Oceanographic boundaries constrain microbial diversity gradients in the South Pacific Ocean

Marine microbes along with micro eukaryotes are key regulators of oceanic biogeochemical pathways. Here we present a high-resolution (every 0.5° of latitude) dataset describing microbial pro- and eukaryotic richness, in the surface and just below the thermocline, along a 7000km transect from 66°S at the Antarctic ice edge to the equator in the South Pacific Ocean. The transect, conducted in Austral winter, covered key oceanographic features including crossing of the polar front (PF), the subtropical front (STF) and the equatorial upwelling region. Our data indicate that temperature does not determine patterns of marine microbial richness, complementing the global model data from Ladau, et al. 2013. Rather, NH₄⁺, nanophytoplankton and primary productivity were the main drivers for archaeal and bacterial richness. Eukaryote richness was highest in the least productive ocean region, the tropical oligotrophic province. We also observed a novel diversity pattern in the South Pacific Ocean; a regional increase in archaeal and bacterial diversity between 10°S and the equator. The Rapoport's rule describes the tendency for the latitudinal ranges of species to increase with latitude. Our data showed that the mean latitudinal ranges of archaea and bacteria decreased with latitude. We show that permanent oceanographic features, such as the STF and the equatorial upwelling can have a significant influence on both alpha-diversity and beta-diversity of pro- and eukaryotes.