Photosynthesis in Bacteria

Although this chapter covers it last, bacterial photosynthesis is, in evolutionary terms, the oldest form of photosynthesis known - it predates the chloroplast-based photosynthesis of plants and algae by a wide margin. Bacteria lack any specialised, membrane-bound organelle equivalent to a chloroplast; instead, they carry out photosynthesis using much simpler structures called chlorosomes and chromatophores. Van Niel discovered, in 1930, a bacterium that releases sulphur rather than oxygen during photosynthesis; in these organisms, the electron (hydrogen) donor is hydrogen sulphide (H2S) rather than water, and only a single photosystem is involved, equivalent in role to Photosystem I, with a reaction centre called P870 (rather than the P700/P680 pair used in plants). The pigments involved also differ from plants: bacteria use bacteriochlorophylls (types a, b, c, d, e and g) together with carotenoids, rather than the chlorophyll a/chlorophyll b/carotenoid-xanthophyll combination plants rely on. Photosynthetic bacteria are grouped into three categories: green sulphur bacteria, such as Chlorobacterium and Chlorobium; purple sulphur bacteria, such as Thiospirillum and Chromatium; and purple non-sulphur bacteria, such as Rhodopseudomonas and Rhodospirillum. Table 13.6 sets out the point-by-point contrast with plant photosynthesis: bacteria run only cyclic phosphorylation (plants run both cyclic and non-cyclic), use only one photosystem (plants use two), never evolve oxygen (plants do), use a single reaction centre, P870 (plants use two, P700 and P680), use NADH+H+ as their reducing agent (plants use NADPH+H+), respond to PAR wavelengths above 700 nm (plants respond across 400-700 nm), and rely on chlorosomes and chromatophores as their photosynthetic apparatu …