Coral mucus as an efficient trap for picoplanktonic cyanobacteria: implications for pelagic-benthic coupling in the reef ecosystem

Although the planktonic community of coral reefs is dominated by picoplankton (e.g. the cyanobacterium Synechococcus), it was long believed to play only a marginal role in pelagic-benthic coupling, as its minute size (0.2 to 2.0 mu m) and negligible sinking rate render if target), unavailable for the filter-feeding reef benthos. However, scleractinian corals have been shown to continuously release mucus that functions as an efficient trap and important carrier for particulate matter within the reef ecosystem. This study investigates the trapping potential of coral mucus for picoplankton in the laboratory and in the field, Freshly released mucus of Fungia corals already contained background levels of pelagic and/or associated synechococcoid cyanobacteria (1.0 +/- 0.2 x 10(4) cells ml(-1)). Mesocosm experiments in flow-through tanks revealed up to 46-fold picoplankton enrichment in aged Mucus aggregates, while laboratory experiments with rotated chambers confirmed the pelagic source of these mucus enrichments. Addition of coral mucus resulted in a significant increase in clearance rates (32 to 52% h(-1)) of the initial Synechococcus spp. population compared to clearance found in non-mucus chambers (6 to 18%, h(-1)). Drilling mucus aggregates originating from Acropora corals collected in situ exhibited high Synechococcus enrichment (up to 4.6 x 10(6) cells ml(-1)) compared to the surrounding seawater (2.1 +/- 0.8 x 10(4) cells ml(-1)), indicating efficient picoplankton enrichment by 2 orders of magnitude. The ensuing rapid sedimentation (0.5 to 1 cm s(-1)) of enriched aggregates highlights the importance of coral mucus as a so far overlooked vector enhancing the flux of pelagic picoplankton to the coral reef benthos.