Algology - Kostikov I.Yu. - 2009-2013

Chapter 12. Raphidophyte Algae – Raphidophyta

Until the 1970s, raphidophyte Algae, under the name Chloromonadophyceae, were grouped with cryptophyte and dinophyte algae within the division Pyrrophyta, or with euglenophyte algae within the division Euglenophyta. The accumulation of data regarding the biochemical features and cytological specificities of species previously included in Chloromonadophyceae led to the Separation of this group into an independent division named Chloromonadophyta. Later, in accordance with the requirements of the International Code of Botanical Nomenclature, the division was renamed Raphidophyta. Today, about 50 species belonging to 10 genera are known within this division; all representatives are exclusively unicellular monadoid algae inhabiting freshwater continental Water bodies and seas.

Class="center">Taxonomic CHARACTERISTICS OF THE Division

Pigments and Reserve Nutrients

The Biochemical characteristics of the division include the presence of chlorophylls a and c, β-carotene, and xanthophylls of the lutein series (violaxanthin, lutein). In addition, freshwater raphidophytes contain diadinoxanthin, dinoxanthin, heteroxanthin, and vaucheriaxanthin, whereas marine forms contain fucoxanthin. Consequently, the METABOLISM/14.html">Chloroplasts of freshwater species are usually green or yellowish-green in color, while those of marine species are yellow or yellowish-brown.

The assimilation product is oil, which accumulates in the Cytoplasm¹. In some cases, small single oil droplets are also found in the chloroplast stroma.

Cytological Characteristics

Raphidophyte Cells are relatively large (30–100 µm), typically dorsiventral in Structure, with a triangular gullet, two heterokont and heteromorphic flagella, and a narrow longitudinal groove located on the ventral side of The Cell.

Cell coverings consist of a Plasmalemma, beneath which mucilaginous bodies and/or trichocysts may be located. When irritated, trichocysts discharge mucous threads outward.

The nuclear apparatus of raphidophyte algae has a typical eukaryotic structure. There is a single large nucleus with one or more nucleoli. A so-called supranuclear apparatus is positioned above the nuclear envelope in the shape of a wide cap. Electron microscopic studies have shown that the supranuclear apparatus consists of several Golgi complexes closely adhering to the nuclear envelope. Microfibrillar flagellar roots connect to the supranuclear apparatus (Fig. 12.1).

Fig. 12.1. STRUCTURE OF THE cell (a) and chloroplast (b, c) of raphidophyte algae: 1 - flagellar basal bodies, 2 - tinsel flagellum, 3 - tripartite mastigonemes, 4 - gullet, 5 - microtubular roots connecting basal bodies to the vacuolar apparatus, 6 - smooth flagellum, 7 - large contractile vacuole, 8 - small contractile vacuoles, 9 - one of the Golgi complexes of the supranuclear apparatus, 10 - nucleolus, 11 - nucleus, 12 - oil droplet, 13 - mitochondrion with tubular cristae, 14 - channel of The Endoplasmic reticulum, 15 - fragment of the perinuclear cisterna of the endoplasmic reticulum, 16 - chloroplasts, 17 - striated microfibrillar ROOT, 18 - smooth microfibrillar root, 19 - microtubular root with a multilayered structure, 20 - mucilaginous bodies, 21 - plasmalemma, 22 - channel of the endoplasmic reticulum transitioning into the outer chloroplast membrane, 23 - first outer chloroplast membrane, 24 - periplastidial reticulum, 25 - second outer chloroplast membrane, 26 - two inner chloroplast membranes, 27 - girdle lamella, 28 - small oil droplet, 29 - threetylakoid lamellae, 30 - pyrenoid, 31 - single and paired thylakoids penetrating the pyrenoid stroma, 32 - genophore (a, b - freshwater representative (Vacuolaria), c - marine (Fibrocapsa)). Schematized after Heywood, 1972, 1977, 1990; Hara, Chihara, 1985).

Mitosis in raphidophytes is closed: the nuclear envelope remains intact during nuclear division, and the spindle is intranuclear. The Functions of centrioles are performed by the basal bodies of the flagella.

Photosynthetic apparatus. The chloroplasts in Raphidophyta are small, numerous, and located at the cell periphery. Each chloroplast is covered by a four-membraned envelope. The two outer membranes form the chloroplast endoplasmic reticulum, which transitions into channels of the endoplasmic reticulum, although it is not continuous with the nuclear membrane. Between the two outer and two inner chloroplast membranes lies the periplastidial space with the periplastidial reticulum.

Thylakoids are grouped in threes to form several lamellae. In freshwater species, chloroplasts lack pyrenoids and possess a girdle lamella. In marine representatives, pyrenoids are present; they are semi-embedded and typically traversed by two thylakoids, while the girdle lamella is absent. The chloroplast DNA-genophore is closed into a ring and located near the chloroplast envelope.

Mitochondria are branched, forming a mitochondrion. Mitochondrial profiles in cross-section exhibit tubular cristae.

Flagellar apparatus. Two flagella of unequal length are located at the anterior end of the cell or slightly laterally. The longer flagellum is tinsel-type, typically spirally coiled at the tip, and provides cell locomotion. It is covered with mastigonemes of tripartite (so-called stramenopile) structure. Mastigoneme formation occurs within the cisternae of the endoplasmic reticulum. The short flagellum is smooth, usually functions as a rudder, and lies within the longitudinal ventral groove. Parabasal swellings on the flagella and an eyespot have not been detected in raphidophytes. The transition zone of the flagellum lacks a helical structure.

Two root systems—gullet and nuclear—arise from the basal bodies of the flagella. The gullet system consists of three to four microtubular roots located in the cytoplasm near the gullet. The nuclear root system includes one microtubular root with a multilayered structure and two microfibrillar roots: smooth and striated. The microfibrillar roots connect the flagellar basal bodies to the supranuclear apparatus, while the microtubular root runs along The Nucleus toward the posterior end of the cell.

The vacuolar apparatus is represented by one large and several small contractile vacuoles that discharge their contents into the gullet.

Type of Body Structure

All raphidophyte algae have a monadoid type of body structure. The cells are slightly metabolic, predominantly dorsiventral or laterally compressed, with a longitudinal groove from which rhizopodia may extend in colorless forms.

Reproduction and Life Cycles

Reproduction occurs by longitudinal Cell Division in the motile state. Before division begins, the smooth flagellum is usually shed, and the tinsel flagellum replicates. Upon completion of division, each cell possesses a tinsel flagellum and develops a new smooth flagellum. Zoospores and sexual reproduction are unknown in Raphidophyta.

The life cycle is simple, similar to those of cryptophyte algae. Under unfavorable conditions, cells transition into a palmelloid state. Spherical cysts are also observed, which sometimes feature an opening plugged by a stopper.

Fig. 12.2. Life Cycle of Raphidophyta: 1 - monad vegetative cell, 2 - cell division into two in the monad state, 3 - cell in the palmelloid state, 4 - cyst, 5 - cyst germination into a monad vegetative cell.

Feeding Characteristics

The vast majority of raphidophytes are photoautotrophs. However, some Representatives of the division lack chloroplasts and are obligate heterotrophs. Nutrition in heterotrophic species occurs via an osmotrophic pathway. Species capable of holozoic feeding are also found. They capture food (Bacteria, detritus particles, other algae) using rhizopodia; Digestion takes place within digestive vacuoles.

Taxonomy of the Division

All Raphidophyta belong to a single class, Raphidophyceae, and two orders, Raphidiales (=Vacuolariales) and Chattonellales. The division into orders is based on ecological characteristics, the composition of accessory xanthophylls, and Structural Features of the chloroplast, specifically the presence of pyrenoids and a girdle lamella.

The order Raphidiales comprises freshwater phototrophic and heterotrophic raphidophyte algae, in which the pigments diadinoxanthin and dinoxanthin predominate, girdle thylakoids are present, and pyrenoids are absent. Subdivision into families and genera is primarily based on the type of nutrition, presence or absence of trichocysts, position of the gullet, cell shape, and ability to form pseudopodia. The main genera are Vacuolaria, Goniostomum, Merotrichia, Thaumatomastix, and Hyaloselene (Fig. 12.3).

The genera Vacuolaria, Goniostomum, and Merotrichia encompass photoautotrophic algae. Vacuolaria possesses mucous bodies but lacks trichocysts. In contrast, Goniostomum and Merotrichia possess trichocysts; in the former, the flagella and gullet are nearly apical, whereas in the latter, they are displaced ventrally. The genera Thaumatomastix and Hyaloselene represent heterotrophic algae that feed both osmotrophically and holozoically, capturing food particles (detritus, bacteria, other algae) via pseudopodia.

Order Chattonellales comprises marine autotrophic raphidophytes whose main xanthophyll is fucoxanthin, with pyrenoids present and girdle thylakoids absent. Characteristic marine representatives include the genera Chattonella, Heterosigma, and Fibrocapsa (Fig. 12.4).

Fig. 12.3. Some representatives of the order Raphidiales: 1 - Vacuolaria, 2 - Goniostomum, 3 - Merotrichia, 4 - Thaumatomastix, 5 - Hyaloselene (after Matvienko, Litvinenko, 1977).

Fig. 12.4. Some marine raphidophyte algae of the order Chattonellales: 1 - Chattonella, 2 - Fibrocapsa (schematized after Hara, Chihara, 1982, 1985; Khan et al., 1996).

Distribution, Ecology, and Significance

All raphidophyte algae are planktonic inhabitants of freshwaters and seas. Freshwater raphidophytes are predominantly acidophiles, most frequently found in Sphagnum bogs in spring and autumn. Marine raphidophytes are neutrophiles.

For a long time, it was believed that raphidophyte algae had no practical significance. However, in the early 1980s, the first reports emerged regarding toxic "red tides" off the coast of Japan caused by the raphidophytes Chattonella antiqua and Ch. marina. These algal blooms caused mass mortality of fish in marine fish-farming cages and inflicted significant economic damage on the aquaculture industry. Later, toxic species were also discovered among other genera of marine Raphidophyta.

Currently, three neurotoxins, hemolytic toxins, and hemagglutinating toxins have been identified in raphidophytes. Even at low cell densities (3–10 thousand cells/mL), raphidophytes disrupt motor coordination in fish, induce abnormal behavior, and lead to mortality within 50–80 minutes. Raphidophytes exhibit the highest toxicity during their active vegetative stage—the so-called logarithmic growth phase.

Position of Raphidophyta in the System of the Organic World

Relatively recently—back in the 1980s—raphidophytes were viewed as a taxon with problematic phylogenetic affinities: some authors placed Raphidophyta close to cryptophyte algae, while others favored euglenophytes. However, following the Discovery of the stramenopile flagellar mastigoneme architecture, raphidophytes began to be considered primitive stramenopiles. Subsequent cytological studies (in particular, elucidating chloroplast structure and mitochondrial cristae Morphology) confirmed the validity of this viewpoint.

In 1996, The nucleotide sequence of the nuclear Gene encoding the 18S ribosomal RNA subunit was deciphered in the raphidophyte alga Heterosigma carterae. This made it possible to include raphidophytes in molecular phylogenetic analyses. Based on molecular phylogenetic reconstructions, the latter hypothesis was confirmed, and data were obtained indicating that Raphidophyta are most closely related to Chrysophyceae and Eustigmatophyceae.

¹ Starch, previously thought to be characteristic of raphidophyte algae, was known only in a single genus, Monomastix, which has now been excluded from Raphidophyta and transferred to green algae.