Release of dissolved and particulate organic matter by the soft coral Lobophytum and subsequent microbial degradation

Understanding the release and remineralization of organic matter by benthic macroorganisms provides insight into nutrient cycling and microbial metabolism in coral reef environments. The release rate of particulate (POC) and dissolved organic carbon (DOC) by the soft coral Lobophytum crassum was quantified and subsequent bacterial growth rates determined in response to this resource, and compared with results from those of the common hard coral Acropora intermedia. The results of this study show that the soft coral released more DOC than POC into the surrounding seawater, similar to what was measured for the hard coral species. However, the soft coral-derived organic matter fostered a lower microbial growth rate with a lower growth efficiency compared to DOC and POC of hard corals, likely due to the lower C:N ratio of the organic matter derived from soft corals. These results suggest that soft coral exudates are relatively refractory compared to the mucus of hard corals. Possible phase shifts from hard to soft corals on degraded reefs may represent very different changes in microbial community dynamics and metabolism as compared to the widely studied coral-algal phase shifts.