Phagotrophy in photosynthetic dinoflagellates from the Chesapeake Bay, with focus on Gyrodinium galatheanum, was investigated both from physiological and ecological perspectives. Fluorescent-labeled protists and unstained, phycoerythrin containing cryptophytes were used to detect phagotrophy in photosynthetic dinoflagellates. Through comprehensive field observations and a series of laboratory experiments, several bloom-forming dinoflagellates, including Gyrodinium galatheanum, Prorocentrum minimum, and an unidentified gonyaulacoid dinoflagellate, were shown for the first time to be mixotrophic. Gyrodinium galatheanum was documented to use a peduncle-like protrusion formed near the flagellar pore in the sulcus region to capture and engulf cryptophyte prey. Experimental results showed that G. galatheanum has higher growth rates and higher cellular carbon contents when it was grown mixotrophically than when it was grown under strict phototrophic conditions, but could not survive in prolonged darkness even when fed. Results also indicated that phagotrophy lead to increases in cellular chl a, photosynthetic capacity (Pmax/cell), and photosynthetic efficiency (alpha/cell). These results suggest that G. galatheanum is an obligately phototrophic species and that phagotrophy contributes to its ability to grow rapidly both by enhancing its photosynthetic performance and by supplying an organic source of C, N, P, and/or other trace factors for growth.

The factors that regulate feeding in Gyrodinium galatheanum were also investigated through field surveys and laboratory experiments. Results showed that prey availability partially controls feeding. However, feeding by this dinoflagellate was light dependent and was stimulated by inorganic nutrient limitation. The feeding capability of this dinoflagellate was positively correlated with its cellular C contents but negatively correlated with chl a:C.

The spatial and temporal distribution patterns of this mixotrophic dinoflagellate in the Chesapeake Bay were characterized, and the grazing impact of this dinoflagellate on cryptophyte populations was estimated as well. Gyrodinium galatheanum was found to be widely distributed in the Bay. Abundance of this dinoflagellate could reach up to 4 x 10^6 cells/L in the mid- and upper Bay during late spring and early summer. Ingestion of cryptophytes was common, but grazing by G. galatheanum generally has a minor impact on cryptophyte populations in the Bay. However the nutritional contribution of cryptophytes to the growth of the dinoflagellates could be significant. Based on in situ data and experimentally determined digestion rates, it is estimated that G. galatheanum in the Chesapeake Bay may meet 10% of its C, 11% of its N and 17% of its P requirements for growth through phagotrophy.