Interannual Differences in Nutrient Dynamics During a Brown Tide Bloom (\u3ci\u3eAureococcus anophagefferens\u3c/i\u3e) and the Interaction of \u3ci\u3eA. anophagefferens\u3c/i\u3e with Heterotrophic Bacteria

Blooms of Aureococcus anophagefferens (Brown Tides) in Chincoteague Bay were observed over a six-year period (2002–2007) during which interannual differences in nitrogen and carbon uptake and concentrations of dissolved constituents were compared at two sites, one in Maryland and the other in Virginia. Overall, I observed an increase in bloom intensity and duration over time. No single nitrogen compound was responsible for fueling blooms. Instead, A. anophagefferens demonstrated the ability to use a wide range of nitrogen compounds to meet its nutritional demands. Results show that NO3-, NH4+, urea, and DFAA were taken up simultaneously during blooms and the dominant source of N varied between years. Although photosynthesis was the dominant form of carbon acquisition, organic carbon uptake contributed up to 30% of the total carbon uptake. The contribution of A. anophagefferens and heterotrophic bacteria to total carbon and nitrogen uptake rates was also examined by using flow cytometry. Results demonstrated that it is possible to distinguish and quantify taxon-specific uptake of C and N by A. anophagefferens versus heterotrophic bacteria during incubations of natural assemblages using stable isotopes as tracers coupled with flow cytometry. Bacteria and A. anophagefferens cell-specific uptake rates reported here confirm that A. anophagefferens uses a wide range of N sources during blooms including NO3-, NH4+, urea, and DFAA-N and it, and not bacteria, are the dominant consumers of these resources in the environment. This finding has important implications for bacterial productivity studies that assume bacteria are the primary consumers of the amino acids C and N uptake was also examined over many diet light cycles to determine if dark C and N uptake augments photosynthetic C uptake and DIN uptake by A. anophagefferens during the day. Results demonstrated that A. anophagefferens actively takes up both organic C and organic and inorganic N during the day and night. This finding is critical for understanding the N and C nutrition of this organism because current dogma is that C uptake by photoautotrophs is limited to daylight hours and N uptake at night is low and limited to particular N compounds and environmental conditions