Intellectual Merit: Over the course of this century, all tropical coral reef ecosystems, whether fringing heavily populated coastlines or lining remote islands and atolls, face unprecedented threat from ocean acidification caused by rising levels of atmospheric CO2. In many laboratory experiments conducted to date, calcium carbonate production (calcification) by scleractinian (stony) corals showed an inverse correlation to seawater saturation state OMEGAar), whether OMEGAar was manipulated by acid or CO2 addition. Based on these data, it is predicted that coral calcification rates could decline by up to 80% of modern values by the end of this century. A growing body of new experimental data however, suggests that the coral calcification response to ocean acidification may be less straightforward and a lot more variable than previously recognized. In at least 10 recent experiments including our own, 8 different tropical and temperate species reared under nutritionally-replete but significantly elevated CO2 conditions (780-1200 ppm, OMEAGar ~1.5-2), continued to calcify at rates comparable to conspecifics reared under ambient CO2. These experimental results are consistent with initial field data collected on reefs in the eastern Pacific and southern Oman, where corals today live and accrete their skeletons under conditions equivalent to 2X and 3X pre-industrial CO2. On these high CO2, high nutrient reefs (where nitrate concentrations typically exceed 2.5 micro-molar), coral growth rates rival, and sometimes even exceed, those of conspecifics in low CO2, oligotrophic reef environments.
The investigators propose that a coral's energetic status, tightly coupled to the availability of inorganic nutrients and/or food, is a key factor in the calcification response to CO2-induced ocean acidification. Their hypothesis, if confirmed by the laboratory and field investigations proposed here, implies that predicted changes in coastal and open ocean nutrient concentrations over the course of this century, driven by both climate impacts on ocean stratification and by increased human activity in coastal regions, could play a critical role in exacerbating and in some areas, modulating the coral reef response to ocean acidification. This research program builds on the investigators initial results and observations. This combined laboratory and field program will test the hypothesis that: 1. The coral calcification response to ocean acidification is linked to the energetic status of the coral host. The relative contribution of symbiont photosynthesis and heterotrophic feeding to a coral's energetic status varies amongst species. Enhancing the energetic status of corals reared under high CO2, either by stimulating photosynthesis with inorganic nutrients or by direct heterotrophic feeding of the host lowers the sensitivity of calcification to decreased seawater OMEGAar. 2. A species-specific threshold CO2 level exists over which enhanced energetic status can no longer compensate for decreased OMEGAar of the external seawater. Similarly, we will test the hypothesis that a nutrient threshold exists over which nutrients become detrimental for calcification even under high CO2 conditions. 3. Temperature-induced reduction of algal symbionts is one stressor that can reduce the energetic reserve of the coral host and exacerbate the calcification response to ocean acidification. 4. Nutrient modulation of the calcification response to ocean acidification already occurs on reefs today and enables corals on naturally high OMEGA/low nutrient reefs to grow as fast as conspecifics on reefs with naturally low OMEGA/high nutrient conditions.
Broader Impacts: The investigator's initial findings highlight the critical importance of energetic status in the coral calcification response to ocean acidification. Verification of these findings in the laboratory and at natural field sites, and identification of nutrient and CO2thresholds for a range of species will have immediate, direct impact on predictions of reef resilience in a high CO2 world. Our project brings together a diverse group of expertise in coral biogeochemistry, chemical oceanography, molecular biology and coral reproductive ecology to focus on a problem that has enormous societal, economic and conservation relevance. In addition to supporting the labs of 4 PIs, it funds a graduate research fellow, a post-doctoral investigator and undergraduate training through the Princeton-BIOS Student Summer Internship Program. All research will be presented at national and international meetings and workshops and data disseminated in a timely manner through publications and archiving in the Biological and Chemical Oceanographic Data Management office.
The skeletal growth of corals is critically important for the building and maintenance of coral reef ecosystems. Ocean acidification (OA), the lowering of ocean pH caused by rising levels of atmopsheric CO2, threatens reefs by reducing the availabilty of carbonate ions that corals need to produce their skeletons faster than they can be eroded by the sea or by boring organisms. Multiple experiments show that skeletal growth and calcification rates decline when pH declines. However in some experiments, and on some naturally more acidic reefs, growth rates are maintained under low pH and the question is why? We proposed that the energetic cost of skeletal growth increases under OA. We hypothesized that coral nutrition, whether derived by autotrophy or heterotrophy, would play a major role in modulating the coral calcification response to OA (OA). In a series of laboratory-based manipulation experiments conducted at Woods Hole Oceanographic Institution and the Bermuda Institute of Ocean Sciences (BIOS) over 3 years, we quantified coral calcification under OA combined with i) inorganic nutrient enrichment ii) heterotrophic feeding and iii) elevated light (PAR). Our results indicate that all forms of nutritional enhancement significantly modulate/weaken the impact of OA on coral calcification. Conversely, we found that spawning female corals exhibit heightened sensitivity to OA. This result is consistent with our hypothesis that depletion of coral energy reserve caused by spawning, bleaching, disease, or alteration of the reef nutritional enviroment due to global warming, will exacerbate the impact of OA on coral calcification and thus reef health. 12 peer-reviewed papers have been published and two chapters contributed to the IPCC Workshop Report on Impacts of Ocean Acidification on Marine Biology and Ecosystems.The project supported PhD thesis research of 4 graduate students in the MIT-WHOI Joint Program in Oceanography (Michael Holcomb, Elizabeth Drenkard, Hannah Barkley, Thomas DeCarlo), 3 post-doctoral researchers (Justin Ries, Neal Cantin, Katie Shamberger) and 10 undergraduate/high school interns and Summer Student Fellows. More than 35 talks and plenary lectures have been given by PIs and students/post-docs, and more than 20 abstracts published. This work was shared through multiple education and outreach events including WHOI’s Ocean Acidification Public Event, the Cambridge Science Festival, the New England Aquarium, television (NOVA), newspaper articles (CC Times) and radio (NPR’s The World). Our work on the naturally acidified reefs of Palau was presented at John Kerry's Our Ocean conference in Washington DC in June 2014 (www.state.gov/documents/organization/225857.pdf).
