Feeding, pigmentation, photosynthesis and growth of the mixotrophic dinoflagellate Gyrodinium galatheanum

Gyrodinium galatheanum is a photosynthetic, mixotrophic dinoflagellate that is capable of ingesting other protists, including cryptophytes. Ingestion of cryptophycean prey involves formation of a protrusion near the flagellar pores in the sulcus region of the dinoflagellate, by which prey are captured and phagocytized. In phototrophically growing G. galatheanum, a total of 12 chlorophylls and carotenoids are detected using HPLC pigment analysis. In G. galatheanum that had been fed cryptophycean prey for 41 h, traces of pigments that were derived from prey were found. This suggests that ingested prey were not fully digested or that some chloroplasts from prey were retained by the dinoflagellate. G, galatheanum cultured in nutrient-replete medium had net positive growth under phototrophic conditions (i.e, without addition of prey). It could not survive in prolonged darkness even with sufficient food supply, and thus is incapable of strictly heterotrophic growth. Under mixotrophic conditions (i.e. in the light with addition of a saturating concentration of prey), growth rates of G. galatheanum were 2- to 3-fold higher than under strictly phototrophic conditions at the same irradiances. Mixotrophically grown G. galatheanum had higher cellular chi a, cell volume, and cellular car bon content than cultures grown without particulate food. Phagotrophy also leads to enhanced photosynthetic performance of G. galatheanum due to increased photosynthetic capacity (P-max(cell)), and/or by increased photosynthetic efficiency (alpha(cell)), particularly when the cells were grown under low light and/or nutrient-limited conditions. These results indicate that G, galatheanum is an obligately phototrophic species and that both photosynthesis and phagotrophy play significant roles in supporting the higher growth rates associated with mixotrophic than with strictly autotrophic growth.