Enrichment of microbial abundance in the sea-surface microlayer over a coral reef: implications for biogeochemical cycles in reef ecosystems

In order to test the hypothesis that the microbial enrichment in the sea-surface microlayer (SML) over coral reefs is significant, we investigated the abundance and production of the microbial community in the SML and sub-surface water (SSW) at a fringing coral reef of Malaysia. Samples were taken at 2 coral sites with different live coral coverage as well as one offshore site. We detected substantially high enrichment factors (EFs) in abundance (where EF = SML/SSW: the ratio of microbial abundance in the SML relative to SSW) for all biological groups at all sites: up to 6.0 for heterotrophic bacteria, 4.2 for cyanobacteria, 18.8 for autotrophic nanoflagellates and 22.6 for heterotrophic nanoflagellates. We also found that the enrichment factors of microorganisms in coral reefs were remarkably higher than in other marine ecosystems, and a higher concentration of microorganisms was observed in the higher coral coverage site, probably due to higher organic matter released by corals. The higher microbial abundance in the SML over coral reefs may enhance gaseous exchange and carbon flow in the food web through the air-sea interface.