Enhancement of pico- and nanoplankton growth by coral exudates

The enhancement of pico- and nanoplankton cell biomass by coral exudates was studied in the laboratory. Two types of mesocosms were used, the first one containing only a carbonate sand layer (control mesocosm) and the second one containing a coral layer over the carbonate sand layer (coral mesocosm). During 10 h incubations, we followed the concentration of bacteria, cyanobacteria, and of auto- and heterotrophic flagellates, as well as the concentrations of inorganic (N and P) and organic (dissolved organic carbon, DOG) nutrients. There were no significant differences in inorganic nutrient concentrations between mesocosms. However, DOC concentrations in coral mesocosms exhibited peaks 5- to 13-fold higher than control mesocosm levels; these peaks took place between 13:00 and 17:00 h and lasted for ca 2 h. As a consequence, microbial growth was significantly enhanced in coral mesocosms. At the end of the incubations, bacterial biomass was B-fold higher in coral relative to control mesocosms. Autotrophic biomass was 3 to 5 times higher in coral mesocosms. These results indicate that small amounts of coral exudates (0.5 to 10 % of maximum DOC concentrations) are enough to greatly stimulate microbial growth.