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Smallest photosynthetic prokaryote of global importance: what do we know about Prochlorococcus diversity in the Adriatic sea?

Prochlorococcus is the most ubiquitous and important picocyanobacteria and the smallest photosynthetic organism of oligotrophic marine environments. It has an important ecological function as major contributor to primary production and carbon cycle. Although ecology of Prochlorococcus has been extensively studied in different oceanic regions, little is known about its microdiversity in the oligotrophic Adriatic Sea. For that purpose winter BIOTA cruise (February/March 2015) was conducted in the Southern Adriatic Pit. Genetic diversity and distribution patterns of different Prochlorococcus ecotypes were investigated by molecular tools. Phylogenetic diversity based on clone libraries of the 16S-23S ribosomal DNA Internal Transcribed Spacer (ITS) region indicated presence of 2 different Prochlorococcus populations: high light (HL) ecotype (highest homology to P. marinus MED4/clade HLI) and low light (LL) ecotype (highest homology to P. marinus NATL1A, NATL2A/clade HLI). In addition, Prochlorococcus ecotypes overlapped with rich Synechococcus community and specific sequences showed homology (94-100%) to several uncultured cyanobacteria. As expected dominant Prochlorococcus ecotype was HLI, a clade typical for Mediterranean Sea, followed by less abundant LLI clade. Overall, Prochlorococcus ecotypes have shown to occupy certain depths with overlapping light niches suggesting their horizontal homogenization in the Southern Adriatic Pit.

Prochlorococcus; Diversity; Internal transcribed spacer region; Ecotype; Adriatic Sea

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