Ocean Acidification and Warming Scenarios Increase Microbioerosion of Coral Skeletons

Microbioerosion in corals is a process that is caused by chemical dissolution and driven by metabolic activities of internal microborers within coral skeletons. This study hypothesizes that the increase in temperature and acidity of seawater that is projected to take place in the coming decades is will reduce calcification as well as increase dissolution in corals and crustose coralline algae because it will alter microbioerosion processes on coral skeletons. Reyes-Nivia et al. compared current ocean conditions with two elevated CO2-temperature scenarios to explore how skeletons of branching and encrusting corals respond combined acidification and warming, and to see if these elevated scenarios would alter the microbioerosion processes. They measured the rate of calcification under these varying conditions, and found that dissolution of coral skeletons was driven predominantly by photosynthetic emdolithic algae microborers, and that this was increased with combined acidity and warming. They also found that that the projected acidification and warming scenarios appear to favor the accumulation of endolithic algae, which would lead to significant bioerosion within coral reef frameworks.—Dawn Barlow
ReyesNivia, C., DiazPulido, G., Kline, D., Guldberg, O. H., & Dove, S., 2013. Ocean acidification and warming scenarios increase microbioerosion of coral skeletons. Global change biology 19, 1919-1921

                  Reyes-Nivia et al. conducted this experiment in the lab using a system of custom software that allowed for CO2 and temperature to follow seasonally appropriate fluctuations measured at a reference field site, providing a near-perfect match to the control conditions. In the lab, three different acidity-temperature scenarios were created by bubbling different amounts of CO2 into tanks that were maintained at different temperatures using industrial heater-chillers. As the aim of this study was to examine the activity and abundance of microborers on coral skeletons in these different scenarios, samples of a branching (Porites cylindrica) and an encrusting (Isopora cuneata) coral harboring high amounts of endolithic algae were collected and the coral tissue was removed from the skeletons to simulate recently dead coral substrates before they were placed in the different tanks. A subset of these samples was kept in the dark so as to inhibit the reestablishment of these endolithic algae which require sunlight to thrive. The pH was measured in the water as well as within the coral skeleton through drilled holes throughout the study. Total calcification and bioerosion was measured as change in the amount of calcium carbonate using the buoyant weight method, which was also used to measure monthly microbioerosion rates normalized to the total surface area. The loss-on-ignition method was used to calculate the biomass of the endolithic algae at the end of the duration of the experiment.
                  What Reyes-Nivia et al. found was that, under natural day-night cycles, all of the coral skeletons with endolithic microalgae lost calcium carbonate. Under elevated CO-temperature conditions, the total microbioerosion increased in both types of coral. For the coral skeletons under full dark conditions—those without photosynthetic endolithic algae—there was no sign of dissolution; in fact there was a positive increase in buoyant weight.
                  This study showed that the dissolution of coral skeletons, which is driven predominantly by photosynthetic microborers, increased under combined ocean acidification-warming scenarios, and that this dissolution varied with the specific scenario and the coral substrate. This variance shows that the skeletal structure of the coral—microskeletal architecture, porosity, density, minerology—plays an important role in determining the magnitude of how the acidity-warming scenario will affect coral skeletal dissolution. The projected acidification and warming scenarios appear to heavily favor biomass accumulation of photosynthetic microalgae, which in turn would mean significant bioerosion of coral reef frameworks.

                  The implications of this study are that the conditions within projected future oceans appear to influence both the biological and ecological responses of endolithic microborers, which lead to the dissolution of coral skeletons. This is demonstrated by the enhanced biomass, shifts in community structure, and increased respiration rates of endolithic algae under elevated CO2-temperature conditions. This implies that if the oceans continue to acidify as predicted, not only will the corals not be able to calcify as well in more acidic waters, but the increased abundance of photosynthetic endolithic algae will contribute simultaneously to their dissolution

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