UPTAKE OF HIGH-MOLECULAR-WEIGHT DEXTRAN BY THE DINOFLAGELLATE ALEXANDRIUM-CATENELLA

Carbohydrate macromolecules (dextrans) labeled with fluorescein isothi ocyanate (FITC) were taken up by the dinoflagellate Alexandrium catene lla at a substrate concentration of 5 mg C l(-1) The FITC-labeled dext rans appeared in what resembled food vacuoles inside the dinoflagellat e cells. Between 5 and 50% of the cells contained fluorescent green va cuoles. A. catenella took up dextrans of high molecular weight (2000 k Da) but did not show significant uptake of lower molecular weight (20 kDa) dextrans. The uptake of the 2000 kDa dextran was higher with addi tion of humic substances to the growth medium and did not seem related to the presence of bacteria. Phagocytosis of fluorescent microspheres (0.36 mu m) by A. catenella was also investigated. Although aggregati on of fluorescent microspheres was observed in the sulcal region of th e cells, no evidence was found of phagocytosis of bacterial-size prey by A. catenella. These observations show that A. catenella has the cap acity to take up high molecular weight organic molecules. perhaps by p inocytosis.