Characterizing the Mechanisms Underlying Southern Ocean Diatom Community Composition Shifts

Diatoms play an important role in Southern Ocean (SO) primary production and biogeochemical cycling. We characterized in situ diatom community composition across varying regions of the SO, including a latitudinal transect from Rothera Station to the Ross Sea (RothR), within the Ross Sea Polynya (RSP), and in a bloom observed in the Antarctic Circumpolar Current (ACC). This was accomplished via high-throughput sequencing of the variable V4 region of the 18S rDNA amplified with diatom-targeted primers. Either Pseudo-nitzschia spp. or Fragilariopsis spp. dominated most sampled areas, although community shifts were apparent. A shift from Thalassiosira sp. to Fragilariopsis sp. was detected in the RothR transect. The abundance of Fragilariopsis sp. declined in the RSP, where sequences from Pseudo-nitzschia sp. were the most prevalent and chlorophyll a concentrations were high compared to the RothR transect and the ACC bloom. In the ACC bloom, the diatom community was dominated by two Fragilariopsis spp., while the larger community was composed mainly of haptophytes. We are currently determining how the diatom distributions in these three SO regions correlate with various physicochemical parameters. To develop ecologically relevant laboratory models, we established a culture collection of >300 isolates from two austral summer and winter cruises conducted in 2013-2014. By comparing isolate and community amplicon data, we have determined our culture collection contains representatives of the major taxa identified with molecular barcoding. This will allows us to conduct physiology experiments with relevant and recent culture representatives. With these representatives, we can begin to probe the physiological status of diatoms important in different regions of the SO.