Mixotrophic Noctiluca scintillans will continue to dominate the Arabian Sea with rising hypoxia

In the last decade, the northern Arabian Sea has experienced a dramatic shift in its composition of phytoplankton blooms. Due to convective mixing, diatoms blooms favored by nutrient-enriched have been replaced by widespread winter-time blooms of the large, green mixotrophic dinoflagellate, Noctiluca scintillans. Supporting itself through the carbon fixation by its chlorophyll-containing endosymbionts, Protoeuglena noctilucae,and by ingestion of prey, they have outcompeted microzooplankton for food and autotropic organisms for nutrients. With deleterious impacts on the food chain, local fisheries and an ecosystem which supports a coastal population of approximately 120 million people has suffered. From preliminary experiments in the field, we believe hypoxic conditions caused by an unprecedented influx of oxygen-deficient water into the euphotic zone from the Arabian Sea’s Oxygen Minimum Zone, has caused an unusually high efficiency of carbon fixation by the endosymbiont P. noctilucae. To confirm field observations, an experiment was designed using a laboratory culture of N. scintillans, grown under present day CO₂conditions of 380 ppm, but experimented under three different oxygen concentrations: 0 ppm (anoxia), 1.9 ppm (hypoxia) and 8 ppm (current conditions). The results of these experiments corroborated our field studies, showing that the endosymbiont photosynthesizes and grows faster when ambient seawater dissolved oxygen concentrations are lower.