homologous nuclear genes of Rhodophyta. Thylakoids in cryptomonads are paired (forming paired-thylakoid lamellae).

In other algae with secondary endosymbiotic rhodoplasts—all chromophytic algae (Raphidophyta, Chrysophyta, Eustigmatophyta, Xanthophyta, Phaeophyta, Bacillariophyta, Dictyochophyta), haptophytes, and certain dinoflagellates—the nucleomorph is absent, lamellae are formed by triplets of thylakoids, and phycobilin pigments are lacking.

Secondary endosymbiotic chloroplasts are typical of algae belonging to Euglenophyta, Chlorarachniophyta, and certain representatives of Dinophyta.

In chlorarachniophytes, the chloroplast is enveloped by four membranes, the outer two of which form the chloroplast endoplasmic reticulum. Eukaryotic 80S ribosomes and a nucleomorph are present in the periplastidal compartment. Much like in cryptomonads, the nucleomorph is separated from the periplastidal compartment by a double-membrane envelope and contains DNA that, based on molecular data, represents a highly reduced green algal nuclear genome. Thylakoids are grouped into lamellae of two or three.

Two Types of secondary endosymbiotic chloroplasts have been identified in dinoflagellates. First-type secondary endosymbiotic chloroplasts resemble those of chlorarachniophytes because they are also bounded by four membranes and retain a nucleomorph in the periplastidal compartment. Second-type secondary endosymbiotic chloroplasts lack a nucleomorph and are bounded by only three membranes. Triple-membrane secondary endosymbiotic chloroplasts lacking a nucleomorph are also characteristic of all photoautotrophic euglenids.

Thus, algal plastids are considerably more diverse than those of higher plants. The type and fine Structural Features of plastids serve as important taxonomic markers at the division level and provide insight into THE ORIGIN OF this organelle.

¹ A special type of organic covering—the pseudo-theca—is found in certain green algae belonging to the class Prasinophyceae. Morphologically, this covering resembles a cell wall, but it is formed by the fusion of submicroscopic organic scales on the surface of the plasmalemma.