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Abstract

The purpose of this study was to analyse the marine picoplankton community along the Atlantic Meridional Transect AMT22. The diversity and abundance of dominant taxonomic groups, potential biogeographical patterns in their distribution and their relation to environmental parameters were investigated. We applied catalyzed reporter depositionfluorescence in situ hybridisation (CARD-FISH) and massive parallel tag sequencing. Firstly, the bacterioplanktonic diversity in different biogeographical provinces was analysed using 16S rRNA tag sequencing. This showed a high diversity in all provinces. Eight phyla represented 97% of the relative sequence abundance of all provinces. The Proteobacteria, Cyanobacteria and Bacteroidetes have an average abundance of 39.5 ± 11.5, 28.5 ± 10.5% and 12 ± 7% respectively in all provinces. The Actinobacteria made up a high abundance with 11 ± 2%. Other phyla Planctomycetes, Chloroflexi, Deferribacteres and Planctomycetes made up < 10% of the total sequence abundance. Of the Proteobacteria the alphaproteobacterial clade SAR11 made up nearly half of the abundance with 41.5 ± 15.5%. The Gammaproteobacteria were the second most abundance proteobacterial class with 39 ± 14%. A large portion of this (15 ± 5%) was made up of the SAR86 clade. Secondly the abundance and distribution of the dominate taxon along an Atlantic transect was analysed. The absolute picoplankton abundance (cells ml-1) changed with latitude and with depth and was positively correlated with high chlorophyll a concentrations (r2 0.76). Bacteria dominate the surface waters of all provinces with 72 ± 16%. Archaea had an absolute abundance of 6.1E+04 ± 2.1E+04 cells ml-1 throughout the water column, however their relative abundance showed a significant change from 1% at the surface to 40% at 150 m depth. This study found that the taxonomic groups which dominate the sequence data could also be found and enumerated using CARD-FISH. These groups also made up a significant fraction of the total cell abundance of the bacterioplankton community. Only a few groups (SAR11, SAR86 and Bacteroidetes) could be found at all latitudes and depths and although they were ubiquitous in the sequencing data, their relative abundances did vary. The diversity analysis showed that at different taxonomic level biogeographical distribution patterns were present. They depended on distance (km) or environmental variation between the provinces. The combined analysis of the diversity and abundance of microorganisms based on biogeographical patterns resulted in a more accurate interpretation of the biogeography of microorganisms in the Atlantic Ocean. Understanding the biogeography of microorganisms is a key step in understanding the ecosystem function of specific microbial assemblages.