Growth and grazing responses of the mixotrophic dinoflagellate Dinophysis acuminata as functions of light intensity and prey concentration

Abstract

Dinophysis acuminata, a photosynthetic marine dinoflagellate, possesses plastids of cryptophyte origin and causes diarrhetic shellfish poisoning (DSP). Recent work has shown D. acuminata to be a mixotroph that grows well when feeding on the photosynthetic ciliate Myrionecta rubra. Using established cultures, we examined the effects of light intensity and prey (M. rubra) concentration on growth and ingestion rates of D. acuminata. Growth rates increased with increasing prey concentration under continuous illumination of 60 μE m-2 s-1, with maximum mixotrophic growth (0.91 d-1) almost 5 times higher than growth in the absence of prey (0.19 d-1). The maximum ingestion rate of D. acuminata was 1296 pg C Dinophysis-1 d-1 (3.2 M. rubra cells Dinophysis -1 d-1) for data fitted to a Michaelis-Menten equation. Growth rate also increased with increasing light intensity, an effect even stronger when prey was supplied. Increased growth with increasing irradiance was accompanied by a corresponding increase in ingestion. While D. acuminata continued to grow in semi-continuous food-replete cultures at high (200 μE m-2 s-1) and low (10 μE m-2 s-1) light intensity, it failed to grow in darkness, despite the presence of prey. Our results suggest that D. acuminata is an obligate mixotroph that requires both light and prey for long-term survival. Results indicate that Dinophysis species are typically prey-limited in the field. © Inter-Research 2008.