Several mixotrophic dinoflagellates preferentially ingest ciliates as an alternate source of nutrition

Abstract

Mixotrophy is a common nutritional strategy among dinoflagellates. Especially in enriched coastal regions where dinoflagellates are often abundant, mixotrophy may constitute an important microbial pathway. In Chesapeake Bay, several bloom‐forming dinoflagellates are mixotrophic, including Ceratium furca, Akashiwo sanguinea, and Gymnodinium instriatum. While these three species are mainly photosynthetic, all three are capable of ingesting prey. Analysis of food vacuole content shows that they predominantly feed on oligotrich ciliates. Their grazing impact on prey populations can be substantial at times, with both C. furca and A. sanguinea capable of removing over 100% of the <20‐μm‐oligotrich standing stock. The ingested ciliate carbon, nitrogen, and phosphorus may contribute significantly to the dinoflagellates' nutritional requirements. For example, C. furca could meet up to 65% of its C, 92% of its N, and 57% of its P requirements for reproduction through phagotrophy, theoretically leading to a 40%‐reduction in doubling time. Feeding in these dinoflagellate species is influenced by a number of factors, including prey abundance, irradiance, and nutrients. In C. furca, internal carbon, nitrogen and phosphorus pools and their relative concentrations seem to play a role in feeding regulation. When cellular nutrient ratios deviate significantly from those required for optimum growth, either due to limited inorganic nutrient availability or reduced carbon fixation, feeding is induced in an attempt to balance the nutrient budget. The ability of these dinoflagellates to exploit an organic nutrient source (i.e. ciliates) may thus give them a competitive advantage over purely photosynthetic species, especially when inorganic nutrients are limited.