Chlorophyta

A great range of somatic differentiation occurs within the Chlorophyta, ranging from flagellates to complex multicellular thalli differentiated into macroscopic organs. The different level of thallus organization (unicellular, colonial, filamentous, siphonous, and parenchimatous) have traditionally served as the basis of classification of this division. Prasinophyceae are unicellular motile algae covered on their cell body and flagella by non-mineralized organic scales (Figure 1.41). The class Chlorophyceae comprises flagellated cells even naked or covered by a cell wall termed theca (Figure 1.42). All Ulvophyceae known to date are sessile organisms having walled vegetative cells. Except for a small group of species, the thalli are usually multicellular or coenocytic during at least some part of the life history. Many species have microscopic, filamentous thalli, but most are macroscopic seaweeds, capable of considerable morphological differentiation (Figure 1.22). Cladophorophyceae take the form of branched or unbranched filaments of multinucleate cells with periodic cross walls. The organization of the thallus in the class of Briopsidophyceae is always syphonous; syphonous thalli can combine to form fairly complex tissues (Figure 1.43). The Zygnematophyceae species are either coccoids or filamentous. In all the Trentepohliophyceae the thalli consist of branched or unbranched filaments with uninucleate cells (Figure 1.44).



Klebsormidiophyceae have coccoid and branched or unbranched filamentous forms (Figure 1.45). Charophyceae have macroscopic thalli, which exhibit the characteristic of both the filamentous and syphonous levels of organization (Figure 1.46). Dasycladophyceae have syphonous thalli in many species encrusted with calcium carbonate (Figure 1.47). Chlorophytes show a wide diversity in the number and arrangements of flagella associated with individual cells (one or up to eight in the apical or subapical region). Flagellated cells are isokont, which means the flagella are similar in structure, but could differ in length. These algae are ubiquitous in freshwater, marine, and terrestrial habitats. Chlorophyta possess chlorophylls a and b, β- and γ-carotene, and several xanthophylls as accessory pigments. Chloroplasts are surrounded by a two-membrane envelope without any endoplasmic reticulum membrane. Within the chloroplasts, thylakoids are stacked to form grana.



Pyrenoids, if present, are embedded within the chloroplast and often penetrated by thylakoids. The circular molecules of chloroplast DNA are concentrated in numerous small blobs (1–2 µm in diameter). The most important reserve polysaccharide is starch, which occurs as a grain inside the chloroplasts; glucan is present in the cell wall of Cladophorophyceae and Bryopsidophyceae and β-1,4 mannan in Dasycladophyceae. Eyespot, if present, is located inside the chloroplast, and consists of a layer of carotenoid-containing lipid droplets between the chloroplast envelope and the outermost thylakoids. Chlorophyta are photoautotropic but can be also heterotropic. No sexuality is known in Prasinophyceae but the genus Nephroselmis has a haplontic life cycle. In Chlorophyceae, reproduction is usually brought about through the formation of flagellate reproductive cells. The life cycle is haplontic. In Ulvophyceae the life cycle is haplontic, isomorphic, and diplohaplontic. In Cladophorophyceae and Trentepohliophyceae, the life cycle of reproductive species are diplohaplontic and isomomorphic. In Bryopsidophyceae, Klebsormidiophyceae, Charophyceae, Zygnematophyceae, and Dasycladophyceae life cycle is haplontic. As the advanced land plants and the “modest” Trentepohliophyceae class possess the same mechanism of cell division, that is, using the phragmoplast disc where the cells will divide, plant evolution researchers believe that the land plants derived directly from this fresh-water algae class.