Chlorophyceae

Chlorophyceae constitute a major class of green algae within the phylum Chlorophyta. They form an extensive and diverse group of mainly freshwater and terrestrial organisms, although some species also inhabit marine and brackish environments. As crucial primary producers, they play a significant ecological role by contributing to global oxygen production and forming the base of numerous aquatic food webs. Their remarkable diversity in body structure, life cycles and cellular organisation has made them a central focus in studies on algal evolution and plant ancestry.

Classification and General Characteristics

Chlorophyceae are eukaryotic, photosynthetic organisms characterised by chlorophyll a and b, storage of starch within plastids, and a cell wall composed primarily of cellulose in the inner layer and pectic substances externally. Their habitats are predominantly freshwater, including lakes, ponds, rivers and ephemeral water bodies, although many species survive on moist terrestrial substrates such as tree trunks, damp walls and soil surfaces.
The range of thallus organisation within the group is highly diverse. Simple unicellular species coexist with elaborate colonial and filamentous types, as well as multinucleate coenocytic forms. Advances in molecular phylogenetics have shown that traditional classifications based solely on morphology were often artificial, owing to repeated evolutionary convergence in unrelated taxa.

Thallus Organisation and Cellular Structure

The body structure of Chlorophyceae varies considerably:

• Unicellular forms, as in Chlamydomonas, which are typically motile.
• Colonial forms, such as Volvox, consisting of many cells embedded in gelatinous material.
• Coenocytic forms, exemplified by Characiosiphon, lacking cross-walls and containing multinucleate cytoplasm.
• Filamentous forms, including Chaetophora, with cells arranged in series or branching filaments.

Typical cellular features include a cellulose-rich cell wall, a distinct nucleus, and one or more chloroplasts of diagnostic shapes. Storage occurs in the form of starch and, in some cases, oil droplets. Some vegetative cells are flagellated, others produce flagella only during reproduction, and some never develop flagella at all.

Chloroplasts, Pigments and Storage Products

Chlorophyceae possess well-developed chloroplasts, usually green due to chlorophyll a and b. Carotenoid pigments, such as carotene, may also be present and enhance pigmentation. Chloroplast morphology is extremely varied: cup-shaped forms occur in unicellular taxa, while axial, reticulate or parietal arrangements predominate in filamentous types.
Many species contain one or more pyrenoids, protein-rich bodies involved in carbon concentration and starch formation. Pyrenoids are enclosed by a sheath of starch and serve as important indicators in algal taxonomy. Although most chlorophyceans are photosynthetic, certain forms, such as Polytoma, have non-photosynthetic plastids and depend on other nutrient sources.

Ultrastructure and Flagellar Apparatus

The ultrastructure of Chlorophyceae shows several distinctive traits. Many species possess two or four flagella, although some may have many more. Flagella arise from the anterior end of the cell and are anchored by basal bodies connected to the nucleus through rhizoplasts.
Two major flagellar orientations occur:

• CW (clockwise) arrangement, where basal bodies are positioned in a clockwise pattern.
• DO (directly opposed) arrangement, where basal bodies face each other along a direct axis.

These orientations correspond to major evolutionary lineages and are important in classification. During mitotic division, Chlorophyceae typically exhibit closed mitosis, in which the nuclear envelope remains intact for most of the process. A system of microtubules called a phycoplast appears parallel to the division plane and helps establish the new cell wall.

Modes of Reproduction

Reproductive strategies in Chlorophyceae include both asexual and sexual methods, with considerable diversity among taxa.
Asexual reproduction may involve:

• Autospores, which are non-motile and resemble smaller versions of the parent cell.
• Zoospores, motile spores with a hydrodynamic shape and often an eyespot apparatus for phototaxis.
• Aplanospores, non-motile spores that share internal similarities with zoospores but lack flagella.

Under adverse conditions such as high salinity or predator pressure, certain unicellular algae like Chlamydomonas and Dunaliella may enter a palmella stage, where cells lose their flagella and divide within a shared mucilaginous envelope. Other species, notably Haematococcus, produce thick-walled akinetes, enabling survival under extreme conditions.
Sexual reproduction varies widely and may be:

• Isogamous, involving fusion of similar gametes.
• Anisogamous, where gametes differ in size or form.
• Oogamous, involving a large non-motile egg and a smaller motile sperm, typical of highly organised forms like Volvox.

The life cycle of most Chlorophyceae is zygotic, with the zygote as the only diploid stage. Zygotes often develop into resistant hypnozygotes with thickened walls, enabling survival until favourable conditions return.

Life Cycle and Affinities with Higher Plants

Chlorophyceae display several affinities with terrestrial plants, underscoring their evolutionary significance. These include the presence of chlorophyll a and b, storage of starch within plastids, and the occurrence of certain plant-like biochemical compounds such as phytochromes and flavonoids. Some also produce flagellated cells with asymmetrical structures reminiscent of the motile gametes found in lower land plants.
Although they share these features with higher plants, Chlorophyceae typically do not possess a multicellular alternation of generations. Instead, the haploid phase dominates, and the diploid phase is represented solely by the zygote.

Taxonomy and Phylogeny

Modern taxonomy of Chlorophyceae relies primarily on molecular evidence, supplemented by morphological and ultrastructural observations. Earlier classification systems based purely on visible traits proved inadequate due to extensive parallel evolution in unrelated lineages.
Several well-defined orders exist within the class, including:

• Chlamydomonadales, comprising many unicellular and colonial flagellates.
• Sphaeropleales, including non-flagellated or reduced-flagella algae with characteristic DO-type basal body configurations.