Marine bacterioplankton biogeography over short to medium spatio-temporal scales

UnrestrictedMicrobes are central to ocean food webs and global biogeochemical processes. Understanding their distribution and interactions over space and time is fundamental to understanding their ecology. Even with the advent of molecular techniques, it is difficult to observe their interactions with each other and with their environment. We are in the early stages of expanding our focus from simply identifying microbes, to discovering what they are doing (i.e. how each microorganism interacts and fits within the functioning of the ecosystem). In the ocean, which is in constant lateral and vertical motion, these community patterns and interactions may change on short to medium spatial and temporal scales. The aim of this study was to determine the community patterns in bacterioplankton in the ocean surface waters on hour-day temporal scales and kilometer spatial scales, patterns in bacterial communities on µm to m depth scales, and interactions among bacteria, protists and Archaea on monthly scales. Molecular fingerprinting techniques coupled with clone libraries for identification were used to study changes in bacterioplankton communities in the surface waters from 2000-2005, in the sea surface microlayer and surface layer from 2004-2006, and interactions between bacterioplankton and protists from 2000-2004 in the ocean near Southern California. Investigating bacterioplankton community patterns over km spatial scales and hour-day temporal scales, we found coherent communities at 2-97km2 area scales and at distances along transects of 0-15km, and the major taxa appeared remarkably consistent throughout, while at distances from 50-255km, there were strong differences in the communities which correlated to environmental parameters. Temporally, we found stable communities within a water patch and in the surface mixed layer over 20-30 hours, and also high community similarity between days over a week at a single geographic location.; Together, these results suggest that in the Southern California Bight, a typical 10L oceanographic sample can represent a bacterioplankton community on the scale of 10-100km2 and 4-6 days. Investigating the communities in the sea surface microlayer compared to the communities at 0.5m depth, our results showed a sea surface microlayer community which was distinct from the bacterial community at 0.5m depth in some locations, but showed no difference between the communities at others. We found low and variable enrichment and depletion of bacteria and viruses and strong depletion of bacterial production in the microlayer. The wide range of similarities found between the sea surface microlayer and the underlying water communities and the high number of shared taxa between the communities, suggests a patchy microlayer community that is likely formed by bacteria being transported from the surface waters to the microlayer. Using a network to visualize the statistical correlations between co-occurrence of bacteria, archaea, and protists, along with viruses and other environmental factors in a microbial community over 3 years suggested a succession of microbial communities and identified possible local ecotypes of groups like the SAR11 cluster, Stramenopiles, Alveolates, Cyanobacteria, and microphagotrophs (e.g. ciliates and choanoflagellates). This new approach provides a feel for the natural history of microbes, and should facilitate the inclusion of complex microbial assemblages in community ecology studies; it also points to biogeochemical roles of “unknown” microorganisms, and indicates possible keystone species. These studies provide a solid addition to a more complete understanding of bacterioplankton biogeography and their ecological roles; and will provide insight into the function and response of the bacterioplankton ecosystem in the surface waters