Elevated levels of microbial activity in the coral surface microlayer

Abstract

Mcrobial biomass and activity parameters were measured in the mucus-rich, coralsurface microlayer (CSM) and in the waters overlying coral reefs in the Dry Tortugas and Key WestFlorida, USA. For all parameters measured (thymidine ~ncorporation, bacterial direct counts, primaryproduction, chlorophyll a, dissolved DNA, and particulate DNA), the CSM values were significantlygreater than the overlying water values on a per volume basis. Enrichment in the CSM ranged fromnearly equal to 7.5-fold for bacterial direct counts to 18 to 280-fold for chlorophyll a. Microautotrophicprocesses in the CSM were due almost exclusively to the activities of free-living zooxanthellae, whileactivities in overlying waters were dominated by autotrophic picoplankton. Higher cellular rates ofthymldine incorporation (1.23 to 50 tlmes greater) indicated that bacteria in the CSM were growingfaster 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 to4 m). Size fractionation of the CSM indicated that most of this bacterial activity was associated withparticles >l pm, while overlying waters were dominated by