Algology - Kostikov I.Yu. - 2009-2013

Chapter 14. Eustigmatophyte Algae – Eustigmatophyta

In 1970, during electron microscopic studies, D. Hibberd and G. Leedale noticed that within the division of yellow-green Algae, there were species exhibiting two distinct ultrastructural Cell Organization patterns. Algae with the first pattern corresponded to "typical" yellow-green algae, whereas the second pattern shared certain morphological similarities with golden algae. It was proposed to separate the species of the second pattern into an independent Class of yellow-green algae - Eustigmatophyceae. Later, significant differences between eustigmatophytes and other yellow-green algae were also discovered at the biochemical and molecular levels, leading to the establishment of a new division, Eustigmatophyta, among stramenopile Algae in the 1990s.

The division comprises about 30 species of unicellular, exclusively coccoid organisms, distributed mainly in soils and marine plankton. According to T. Norton and co-workers (Norton & al., 1996), this division is quite diverse, and its expected species richness is estimated to range from 1,000 to 10,000 species.

Taxonomic CHARACTERISTICS OF THE Division

Pigments and Reserve Nutrients

Biochemical characteristics include the presence of chlorophyll a, β-carotene, and xanthophylls of the lutein series, with violaxanthin predominating; other xanthophylls of this group - lutein, neoxanthin, zeaxanthin, and antheraxanthin - occur in trace quantities. Among the specific xanthophylls found in eustigmatophytes is vaucheriaxanthin, which also occurs in yellow-green algae of the genus Vaucheria. Green chlorophylls and yellow violaxanthin give the METABOLISM/14.html">Chloroplasts of eustigmatophytic algae a yellowish-green color.

The assimilation products of eustigmatophytes are chrysolaminarin and oil, as well as a polysaccharide of undetermined nature that is deposited in the Cytoplasm around the pyrenoid and within vacuoles.

Cytological Characteristics

Cell coverings are represented by a pectic wall. The wall can be smooth or ornamented, bearing thickenings, spines, or warts. Monad Cells of eustigmatophytes, represented by zoospores, are bounded only by the Plasmalemma (Fig. 14.1).

Fig. 14.1. Eustigmatophyte algae. A - zoospore of Pseudocharaciopsis, B - vegetative cell of Monodopsis. 1 - pigmented globules of the eyespot (stigma); 2 - parabasal body; 3 - long flagellum with tripartite mastigonemes; 4 - Golgi apparatus; 5 - short smooth flagellum; 6 - Nucleus; 7 - nucleolus; 8 - chloroplast Endoplasmic reticulum; 9 - double-membraned chloroplast envelope; 10 - thrilamellar thylakoid; 11 - vacuole with thin lamellae of reserve polysaccharide; 12 - mitochondrial profile; 13 - oil droplet; 14 - pectic Cell wall; 15 - pyrenoid; 16 - reserve polysaccharide lamella around the pyrenoid (after Lee, Bold, 1973; Santos, Leedale, 1995).

Nuclear apparatus is similar to that of golden algae: The Nucleus is eukaryotic, enclosed in a double-membraned envelope. In vegetative cells, the outer nuclear membrane directly transitions into the outer membrane of the chloroplast endoplasmic reticulum; however, in zoospores, there is no connection between the nucleus and the chloroplast endoplasmic reticulum.

Mitosis, unlike that of golden algae, is closed - the nuclear envelope remains intact. True centrioles are absent, and their function in monad cells is performed by the basal bodies of the flagella.

Photosynthetic apparatus. Chloroplasts are usually single, predominantly dissected or lobed, located at the periphery of The Cell. The chloroplast is surrounded by four membranes. The two inner layers are formed by the chloroplast envelope membranes, and the two outer layers by the membranes of the chloroplast endoplasmic reticulum. A periplastidial space is present between the second and third membranes.

Thylakoids in the chloroplast stroma are grouped in threes, and the girdle lamella is absent. Vegetative cells usually contain a pyrenoid. It is protruding, angular, and not traversed by thylakoids. Outside the chloroplast, around the pyrenoid, thin lamellae of an assimilate of undetermined nature are deposited. The pyrenoid is always absent in zoospores and has not been detected in most marine eustigmatophytes.

Chloroplast DNA, unlike that of Chrysophyta, is organized into numerous small aggregates that interconnect to form a reticulate, dispersed genophore.

Mitochondrial apparatus has been poorly studied so far. Cytology/cytology/93.html">ELECTRON MICROGRAPHS OF various species show from two large to many small mitochondrial profiles with tubular cristae.

Flagellate stages are represented exclusively by zoospores. They possess two flagella of unequal length: a long, hairy, locomotor flagellum bearing two rows of numerous mastigonemes of typical tripartite (stramenopile) Structure, and a short, smooth one. Quite often, the short flagellum is reduced, leaving only its basal body. Tripartite mastigonemes are formed in the same way as in golden algae - within invaginations of the chloroplast Endoplasmic reticulum and the outer membrane of the nuclear envelope. The flagellar transition zone contains a spiral-like structure.

At the Base of the hairy flagellum lies the parabasal body. Its position is coordinated with the Location OF THE eyespot (stigma). The latter lies outside the chloroplast and consists of pigmented lipid globules of varying sizes, which are not surrounded by a Plasma Membrane. The largest globule envelops the parabasal body on one side, followed by smaller globules. An electron-dense, lamellar structure, which serves as the actual photoreceptor, has been discovered within the thickening of the parabasal body. Special studies have shown that the stigma in eustigmatophytes does not act as a shade screen, as observed in most algae, but rather focuses light rays onto the lamellar STRUCTURE OF THE parabasal body. It is precisely due to this unique Structure and function of the stigma that the division received its name - Eustigmatophyta (Fig. 14.2).

Fig. 14.2. Diagram of the photoreceptor apparatus structure in eustigmatophyte algae: 1 - long flagellum, 2 - parabasal body, 3 - lamellar structure, 4 - large globule of the stigma enveloping the parabasal body, 5 - smaller globules of the outer stigma layer (after Santos, Melkonian, Kreimer, 1996).

Unlike vegetative coccoid cells, eustigmatophyte zoospores generally lack a Golgi apparatus (with exceptions in Botryochloropsis similis and Pseudocharaciopsis texensis) and a pyrenoid, and the outer membrane of the nuclear envelope does not continue into the membrane of the chloroplast endoplasmic reticulum. The flagellar ROOT system of eustigmatophyte algae is poorly studied.

Types of Morphological Body Structures

All known eustigmatophytic algae exhibit exclusively a coccoid structural type. Most species have solitary cells, while some representatives form mucilaginous (Chlorobotrys) or non-mucilaginous (Botryochloropsis) colonies.

Reproduction and Life Cycles

Eustigmatophytes reproduce exclusively via zoospores and aplanospores. Sexual reproduction has not been observed. Their life cycles are characterized by cyclomorphosis. Resting stages such as cysts or akinetes are not known in Eustigmatophyta.

Taxonomic System of the Division

The division includes a single class, Eustigmatophyceae, one order, Eustigmatales, and four families. Families are differentiated based on morphological traits such as the presence of zoospores, the number of flagella, and The ability to form mucous colonies.

However, sequence analyses of the Gene encoding the 18S ribosomal RNA subunit indicate that the division comprises two phylogenetically distinct groups of species. The first group unites soil and freshwater eustigmatophycean algae, while the second comprises marine picoplanktonic representatives. Consequently, the family-level Taxonomy within this division is artificial.

Typical soil-dwelling Representatives of the division include Eustigmatos, Vischeria, Pseudocharaciopsis, Monodopsis, and Chlorobotrys, whereas Nannochloropsis is a prominent marine genus. Nearly all of these genera were previously classified under various genera of yellow-green algae (for instance, Eustigmatos was assigned to Pleurochloris, Monodopsis to Monodus, and Pseudocharaciopsis to Characiopsis).

The genus Eustigmatos is widely distributed in various soil types across all regions except the tropics and subtropics. Its most common species, Eustigmatos magnus, is readily isolated into culture from forest, meadow, or steppe soils. Vegetative cells are spherical and solitary, possessing a large bi- or trilobed chloroplast, typically with a prominent angular pyrenoid, formerly referred to as a protein crystal. Reproduction in soils and on Agar media occurs via aplanospores, which form inside the mother cell in groups of 2–4. In liquid cultures, the alga is capable of producing uniflagellate zoospores bearing a relatively large, distinct red stigma located near the flagellar base and unattached to the chloroplast (Fig. 14.3).

Fig. 14.3. Eustigmatophyte algae: 1 - zoospore and vegetative cells of Eustigmatos magnus; 2 - zoospore and vegetative cells of Vischeria helvetica; 3 - Pseudocharaciopsis minuta; 4 - Monodopsis subterranea; 5 - Chlorobotrys regularis (1-4 - orig., 5 - after Pascher, 1939).

Closely related to Eustigmatos is the genus Vischeria. Its distinctive feature is the presence of angular cells with thickened walls at the apices of the angles. Species of this genus are typical inhabitants of mountain soils.

Pseudocharaciopsis occurs both in soils (predominantly forest soils) and in the periphyton of freshwater continental Water bodies. Its cells are spindle-shaped and bear a small stalk with a basal holdfast at one end, attaching the Organism to substrates such as sand grains or filamentous algae. Reproduction proceeds via aplanospores and zoospores equipped with two flagella of unequal length. The most widespread species is P. minuta.

In the genus Monodopsis, cells are solitary, teardrop-shaped, and reproduce exclusively through aplanospores. They are primarily found in terrestrial habitats and occasionally (especially in winter) can trigger green and yellowish-green "blooms" on decaying plant debris.

Species of the genus Chlorobotrys form mucous colonies and reproduce solely by aplanospores. Cells are aggregated in groups of two to four into gleocapsoid clusters, which can mass together to form macroscopic light-green patches. These organisms inhabit primarily damp mountain soils.

A prime example of a marine picoplanktonic eustigmatophyte alga is Nannochloropsis. Its cells are minute (2–4 µm in diameter), solitary, and contain a single chloroplast lacking a pyrenoid (Fig. 14.4). Reproduction occurs via aplanospores.

Fig. 14.4. Marine picoplanktonic eustigmatophyte alga of the genus Nannochloropsis: 1 - cell wall; 2 - mitochondrial profile; 3 - vacuole with reserve polysaccharide; 4 - nucleus; 5 - chloroplast endoplasmic reticulum; 6 - chloroplast envelope; 7 - triple-thylakoid lamella (diagrammatic representation after Maruyama & al., 1986).

Distribution and Significance

The majority of eustigmatophytes inhabit soils of ecologically pristine regions, primarily mountainous areas. Representatives of the division are highly sensitive to any Forms of soil contamination involving heavy metals, petroleum, and petroleum products, as well as soil salinization; upon exposure to such pollutants, they are among the first, alongside yellow-green algae, to disappear from biogeocenoses.

Marine eustigmatophytes are components of planktonic communities in open pelagic Zones of the World Ocean, playing a significant role in primary production alongside other members of the marine picoplankton.