Physiology of the mixotrophic dinoflagellate Fragilidium subglobosum. II. Effects of time scale and prey concentration on photosynthetic performance

Photosynthetic performance and cellular chlorophyll a (chl a) content were studied in the mixotrophic dinoflagellate Fragilidium subglobosum during tr ansition from phototrophic to mixotrophic growth and back again. Experiment s were conducted at an irradiance of 50 mu E m(-2) s(-1) and, for mixotroph ic growth, in the presence of Ceratium tripes or C. lineatum. After 4 to a d in mixotrophic culture, cells of F subglobosum had reduced both their cel lular chi a content to 47 % and the photosynthetic performance to similar t o 42 % of that found in phototrophic cultures. Net production of chi a in p hototrophic and mixotrophic F subglobosum cells was the same, indicating th at feeding in F. subglobosum does not repress chi a production in mixotroph ic cells. Thus, the reduction in cellular chl a content observed in mixotro phic F. subglobosum cells was caused by an increase in growth rate due to f eeding. In cultures close to steady state, the photosynthetic performance a t low prey concentrations was similar to values obtained by monocultures of F subglobosum. However, above a certain prey concentration, photosynthetic performance decreased exponentially with an increase in prey concentration . The application of the traditional C-14 labeled HCO3- technique to measur e photosynthesis in mixotrophically grown cells underestimated rates by 5 t o 12 %, probably due to fixation of CO2 produced from respiration of ingest ed carbon. In phototrophic and mixotrophic F subglobosum cells 46 and 28 % of the assimilated carbon is used for respiration, respectively. This indic ates that F. subglobosum needs more energy to synthesize, maintain and run the photosynthetic apparatus than the heterotrophic apparatus involved in p rey capture, digestion and assimilation.