Reefs devoured by tiny plants as oceans warm and acidify – ‘If we think of the reef as a scaffold, it’s now being taken apart faster than it can re-build’

20 March 2013 (Practical Fishkeeping) – A study has found that, weakened by microscopic borers, the world’s coral reefs will erode more rapidly as the oceans warm and acidify. This phenomenon, combined with a slower growth of coral reefs due to ocean acidification, may make reefs more vulnerable to storms and cyclones, says Ms Catalina Reyes of the ARC Centre of Excellence for Coral Reef Studies (CoECRS) and The University of Queensland (UQ). Ms Reyes explains that corals use calcium carbonate, or limestone, to build the reef structure. As they accumulate carbonate and extend their skeleton, the old, dead parts are eroded by waves, currents, fishes, sponges and by tiny plants that live inside the reef. “There is a fine balance between accumulating and losing carbonate, and healthy reefs are the ones that gain more than they lose,” Ms Reyes says. “Anything that disrupts this balance puts coral reefs in danger.” Coral reefs are already threatened by ocean acidification, caused by human carbon emissions dissolving into the oceans, as this process reduces the amount of carbonate in the seawater, causing the corals to build the reef at a slower pace, says Associate Professor Sophie Dove of CoECRS and UQ. In this latest study, CoECRS researchers found that the lack of carbonate to build coral reefs isn’t the only challenge that these ecosystems face. “Our research shows that when seawater is both acidic and warm – which is predicted to happen under future climate scenarios – coral reefs could be made more fragile by microborers, such as algae, blue-green algae and fungi that inhabit reefs and bore tiny holes in it that undermine the strength of the coral skeleton.” […] “So if we look into the future, not only do corals have less material with which to build their reefs, but the old, dead parts that support them are eroded much faster,” says Dr Dove. “If we think of the reef as a scaffold, it’s now being taken apart faster than it can re-build, which means that it’s at a higher risk of collapsing.” [more]

Reefs devoured by tiny plants as oceans warm and acidify

ABSTRACT: Biological mediation of carbonate dissolution represents a fundamental component of the destructive forces acting on coral reef ecosystems. Whereas ocean acidification can increase dissolution of carbonate substrates, the combined impact of ocean acidification and warming on the microbioerosion of coral skeletons remains unknown. Here, we exposed skeletons of the reef-building corals, Porites cylindrica and Isopora cuneata, to present-day (Control: 400 μatm – 24 °C) and future pCO2–temperature scenarios projected for the end of the century (Medium: +230 μatm – +2 °C; High: +610 μatm – +4 °C). Skeletons were also subjected to permanent darkness with initial sodium hypochlorite incubation, and natural light without sodium hypochlorite incubation to isolate the environmental effect of acidic seawater (i.e., Ωaragonite <1) from the biological effect of photosynthetic microborers. Our results indicated that skeletal dissolution is predominantly driven by photosynthetic microborers, as samples held in the dark did not decalcify. In contrast, dissolution of skeletons exposed to light increased under elevated pCO2–temperature scenarios, with P. cylindrica experiencing higher dissolution rates per month (89%) than I. cuneata (46%) in the high treatment relative to control. The effects of future pCO2–temperature scenarios on the structure of endolithic communities were only identified in P. cylindrica and were mostly associated with a higher abundance of the green algae Ostreobium spp. Enhanced skeletal dissolution was also associated with increased endolithic biomass and respiration under elevated pCO2–temperature scenarios. Our results suggest that future projections of ocean acidification and warming will lead to increased rates of microbioerosion. However, the magnitude of bioerosion responses may depend on the structural properties of coral skeletons, with a range of implications for reef carbonate losses under warmer and more acidic oceans.

Ocean acidification and warming scenarios increase microbioerosion of coral skeletons