Enhancement of pico- and nanoplankton growth by coral exudates

The enhancement of pico- and nanoplankton cell biomass by coral exudates wa s studied in the laboratory. Two types of mesocosms were used, the first on e 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, cyano bacteria, and of auto- and heterotrophic flagellates, as well as the concen trations of inorganic (N and P) and organic (dissolved organic carbon, DOG) nutrients. There were no significant differences in inorganic nutrient con centrations between mesocosms. However, DOC concentrations in coral mesocos ms 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 con sequence, microbial growth was significantly enhanced in coral mesocosms. A t 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 exud ates (0.5 to 10 % of maximum DOC concentrations) are enough to greatly stim ulate microbial growth.