Fig. 8.5 Chloroplast glycolipids. (a) Monogalactosyl diglyceride. (b) Diga-lactosyl diglyceride. (c) Thylakoid sulfolipid: sulfoquinovosyldiacylglycerol.
Prochlorococcus. Van Mooy et al. (2006) found that in the oligotro-phic North Pacific Subtropical Gyre, the world's largest biome, Prochlorococcus - which dominates the phytoplankton - synthesized predominantly SQDG, rather than phospholipids. In cell cultures, the vast majority (94 ± 5%) of membrane lipids in Prochlorococcus and in the related genus, Synechococcus, were composed of the sulfolipid
PHyutfAii listnriEs (r-j and Mue-pesn
Lip iL-i t: laver
Fig. 8.6 Possible mode of organization of the thylakoid membrane in algae and aquatic higher plants.
(36-66% in axenic cultures) and the two galactolipids. The authors suggest that the evolution of this 'sulfur for phosphorus' strategy, taking advantage as it does of the more plentiful supply of sulfate than of phosphate in sea water, may be a large part of the reason for the success of picocyanobacteria in oligotrophic environments.
The light absorption/electron transport system of photosynthesis consists of two subsystems, referred to as photosystem I and photosystem II (see §8.5). It is thought that these exist within the thylakoid membrane as two distinct, but interacting, types of particle, each containing a reaction centre and light-harvesting pigment-proteins, and having specific types of electron transfer components associated with it. The molecular structure of the photosynthetic membrane is likely to vary markedly from one algal class to another, but there are grounds for supposing that the mode of organization is something along the lines indicated in Fig. 8.6.
The task of collecting light energy from the underwater light field is carried out by the photosynthetic pigments - molecules whose structures are such that they efficiently absorb light in different parts of the 400 to 700 nm range. There are three chemically distinct types of photosynthetic pigment: the chlorophylls, the carotenoids and the biliproteins. All photosynthetic plants contain chlorophyll and carotenoids; the red algae, the blue-green algae and the cryptophytes contain biliproteins as well. A comprehensive account of the photosynthetic pigments of algae has been given by Rowan (1989).
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