Contribution of coral rubble associated microbial community to the dissolution of calcium carbonate under high pCO₂

Abstract

The contribution of coral rubble associated microbial community to the dissolution of calcium carbonate at different pCO₂ levels were investigated using incubation experiments at Sesoko Island, Okinawa, Japan. Short incubation (24h under natural illumination) and long incubations (4 days under natural illumination and dark condition) were carried out using white coral skeleton (control), natural rubble, treated rubble (removed epilithic community) and natural rubble with added organic matter (glucose) under different levels of pCO₂ (ambient, 520, 720 and 1120ppm). Net primary production, respiration and dissolution rates of the endolithic communities were much higher than the epilithic communities at all pCO₂ levels. During the long experiment under natural illumination, carbonate dissolution rates varied from 23.7±0.1 (ambient) to 50.5±1.5 (1120ppm)μmol m^-2d^-1 at night time and 3.2±1.1 (ambient) to 5.8±1.2 (1120ppm) μmol m^-2d^-1 at day time. However, in long experiment under complete dark condition dissolution increased to 81.6 ±0.2μmol m^-2d^-1 at high pCO₂. With addition of glucose as source of organic matter, growth of bacteria was enhanced and therefore respiration rate increased. Heterotrophic microbial communities produced more CO₂ and promoted carbonate dissolution. This was reflected by further decrease in aragonite saturation state (Ωa) and increase in pCO₂ levels. The results show that inputs of organic matter in the reefs enhance metabolic activities of microbial communities associated with coral rubble which ultimately increase dissolution of calcium carbonate. These suggest the importance of biological processes (photosynthesis vs. respiration) in determining calcification and dissolution.