The Genus Prochlorococcus
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The Genus Prochlorococcus ANTON F. POST
General Until the mid-1970s the general belief was that oxygenic photosynthesis among prokaryotes was limited to the cyanobacteria with their pigment complement of chlorophyll a and phycobiliproteins. This concept came under fire with the subsequent discoveries of the three so-called “prochlorophytes” Prochloron didemnii (Lewin, 1975a; Lewin and Withers, 1975b), Prochlorothrix hollandica (Burger-Wiersma et al., 1989) and Prochlorococcus marinus (Chisolm et al., 1988), which all lack phycobilisomes but possess chlorophyll b as a light harvesting pigment. An additional discovery of Acaryochloris marina (Miyashita et al., 1996), which contains chlorophyll d as the main pigment, increased appreciation for the plasticity of the photosynthetic pigment-complements of prokaryotes. The photosynthetic apparatus of the above-listed species and that of its cyanobacterial counterpart differed with respect to the genes encoding the light harvesting antenna proteins, the localization of these proteins, and the regulation of photosynthetic activities (Post and Bullerjahn, 1994; LaRoche et al., 1996; Bibby et al., 2003). However, phylogenetic studies unequivocally placed them within the cyanobacterial clade (Morden and Golden, 1989; Urbach et al., 1992). Moreover, later studies have shown that these different photosynthetic machineries are to a large extent different phenotypes of a common gene pool (see below). This then suggests that broad plasticity of photosynthesis is an exclusive property of the cyanobacteria, but that the name may not provide appropriate cover for this group of fascinating organisms. At present, Prochlorococcus is the best-characterized organism among the “atypically pigmented” cyanobacteria, and detailed information is available on natural populations, ecological physiology, photosynthesis, nutrient acquisition, genetics and genomics. Prochlorococcus spp. with a cell diameter of 0.6– 1.0 µm is the smallest photosynthetic organism known to date. Cells are slightly ellipsoid and have typical Gram-negative bacterial structure with a cell envelope consisting of an outer mem-
brane, cell wall, periplasmic space and a cytoplasmic membrane. Except for photosynthetic membranes, intracellular bodies are not seen. The thylakoids are slightly appressed but as in other cyanobacteria (and in contrast to algal and higher plant chloroplasts)—the grana and stroma have no particular organization. A varying number of thylakoid membranes together form a horseshoe-shaped structure in thinsection electron microscopy pictures. Type strains are Prochlorococcus marinus (Chisholm et al. 1992) subsp. pastoris subsp. nov. strain PCC 9511 (available from the Pasteur Institute; Rippka et al., 2000) and Prochlorococcus strain CCMP 1378 (available from the Bigelow Marine Laboratories; Garczarek et al., 1998). Prochlorococcus has further proven to be an excellent tool for the study of genetic diversity and niche adaptation of microbial populations in the ope
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