The role of nutrients in decomposition of a thecate dinoflagellate

The decomposition of freeze-dried whole cells and empty thecae of the dinof lagellate Peridinium gatunense Nygaard originating from dense blooms in Lak e Kinneret (Israel) was followed experimentally under controlled conditions in the lab. The two materials (whole cells; empty thecae) were suspended i n replicate bottles containing nutrient-poor epilimnetic water from the lak e. After 7 d, nutrients (N, P, and trace metals) were added to half the bot tles. We followed the changes with time in dry weight, the dynamics of nutr ients, microbial abundances and a range of microbial activities including l eucine incorporation rates and activities of hydrolytic enzymes. Because of the low N and P content of thecae (C:N: P atomic ratios >3000:19:1) relati ve to protoplasts (276:51:I), the microbial utilization of thecae was expec ted to depend much mon on the availability of external nutrient sources tha n the utilization of protoplasts. Indeed, decomposition of thecae did not o ccur in the absence of external nutrients but was rapid (1-2 d to their dis appearance) after nutrients were added. In contrast, almost no stimulating effect of nutrient addition was observed for the decomposition of whole cel ls. The results suggest that intensive regenerative nutrient cycling or ext ernal nutrient inputs are a necessary precondition for an efficient trophic transfer of the energy stored in blooms of thecate dinoflagellates. The hi gh nutrient demands of microbial degradation imply furthermore a competitio n for nutrients between heterotrophic degradative and phototrophic producti ve processes. Because of the generally assumed stronger competitive ability of heterotrophic bacteria, reduced primary production is expected as an in direct result of dinoflagellate bloom degradation. Indeed, reduced primary production is observed in Lake Kinneret every summer after the decline of t he annual Peridinium bloom in June-July.