Elevated levels of microbial activity in the coral surface microlayer

Mcrobial biomass and activity parameters were measured in the mucus-rich, coral surface microlayer (CSM) and in the waters overlying coral reefs in the Dry Tortugas and Key West Florida, USA. For all parameters measured (thymidine ~ncorporation, bacterial direct counts, primary production, chlorophyll a, dissolved DNA, and particulate DNA), the CSM values were significantly greater than the overlying water values on a per volume basis. Enrichment in the CSM ranged from nearly equal to 7.5-fold for bacterial direct counts to 18 to 280-fold for chlorophyll a. Microautotrophic processes in the CSM were due almost exclusively to the activities of free-living zooxanthellae, while activities in overlying waters were dominated by autotrophic picoplankton. Higher cellular rates of thymldine incorporation (1.23 to 50 tlmes greater) indicated that bacteria in the CSM were growing faster than those in the water column. Bacterial production estimates on a per area basis in the CSM (thickness = several mm) were equivalent to that of the whole water column over the reef (depth 1 to 4 m). Size fractionation of the CSM indicated that most of this bacterial activity was associated with particles > l pm, while overlying waters were dominated by < l pm, free-living bacterioplankton. No significant temporal or spatial variation was noted In these parameters in the overlying waters in a transect across the reef or in a die1 study. However, all values in the CSM (except direct counts) were significantly greater at a midnight sampling than at a noon samphng. Dissolved deoxyribonuclease activity was found in both environments. Collectively these results indicate that the CSM contains an extremely active microbial community, comparable to that found in a eutrophic estuary. These results are discussed in terms of reef production in oligotrophic environments and nutrient dynamics.