There and back again – unraveling mechanisms of bacterial biogeography in 1 the North Pacific Subtropical Gyre to and from station ALOHA 2 3

18 Bacterially-mediated fluxes of energy and matter are dynamic in time and space coupled with 19 shifts in bacterial community structure. Yet, our understanding of mechanisms shaping 20 bacterial biogeography remains limited. Near-surface seawater was collected during transits 21 between Honolulu and Station ALOHA in the North Pacific Subtropical Gyre to examine the 22 shape of occupancy-frequency distributions (the different number of populations occupying 23 different number of sites) and determine bacterial metapopulation dynamics. Bacterial 16S 24 rRNA gene amplicons were sequenced from whole seawater and filter-size fractionated 25 plankton DNA samples while also separating the community into distinct taxonomic groups 26 at phyla/class and analyzing these compartments separately. For the total seawater (i.e. the 27 >0.2 μm size fraction) and picoplankton communities (i.e. the size fraction >0.2 μm and < 3.0 28 μm), but not the large size fraction community (i.e. the >3.0 μm size fraction), most 29 individual operational taxonomic units (OTUs) occupied a single site and the number of 30 OTUs occupying different number of sites followed a significant bimodal pattern with several 31 core OTUs occupying all sites. Nevertheless, only Cyanobacteria (in particular 32 Prochlorococcus sp.) and in a few instances also Alphaproteobacteria (in particular SAR11 33 . CC-BY-NC-ND 4.0 International license not peer-reviewed) is the author/funder. It is made available under a The copyright holder for this preprint (which was . http://dx.doi.org/10.1101/141085 doi: bioRxiv preprint first posted online May. 23, 2017;