| Summary: | The Southern Ocean (SO) is a fragile ecosystem as judged by changes in the timing of the advance and retreat of its ice cover. In the SO, the Antarctic Circumpolar Current (ACC) and the near shore gyres (Weddell Sea and Ross Sea) provide local environments with distinct temperature and salinity attributes associated with varying sea ice history. Phaeocystis antarctica is a prymnesiophyte often dominating polar phytoplankton blooms in the (SO) and is a keystone species there because its abundance can have negative effects on higher trophic levels and it can influence air/sea gas exchange involved in DMSP production, Thus, its ability to survive in response to perturbations in the environments may be linked to its genetic diversity within its populations as they move around the SO. Here we apply increased (70 PSU) and decreased (18 PSU) salinity treatments to five genetically different P. antarctica strains isolated from three different water masses to test whether genetic similarity or water mass physical features were more important in determining responses to salinity changes, such as those encountered by inclusion into sea ice brine channels and/or its subsequent melt water in those water masses that have annual ice cover. Strains that were geographically close (isolated from the same water mass), but genetically distinct (ca. 30% similar and from different gene pools as judged by microsatellite (MS) and amplified fragment linked polymorphisms (AFLP) analyses responded similarly to higher and lower salinity regimes, whereas genetically close strains (ca. 95% identical or from the same gene pool) that originated from different water masses and hence different environmental conditions responded differently. Dimethylsulphoniopropionate (DMSP) production in response to these salinity changes were not significantly different between any of the strains/treatments. Considering the presence of highly similar genotypes in ice-free as well as seasonally ice covered sampling sites, the observed phenotypic differences ...
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