Page 46 - Algae

General Overview 29

FIGURE 1.43 Thallus of Codium sp. (Bar: 2 cm.) FIGURE 1.44 Thallus of Trentepohlia arborum.

stacked to form grana. Pyrenoids, if present, are embedded within the chloroplast and often pene-
trated by thylakoids. The circular molecules of chloroplast DNA are concentrated in numerous
small blobs (1–2 mm 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 b-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 hap-
lontic 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, Klebsormidio-
phyceae, 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.

ENDOSYMBIOSIS AND ORIGIN OF EUKARYOTIC ALGAE
Within the algae, different evolutionary lineages are discernable. Three major eukaryotic photosyn-
thetic groups have descended from a common prokaryotic ancestor, through an endosymbiotic
event. Therefore, these algae possess primary plastid, that is, derived directly from the prokaryotic
ancestor. Other algal groups have acquired their plastids via secondary (or tertiary) endosymbiosis,
where a eukaryote already equipped with plastids is preyed upon by a second eukaryotic cell.