Calcification in a changing ocean

Abstract

Hard parts are emblematic features of marine calcifiers. These calcium carbonate structures can be instrumental for supporting body mass, providing defense against predatory attempts, or protecting from environmental extremes. Some of the most iconic calcifiers (mussel beds, oyster reefs, and coral reefs) are ecosystem engineers and provide substantial ecosystem services. Albeit the vast diversity of calcifying taxa in the ocean, corals, and to a lesser extent bivalves, were the focus of most research presented in the 2016 Ocean Sciences Meeting session, “Coral Reef Calcification in a Changing Ocean: From Microscale Mechanisms to Macroscale Responses.” This emphasis reflects the realized effects of changing ocean chemistry for these particular animals. The day began with a focus on the mechanistic impacts of climate change on calcification—the means by which the organism can mediate the precipitation of calcium carbonate in a favorable or unfavorable environment. Calcification is more than just the facilitated movement of calcium ions and inorganic carbon by the organism; unanticipated enzymes are suspected to play a role as well. For example, Joshua Rosenthal from the University of Puerto Rico demonstrated that during early calcification in the coral, Porites astreoides, vitamin C is transported via a sodium-dependent transporter and seems to play a critical role in collagen formation. Collagen is considered an integral backbone of calcium carbonate. Other mechanistic impacts of changing ocean chemistry on calcification include the ability for both bivalves and corals to increase pH at the site of calcification. Increasing pH can favor the calcification reaction. Observations of pH were mostly accomplished through the use of micro-electrodes, where progress in micromanipulation of the electrode through the animal tissue and into the calcifying space is being mastered. The impact of changing climate on bivalves has been another area of emphasis given the economic value of the aquaculture industry. The majority of talks on bivalves considered the larval stage during which the shell is first calcified. Larval production is a bottleneck in the aquaculture industry, and climate change has been observed to have direct impacts on larval production through effects on first shell formation. Research presented by Kirti Ramesh from GEOMAR targeted identification of the calcium carbonate polymorph first precipitated, which is important for both calcification and dissolution kinetics. Members of Donal Manahan's lab at the University of Southern California (including the author) presented a calcification budget for first shell formation to demonstrate why shells are smaller at undersaturation, and also developed a bioenergetics framework for the energetic cost of calcification. The impact of calcification on biogeochemistry on coral reefs was another major theme of the session. The autonomous sensing capabilities and the creativity of the scientists used to unravel the interplay between calcification and biogeochemistry are becoming impressive. The purview spanned past, present, and future climate change perspectives. A few of the highlights included: Rebecca Albright from the Carnegie Institution for Science presented work in which she and colleagues turned back the clock on coral reefs by manipulating seawater in situ to closely reflect pre-industrial conditions. This ‘reversal of ocean acidification' enhanced net coral reef calcification indicating that ocean acidification may already be impairing the state of coral reefs. Present-day climate oscillations, in particular the 2015 “Godzilla” El Niño, which was ongoing at the time, also made the list of hot topics. Wade McGillis from Lamont-Doherty Earth Observatory utilized biogeochemical sensing packages at Panama coral reefs to quantify the negative impact of El Niño on net calcification rates. The final talk of the session considered coral reefs into the future. Dennis Hubbard from Oberlin College discussed whether coral reefs can maintain accretion rates to keep pace with rising sea levels. The take away: greater than 50% of reefs appear to be lagging behind sea level rise. The implication is that low-lying island communities will become increasingly vulnerable to storms as the reef's capacity to temper wave energy becomes capacitated. These sobering findings led Hubbard to refer to humans as “Homo stupidus.” Can research on calcification in a changing ocean keep up with the pace of change in the ocean? There are still many uncertainties with regards to resilience and adaptation potential that will regulate the actual impact of anthropogenic climate change on marine calcifiers. And while resilience and adaptation have proved challenging to assess, it is a much-needed next-step forward for research efforts. Indications from this session are that these topics should be on the agenda at future Ocean Sciences Meetings. Christina A. Frieder, University of Southern California, Los Angeles, California; cafriede@usc.edu