Microbial degradation of cold-water coral-derived organic matter: potential implication for organic C cycling in the water column above Tisler Reef

Cold-water corals release organic matter, in particular mucus, but its role in the ecological functioning of reef ecosystems is still poorly understood. The present study investigates the planktonic microbial degradation of mucus released by Lophelia pertusa colonies from Tisler Reef, Skagerrak. Results are compared to the degradation of dissolved and particulate organic substrates, including the carbohydrates glucose and starch, as well as gum xanthan and the cyanobacterium Spirulina spp. as the model organism for phytoplankton. Resulting microbial organic C degradation rates for the dissolved fraction of L. pertusa-derived mucus showed nearly linear progression over time and revealed similar degradation rates compared to glucose and starch. Degradation of the particulate mucus fraction, in contrast, displayed exponential progression and was much faster than degradation of the dissolved fraction. In addition, particulate mucus degradation showed a 4-fold increase compared to that of the added Spirulina spp. suspension. Mucus-associated microbial communities apparently play a key role in organic matter recycling, as degradation rates more than doubled in untreated compared to sterile coral-derived mucus over 3 d of incubation. Quantification of O2 consumption in the water column above Tisler Reef showed significantly increased values in the direct vicinity of the reef. C-stable isotope signatures of suspended particulate organic matter close to Tisler were close to those of L. pertusa-derived mucus, and high dissolved organic carbon (DOC) concentrations were detected above Tisler Reef. These findings demonstrate the stimulating effect of cold-water coral reefs on microbial activity in the adjacent water column and may indicate some control over organic C cycling.