Food selectivity and processing by the cold-water coral Lophelia pertusa

Cold-water corals form prominent reef ecosystems along ocean margins that depend on suspended resources produced in surface waters. In this study, we investigated food processing of 13C and 15N labelled bacteria and algae by the cold-water coral Lophelia pertusa. Coral respiration, tissue incorporation of C and N and metabolic-derived C incorporation into the skeleton were traced following the additions of different food concentrations (100, 300, 1300 µg C/l) and two ratios of suspended bacterial and algal biomass (1:1, 3:1). Respiration and tissue incorporation by L. pertusa increased markedly following exposure to higher food concentrations. The net growth efficiency of L. pertusa was low (0.08±0.03), which is consistent with their slow growth rates. The contribution of algae and bacteria to total coral assimilation was proportional to the food mixture in the two lowest food concentrations, but algae were preferred over bacteria as food source at the highest food concentration. Similarly, the stoichiometric uptake of C and N was coupled in the low and medium food treatment, but was uncoupled in the high food treatment and indicated a comparatively higher uptake or retention of bacterial carbon as compared to algal nitrogen. We argue that behavioural responses for these small-sized food particles, such as tentacle behaviour, mucus trapping and physiological processing, are more likely to explain the observed food selectivity as compared to physical-mechanical considerations. A comparison of the experimental food conditions to natural organic carbon concentrations above CWC reefs suggests that L. pertusa is well adapted to exploit temporal pulses of high organic matter concentrations in the bottom water caused by internal waves and down-welling events.

Seawater carbonate chemistry and calcification during an experiment with a coral Porites lutea, 2004

Using living corals collected from Okinawan coral reefs, laboratory experiments were performed to investigate the relationship between coral calcification and aragonite saturation state (W) of seawater at 25?C. Calcification rate of a massive coral Porites lutea cultured in a beaker showed a linear increase with increasing Waragonite values (1.08-7.77) of seawater. The increasing trend of calcification rate (c) for W is expressed as an equation, c = aW + b (a, b: constants). When W was larger than ~4, the coral samples calcified during nighttime, indicating an evidence of dark calcification. This study strongly suggests that calcification of Porites lutea depends on W of ambient seawater. A decrease in saturation state of seawater due to increased pCO2 may decrease reef-building capacity of corals through reducing calcification rate of corals.