MEPS 201:129-136 (2000)  -  doi:10.3354/meps201129

Physiology of the mixotrophic dinoflagellate Fragilidium subglobosum. I. Effects of phagotrophy and irradiance on photosynthesis and carbon content

Alf Skovgaard1,*, Per J. Hansen1, Diane K. Stoecker2

1Marine Biological Laboratory, University of Copenhagen, Strandpromenaden 5, 3000 Helsingør, Denmark
2University of Maryland Center for Environmental Science, Horn Point Laboratory, PO Box 775, Cambridge, Maryland 21613, USA

ABSTRACT: Photosynthesis was measured for the mixotrophic dinoflagellate Fragilidium subglobosum grown with and without prey (Ceratium lineatum), at 5 different irradiances. Photosynthetic rates were measured in phototrophically and mixotrophically grown cultures by use of the single-cell 14C uptake technique. Photosynthetic rates were reduced by 48 to 69% and cellular chlorophyll a (chl a) content was reduced by 13 to 67% in food-replete mixotrophic cultures as compared with strictly phototrophic cultures grown at the same irradiances. Under the same conditions, chl a-normalized photosynthetic rates were thus 7 to 49% lower in mixotrophic cultures than in strictly phototrophic cultures. It is, therefore, suggested that either the photosynthetic apparatus becomes less efficient when the cells are phagotrophically active or the chl a-normalized photosynthesis is reduced because fed cells contain photosynthetically inactive chl a in their food vacuoles. When food is abundant, the greater part of the gross carbon uptake of F. subglobosum is acquired through phagotrophy even at irradiances saturating for photosynthesis.


KEY WORDS: Mixotrophy · Dinoflagellate · Photosynthesis · Single-cell 14C technique · Chlorophyll a · Ingestion · C content · N content · C budget


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