Bacterioplankton Biogeography of the Atlantic Ocean: A Case Study of the Distance-Decay Relationship

In order to determine the influence of geographical distance, depth and Longhurstian province on bacterial community composition and compare it with the composition of photosynthetic micro-eukaryotes, 382 samples from a A depth-resolved latitudinal transect (51°S to 47°N) from the epipelagic zone of the Atlantic ocean with a total of 382 samples was analyzed by Illumina amplicon sequencing was used to determine the influence of geographical distance, depth and Longhurstian province on bacterial community similarity and compare it with the composition of photosynthetic micro-eukaryotes. In the upper 100 m of the ocean, community similarity decreased towards the equator for 6.000 km, but subsequently increased again, reaching similarity values of 40-60% for samples that were separated by ~12.000 km, resulting in a U-shaped distance-decay curve. We conclude that adaptation to local conditions can override the linear distance-decay relationship in the upper epipelagial of the Atlantic Ocean whichand is apparently not restrained by barriers to dispersal, since the same taxa were shared between the most distant communities. The six Longhurstian provinces covered by the transect were comprised of distinct microbial communities;  30 % of variation in community composition could be explained by province. Bacterial communities belonging to the deeper layer of the epipelagic zone (140 - 200 m) lacked a distance-decay relationship altogether and showed little provincialism. Interestingly, those biogeographical patterns were consistently found for bacteria from three different size fractions of the plankton with different taxonomic composition, indicating conserved underlying mechanisms. Analysis of the chloroplast 16S rRNA gene sequences revealed that phytoplankton composition was strongly correlated with both free-living and particle associated bacterial community composition (R between 0.51 and 0.62, p < 0.002). The data show that biogeographical patterns commonly foundconserved in macroecology do not hold for marine bacterioplankton, most likely because dispersal and evolution occur at drastically different rates in bacteria.