The ecology of planktonic bacteria in oligotrophic marine systems

Abstract

A broad synthesis regarding bacterioplankton ecology in the oligotrophic ocean was obtained through investigations along three fundamental ecological units--the community, the population and the organism. Community investigations focused on the spatial and temporal dynamics of the bacterioplankton at Station ALOHA, a site representative of the oligotrophic North Pacific subtropical gyre. Microbial DNA was harvested from water samples collected at monthly intervals over a 4-year period from surface to 4000 m depths. A suite of 16S ribosomal RNA gene-based techniques including massively parallel pyrosequencing, terminal restriction fragment length polymorphism and clone library sequencing were used to interrogate the bacterioplankton community. Distinct communities were observed stratified within the water column, with a major distinction near the base of the euphotic zone and finer distinctions through water mass association. These communities appeared structured through K-selection, as they were dominated by a surprisingly few number of specific taxa. Weak seasonality was detected in surface communities that resulted from fluctuations in relative abundances of rare populations. These rare populations were associated with the phytoplankton community and correlated with mild seasonal environmental disturbances. Low diversity was observed in both surface and bottom waters and highest in mesopelagic waters near the oxygen minimum zone. These observations fit an intermediate disturbance model that is based upon high frequency environmental disturbances at the surface and lower frequency disturbances at depth in the form sinking particulate matter, as taxonomic evidence supports. Taken together, the oligotrophic bacterioplankton community is viewed to consist of bulk members that are highly competitive for limited resources and rare opportunists that exploit habitat patchiness. The overall stability of these communities in time and space is dependent upon the integration of autochthonous (water mass) community with the allochthonous (particle associated) community. Major populations representative of the autochthonous community were investigated in further detail, yielding results that suggest their abundances may be controlled by basic physical parameters, such as temperature. Investigations at the organismal level focused on an isolate representative of the allochthonous community at T=0, as its abundances are correlated with diatom blooms. Genomic interrogation uncovered physiological features that enhance our view of oligotrophic bacterioplankton ecology.