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Zusammenfassung:
Members of the genus Prochlorococcus Chisholm et al., 1992 R8 (Chisholm et al., 1992 R8 ) are the most abundant photosynthetic organisms in the world’s oceans due to their high cell densities and wide distribution within the 40° N–40° S latitudinal band (Partensky et al., 1999 R22 ). The remarkable range of depths colonized by these oxygenic photosynthetic prokaryotes is achieved by the existence of multiple ecotypes which are adapted for growth in high-light (HL) or low-light (LL) environments (Ferris & Palenik, 1998 R9 ; Moore et al., 1998 R19 ; Urbach & Chisholm, 1998 R8 ; West & Scanlan, 1999 R34 ). Although only a single species, Prochlorococcus marinus, the nomenclatural type, has been described on the basis of strain SS120 (Chisholm et al., 1992 R8 ), the different light-dependent physiologies exhibited by this and numerous other isolates (Moore & Chisholm, 1999 R18 ; Moore et al., 1998 R19 ) has led to the description of a new subspecies (Rippka et al., 2000 R24 ). This is in agreement with comparative sequence analyses of 16S rRNA genes which demonstrated that HL- and LL-adapted representatives of the genus form three distinct subclusters (HLI, HLII and LL) in phylogenetic trees (Moore et al., 1998 R19 ; Urbach et al., 1998 R30 ; West & Scanlan, 1999 R34 ). However, due to a lack of sufficient axenic strains no DNA–DNA hybridization studies are available to correlate 16S rRNA gene sequence divergence with the boundaries of species. Hence, with the exception of the type strain, we shall refer to all isolates or genotypes of this genus as Prochlorococcus sp. Despite the relatively high similarity (97·3 %) of the most distantly related HL and LL Prochlorococcus sp. genotypes (Moore et al., 1998 R19 ), there are variable regions of the 16S rRNA which can be targeted by oligonucleotides in order to distinguish them. In a recent study (West & Scanlan, 1999 R34 ), these genotype-specific probes were hybridized to PCR-amplified Prochlorococcus sp. 16S rDNA from natural samples. This confirmed the genetic distinctions between HL and LL ecotypes and revealed that nutrient concentrations and hydrological conditions may be important for their partitioning in surface and deep water, respectively.
