Closely related Prochlorococcus genotypes show remarkably different depth distributions in two oceanic regions as revealed by in situ hybridization using 16S rRNA-targeted oligonucleotides

An in situ hybridization method was applied to the identification of marine cyanobacteria assignable to the genus Procholorococcus using harseradish-p eroxidase-labelled 16S rRNA-targeted oligonucleotide probes in combination with tyramide signal amplification (TSA). With this method very bright sign als were obtained, in contrast to hybridizations with oligonucleotides mono labelled with fluorochromes, which failed to give positive signals. Genotyp e-specific oligonucleotides for high light (HL)- and low light (LL)adapted members of this genus were identified by 16S rRNA sequence analyses and the ir specificities confirmed in whole-cell hybridizations with cultured strai ns of Prochlorococcus marinus Chisholm et al., 1992, Prochlorococcus sp. an d Synechococcus sp. In situ hybridization of these genotype-specific probes to field samples from stratified water bodies collected in the North Atlan tic Ocean and the Red Sea allowed a rapid assessment of the abundance and s patial distribution of HL- and LL-adapted Prochlorococcus. In both oceanic regions the LL-adapted Prochlorococcus populations were localized in deeper water whereas the HL-adapted Prochlorococcus populations were not only dis tinct in each region but also exhibited strikingly different depth distribu tions, HLI being confined to shallow wafer in the North Atlantic, in contra st to HLII, which was present throughout the water column in the Red Sea.