On-board sequencing of the microbial community of the South Pacific Gyre

Fluorescence in situ hybridization (FISH) with 16S rRNA-targeted oligonucleotide probes was used to investigate the phylogenetic composition of the seven most abundant bacterial clades (determined by 16S rRNA tag sequencing) of the South Pacific Gyre (SPG). Seawater samples were collected aboard the RV Sonne SO-245 "UltraPac"cruise from Antofagasta, Chile (17.12.2015) to Wellington, New Zealand (28.01.2016). A total of 15 stations were sampled at multiple depths from surface (20 m) to ~5000 m. Total cell counts (TCC, by DAPI staining) and FISH were carried out as described in Bennke et al. (2016). DAPI and FISH stained cells were visualised and counted automatically using a fully automated image acquisition and cell enumeration system (Bennke et al., 2016). The cellular abundance of SAR11 clade, Prochlorococcus, AEGEAN-169 marine group, SAR86, SAR202, SAR324 and SAR406 were enumerated and are shown here in total (cell ml-1) and relative abundance (% TCC). For this study, a new probe specific for the AEGEAN-169 clade was designed and tested, based on the latest SILVA 16S rRNA database (refnr 128). We found that the microbial community within the SPG was highly similar to that of other oceanic gyres and showed a pronounced vertical distribution pattern. Two major differences which we observed, in comparison to previous studies of both the SPG and other oceanic Gyres, was a high abundance of the AEGEAN-169 marine group, a sister group of the SAR11 clade, and a low abundance of Prochlorococcus specifically in the surface waters of the central gyre