Phycology - Kostikov I.Yu. - 2009-2013

Chapter 11. Chlorarachniophytes – Chlorarachniophyta

The division of Chlorarachniophytes (Chlorarachniophyta) was established in 1984 during the investigation of the ultrastructure of Chlorarachnion reptans, an amoeboid alga previously included in the division Xanthophyta. This species was first isolated from marine mud cultures in the 1920s. In 1987, a second representative of the division was described, Cryptochlora perforans, isolated into culture from dead thalli of marine siphonous Algae. Later, several more species from this phylum were discovered. Today, chlorarachniophytes comprise four genera with six species.

Biochemical and, subsequently, molecular-biological studies have confirmed the independence of chlorarachniophytes as a higher-level taxon.

Class="center">Taxonomic CHARACTERISTICS OF THE Division

Pigments and Reserve Nutrients

The Biochemical characteristics of the division include the presence of chlorophylls a and b, which are not masked by accessory pigments. Consequently, the METABOLISM/14.html">Chloroplasts of chlorarachniophytes are green in color. Their reserve nutrient, similar to that of euglenophytes, is paramylon.

Cytological Characteristics

The Cells of chlorarachniophytes are amoeboid and produce A large number of thin, branched rhizopodia. These connect the cells into reticulate colonies, sometimes referred to as a plasmodium. The number of cells in a colony ranges from a few to 150. Each Cell in the colony is capable of active movement via pseudopodia (Fig. 11.1).

Fig. 11.1. Chlorarachniophytes (using Chlorarachnion as an example): 1 - colony; 2 - individual amoeboid cell; 3 - zoospore; 4 - coccoid stage (after Geitler, 1930; Pascher, 1939).

Cell coverings are represented solely by the Plasmalemma. Diffusely distributed trichocysts are located beneath the plasmalemma.

Nuclear apparatus. The cells of chlorarachniophytes are uninucleate. The Nucleus has a eukaryotic Structure with chromosomal Organization; the nuclear DNA is bound to Histones. The nucleus is separated from the Cytoplasm by a double-membrane envelope and contains a single nucleolus.

Photosynthetic apparatus. The Cell contains several parietal, disc-shaped, bilobed chloroplasts, each of which possesses a pyrenoid. The chloroplasts are enveloped by four membranes. The two inner membranes are the chloroplast's own membranes, while the two outer membranes form the chloroplast Endoplasmic reticulum, usually with at least one large invagination. Between the inner chloroplast membranes and the membranes of the chloroplast endoplasmic reticulum lies a fairly large, so-called periplastidial space, which houses 80S Ribosomes and a unique DNA-containing structure known as the nucleomorph. The nucleomorph is separated from the periplastidial space by its own double-membrane envelope (Fig. 11.2).

Fig. 11.2. Diagram of the chloroplast structure in chlorarachniophytes: 1 - two inner membranes of the chloroplast envelope; 2 - two membranes of the chloroplast endoplasmic reticulum; 3 - lamellae consisting of three thylakoids; 4 - periplastidial space with 80S ribosomes; 5 - pyrenoid; 6 - paramylon; 7 - nucleomorph; 8 - double-membrane envelope of the nucleomorph; 9 - invagination of the chloroplast endoplasmic reticulum housing the nucleomorph; 10 - chloroplast 70S ribosomes; 11 - cytoplasmic 80S ribosomes (according to Hibberd, Norris, 1984).

Beneath the chloroplast envelope lie thylakoids, grouped predominantly in pairs or triplets, or more rarely single. A girdle thylakoid is absent. The chloroplast stroma also contains chloroplast DNA, 70S ribosomes, and a large pyrenoid projecting beyond the bounds of the chloroplast. Unlike the pyrenoids of most other algae, this pyrenoid is not traversed by thylakoids. Paramylon accumulates around the pyrenoid, but always outside the chloroplast.

Mitochondrial apparatus is represented by a branched mitochondrion with tubular cristeae.

Flagellate stages in chlorarachniophytes are represented by zoospores with a unique structural plan unknown in other algae: there is always a single, subapical flagellum that is spirally coiled around the cell and, unlike in all other algae, directed backwards. The axoneme of the flagellum bears simple, very thin, pinnately arranged mastigonemes. An eyespot (stigma) is always absent.

Other Organelles. The cytoplasm contains a well-developed Golgi apparatus, as well as digestive vacuoles formed when rhizopodia capture small unicellular algae, Bacteria, or detritus particles.

Reproduction and Life Cycle

Reproduction occurs through cell binary fission or via zoospores. Under unfavorable conditions, amoeboid cells can become spherical, develop a Cell wall, and enter a coccoid state.

Sexual reproduction has been observed in Chlorarachniophyta, but its specific features, the site of Meiosis, and the alternation of nuclear phases remain unclear.

The life cycle is simple, representing a cyclomorphosis with the alternation of solitary amoeboid cell stages, reticulate colonies, and zoospores. Under unfavorable conditions, colonies break down into individual cells; each cell retracts its rhizopodia, becomes spherical, and transforms into a resting coccoid akinete.

Systematics of the Division

All known chlorarachniophytes belong to the class Chlorarachniophyceae, order Chlorarachniales, and family Chlorarachniaceae. A typical representative is the genus Chlorarachnion (see Fig. 11.1).

Origin of Chlorarachniophytes and Their Position in the Eukaryotic System

Unlike most eukaryotes, chlorarachniophytes possess four genomes: nuclear, mitochondrial, plastid, and nucleomorph. The Water/144.html">Origin of the first and the last of these genomes has been investigated using molecular biology Methods, specifically using Chlorarachnion reptans as a model.

Based on nucleotide sequence Analysis of the Gene encoding the small subunit of cytoplasmic ribosomal RNA, the nuclear genome was found to be closely related to the genomes of testate amoebae (Testacea), particularly those of the genera Euglypha and Paulinella (Fig. 11.3). The nucleomorph genome proved to be related to the nuclear genomes of green algae from the order Volvocales.

Based on these findings, an evolutionary pathway for chlorarachniophytes was proposed, which is also well supported by a set of specific phenotypic traits characteristic of this phylum (Fig. 11.4). According to this model, chlorarachniophytes arose through the endosymbiosis of a heterotrophic amoeboid cell, closely related to Testacea, and a photoautotrophic green alga from the Volvocales. The process occurred in two stages.

Fig. 11.3. Examples of testate amoebae most closely related to Chlorarachniophyta: Euglypha (left) and Paulinella (right). Interestingly, the cytoplasm of Paulinella contains symbiotic cyanobacteria, or cyanelles (after Dogiel, 1981; Kies, 1974).

Fig. 11.4. Hypothetical scheme of THE ORIGIN OF chlorarachniophyte algae: A – heterotrophic amoeboid cell engulfing a green alga; B – digestive vacuole containing the green alga within the cytoplasm of the host cell; C – endosymbiont transforms into a chloroplast, accompanied by the reduction of its nucleus. 1 – Nucleus of the amoeboid host cell, 2 – nucleus of the green alga transforming into a nucleomorph, 3 – chloroplast (after McFadden & al., 1997).

In The First stage, the alga is engulfed by the amoeba's pseudopodia, forming a digestive vacuole. This explains the presence of four membranes surrounding the chloroplast: the two innermost are the membranes of the green algal chloroplast, the third is its Plasma Membrane, and the fourth is the membrane of the amoeba's digestive vacuole. The cytoplasm of the green alga, containing 80S ribosomes, forms the periplastidal compartment.

In the second stage, partial reduction of the green algal nucleus occurs, converting it into a nucleomorph. For instance, the nucleomorph of Chlorarachnion reptans contains only three Chromosomes, consisting of 145, 140, and 95 kbp. These chromosomes retain genes encoding periplastidal ribosomal RNA and at least ten genes encoding Enzymes involved in Replication, Transcription, and Translation Processes.

Thus, molecular and cytological data indicate that Chlorarachniophyta belong to the kingdom Tubulicristata, forming, along with phylosean amoebae and certain other amoeboid organisms, a group of primitive tubular-cristate organisms. This group has been assigned the status of the subkingdom Amoeboflagellata.