Nitrogenous Nutrient Preferences and Intracellular Ammonium Accumulation by the Bloom-Forming Mixotroph Green Noctiluca scintillans

Green Noctiluca scintillans (green Noctiluca) is an unusually large mixotrophic dinoflagellate that forms widespread blooms in tropical coastal marine ecosystems. Its recent emergence as the dominant winter-monsoon bloom forming organism in the Arabian Sea has been attributed to the unusual ability of its endosymbionts Protoeuglena noctilucae present within its symbiosome to photosynthesize more efficiently under suboxic conditions. What also distinguishes green Noctiluca from most mixotrophs and other bloom forming organisms, is its ability to persist and thrive as large blooms even under the most unfavorable nutrient conditions. In this study, we report the results of: 1) nitrogenous nutrient enrichment experiments undertaken with a laboratory strain of green Noctiluca isolated from the Arabian Sea, and 2) nitrogenous nutrient uptake experiments undertaken with natural populations of green Noctiluca sampled off the coast of Oman. Both investigations revealed that this organism has a greater preference for regenerated nitrogenous nutrients, i.e. urea and ammonium as compared new nitrogen, nitrate. What was particularly surprising about this mixotroph however, was its ability to grow and survive for extended periods of time even in the absence of nitrogenous nutrients. In the field, green Noctiluca bloom outbreaks were invariably preceded by extremely high concentrations of urea and ammonium. We also observed that irrespective of the nitrogenous nutrients available to them, green Noctiluca cells always accumulated large amounts of ammonium within their symbiosome, which resulted in sharp increases in seawater ammonium concentrations (30 to 200 μM) following their demise.