INTERACTIONS BETWEEN A MARINE DINOFLAGELLATE (ALEXANDRIUM-CATENELLA) AND A BACTERIAL COMMUNITY UTILIZING RIVERINE HUMIC SUBSTANCES

Dissolved organic matter in the form of riverine humic substances stim ulated the growth of both axenic nitrogen-limited Alexandrium catenell a cultures and nitrogen-limited cultures with a marine bacterial commu nity present. The biomass increase of A. catenella could not be accoun ted for by utilization of inorganic nitrogen compounds. However, there was a considerable release of dissolved free and combined amino acids from the humic substances that was utilized by A. catenella. About 40 % of the nitrogen used by A. catenella in the axenic treatment with hu mic substances added was taken up as organic nitrogen. Bacterial amino peptidase and beta-glucosidase activity was stimulated by the addition of humic substances and bacterial growth increased several-fold. Bact eria also utilized the released amino acids from the humic substances, but did not remineralize nitrogen, since no increase in ammonium conc entrations could be detected in the bacteria treatments with humic sub stances added. In the axenic A. catenella treatment there was no signi ficant aminopeptidase activity, suggesting that A. catenella was able to utilize the dissolved combined amino acids directly. Moreover. larg e fluorescently labeled dextran molecules (2000 kDa) were taken up by A. catenella in the humic treatments, showing up in vacuoles inside th e cells. These results suggests that A. catenella can grow well utiliz ing macromolecular organic compounds containing nitrogen, probably by a direct uptake.