Niche-partitioning of Prochlorococcus populations in a stratified water column in the eastern North Atlantic Ocean

The in situ community structure of Prochlorococcus populations in the easte rn North Atlantic Ocean was examined by analysis of Prochlorococcus 16S rDN A sequences,vith three independent approaches: cloning and sequencing, hybr idization to specific oligonucleotide probes, and denaturing gradient gel e lectrophoresis (DGGE). The hybridization of high-light (HL) and low-light ( LL) Prochlorococcus genotype-specific probes to two depth profiles of PCR-a mplified 16S rDNA sequences revealed that in these two stratified water col umns, an obvious niche-partitioning of Prochlorococcus genotypes occurred. In each,vater column a shift from the HL to the LL genotype was observed, a transition correlating with the depth of the surface mixed layer (SML). On ly the HL genotype was found in the SML in each water column, whereas the L L genotype was distributed below the SML. The range of in situ irradiance t o which each genotype was subjected within these distinct niches was consis tent with growth irradiance studies of cultured HL- and LL-adapted Prochlor ococcus strains. DGGE analysis and the sequencing of Prochlorococcus 16S rD NA clones were in full agreement with the genotype-specific oligonucleotide probe hybridization data. These observations of a partitioning of Prochlor ococcus genotypes in a stratified water column provide a genetic basis for the dim and bright Prochlorococcus populations observed in flow cytometric signatures in several oceanic provinces.