Feeding by the mixotrophic red-tide dinoflagellate Gonyaulax polygramma: mechanisms, prey species, effects of prey concentration, and grazing impact

The red-tide dinoflagellate Gonyaulax polygramma (GenBank accession number = AJ833631), previously known as an exclusively autotrophic dinoflagellate, has been found to be a mixotrophic species. We investigated feeding mechanisms, types of prey species, and the effects of prey concentration on the growth and ingestion rates of G. polygramma when feeding on an unidentified cryptophyte species (equivalent spherical diameter, ESD = 5.6 mu m). We also calculated grazing coefficients by combining field data on abundances of G. polygramma and co-occurring cryptophytes with laboratory data on ingestion rates obtained in the present study. Among the phytoplankton prey offered, G. polygramma ingested small phytoplankton species with ESD less than or equal to 17 mu m, but did not feed on large phytoplankton species with ESD > 22 mu m. G. polygramma fed on prey cells by engulfing them through the apical horn, a previously unknown mechanism, as well as through the sulcus. The feeding mechanism of G. polygramma on phytoplankton mainly depended on the prey species. Specific growth rates of G. polygramma on a cryptophyte increased with increasing mean prey concentration, with saturation occurring at a mean prey concentration of approximately 600 ng C ml super(-1). The maximum specific (mixotrophic) growth rate of G. polygramma on a cryptophyte was 0.278 d super(-1), under a 14:10 h light:dark cycle of 50 mu E m super(-2) s super(-1), while its (phototrophic) growth rate under the same light conditions without added prey was 0.186 d super(-1). Its maximum ingestion and clearance rates were 0.18 ng C grazer super(-1) d super(-1) (10.6 cells grazer super(-1) d super(-1)) and 0.18 mu l grazer super(-1) h super(-1), respectively. The grazing coefficients of G. polygramma on cryptophytes were up to 0.479 h super(-1). The results of the present study suggest that G. polygramma can have a considerable grazing impact on cryptophyte populations.