Mixotrophy in Gyrodinium galatheanum (Dinophyceae): Grazing responses to light intensity and inorganic nutrients

This paper presents results of field and laboratory studies on mixotrophy i n the estuarine dinoflagellate Gyrodinium galatheanum (Braarud) Taylor, We tested the hypotheses that this primarily photosynthetic organism becomes p hagotrophic when faced with suboptimal light and/or nutrient environments, In Chesapeake Bay, incidence of feeding of this species on cryptophytes is positively correlated with prey density and concentrations of nitrate and n itrite, but negatively correlated with depth, salinity, and phosphate conce ntration. Feeding in natural assemblages and cultures increased hyperbolica lly with light intensity, The stoichiometric proportions of dissolved inorg anic P and N (DIP:DIN) at the stations where G. galatheanum was present wer e far below the optimal growth P:N (1:10), Incidence of feeding was negativ ely related to the ratio of DIP to DIN, suggesting that P limitation may ha ve induced feeding. Addition of nitrate, or addition of both nitrate and ph osphate, inhibited feeding in a natural population, indicating that N limit ation may also induce feeding, Ingestion of the cryptophyte, Storeatula maj or, by cultured G, galatheanum was higher in media low in nitrate or phosph ate or both, but moderate rates of feeding occurred in nutrient-replete cul tures, When cells were grown in media with varying concentrations of nitrat e and phosphate, N deficiency resulted in greater cellular N and Chi a loss es than did P deficiency, but P deficiency stimulated feeding more than N d eficiency, Both N and P deficiency, or P:N ratios that deviated greatly fro m 1:10, result in an increase of cellular carbon content and an increase in propensity to feed, Our results suggest that feeding in G, galatheanum is partly a strategy for supplementing major nutrients (N and P) that are need ed for photosynthetic carbon assimilation, Feeding in G, galatheanum may al so be a strategy for supplementing C metabolism or acquiring trace organic growth factors, since feeding occurs, although at a reduced rate, in nutrie nt-replete cultures.