| Summary: | Primary production (PP) of the Ross Sea Polynya (RSP) contributes a significant proportion of the total PP of the Southern Ocean. As a result, the photosynthetic activities of phytoplankton communities in the RSP play important roles in the overall biochemical cycling of carbon and nutrients and structuring the marine food-web. Environmental change, especially in light and iron regimes, regulates variations in PP and affects the phytoplankton community dynamics. Since individual phytoplankton species have unique nutrient requirements, the limited resources impact these algal taxa in different ways. In this study, we used metatranscriptomic analysis to examine how the algal taxa Fragilariopsis, Thalassiosira, Psuedo-nitzschia, Micromonas, and Phaeocystis antarctica differ in their acclimation to their shared environment during the bloom season in the RSP. During the austral spring and summer of 2013-2014, the phytoplankton communities in the RSP were iron limited across all sampling sites and exposed to high-light conditions at some sampling sites. However, the acclimation of individual algal taxa to these conditions was different. Niche partitioning between the diatom group and the haptophyte Phaeocystis antarctica was detected. Phaeocystis dominated at the greater depths (80-100m) and showed relatively low abundances at the surface (average 8.9±5.8%). Pseudo-nitzschia showed optimal niche adaptation among near the surface with a largest population size (average relative abundance 32.0±18.0%) and increased genetic activity. The expression of key-genes of Pseudo-nitzschia showed potential acclimation to limited iron and cobalamin conditions. However, the expression levels of those genes were not only controlled by the related environmental parameters, but also by a set of physical and biochemical environmental factors. Our study is limited to examining the interaction between the phytoplankton communities and the environments. The potential effects of bacterial communities, viruses and predators should be included to further the understanding of individual algal species’ niche adaptation in the future. This study demonstrates that, although some environmental factors control the overall bulk PP, those factors have variable effects on the individual algal species. We also show the need to consider a combination of environmental parameters to accurately predict shifts in phytoplankton community composition within the context of long-term climate change.
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