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Fig. 8.4 Chloroplast structure in green aquatic plants. (a) Part of a chloroplast of the green unicellular flagellate Dunaliella tertiolecta (Chlorophyta) showing the somewhat irregular stacking of thylakoids to form rudimentary grana (by courtesy of Dr M. Vesk). Bar = 0.5 mm.

not contain biliproteins and so lacks phycobilisomes. It was proposed by Lewin (1976) that these algae should be placed in a new algal division, the Prochlorophyta, but the counterproposal has been put by Antia (1977) that the criteria defining the Cyanophyta be widened so that the group could include algae such as Prochloron. Different isolates of Prochloron have thylakoids lying within the cytoplasm singly, in pairs or in stacks of up to 12 thylakoids.1356,1459 Free-living prochlorophyte species also exist: a filamentous form, Prochlorothrix, has been found in a lake, and coccoid forms are abundant in the sea. , These marine coccoid forms, all currently referred to as Prochlorococcus marinus,225 are now known to be of great ecological importance, contributing substantially to ocean primary productivity, particularly in the lower regions of the euphotic zone. The cells contain two or three thylakoids appressed together, adjacent and closely parallel to the cytoplasmic coin membrane, and extending most of the way around the cell. , Pro-chlorococcus marinus does not possess phycobilisomes, although some strains do contain biliproteins.

The thylakoid membrane in all plants consists of a polar lipid bilayer with protein particles embedded within it. The thylakoids of higher plant chloroplasts are made up of about 32% colourless lipid, 9% chlorophyll, 2% carotenoid and 57% protein, and it is likely that algal thylakoids have a broadly similar average chemical composition. The nature of the lipids constituting the bilayer varies from one class of alga to another, but it seems that in every case, major components are the two galactolipids -mono- and digalactosyl diglyceride (Fig. 8.5a and b), which are polar but uncharged. Functional membranes also need ionized lipids, and in the thylakoid these are present partly in the form of phospholipids, such as phosphatidyl glycerol and phosphatidyl choline, common constituents of most biological membranes. In addition, thylakoid membranes contain a sulfolipid - sulfoquinovosyldiacylglycerol (SQDG, Fig. 8.5c) -which is a sulfonic acid and exists in the membrane in its ionized, negatively charged, form. Functionally, the sulfolipid can in some situations substitute for phospholipids. This substitution appears to have been successfully achieved by the important marine prochlorophyte,

(b) Chloroplasts in the leaf of the submerged aquatic plant Vallisneria spiralis (by courtesy of Mr D. Price and Dr S. Craig), showing the stacking of thylakoids to form grana (g), and the stroma thylakoids connecting the grana. A large starch grain is evident in the left-hand chloroplast.

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