Why corals care about ocean acidification : uncovering the mechanism

Author Posting. © Oceanography Society, 2009. This article is posted here by permission of Oceanography Society for personal use, not for redistribution. The definitive version was published in Oceanography 22 no. 4 (2009): 118-127.Stony corals build hard skeletons of calcium carbonate (CaCO3) by combining calcium with carbonate ions derived, ultimately, from seawater. The concentration of carbonate ions relative to other carbonate species in seawater is rather low, so corals expend energy to raise the pH of seawater sequestered in an isolated, extracellular compartment where crystal growth occurs. This action converts plentiful bicarbonate ions to the carbonate ions required for calcification, allowing corals to produce CaCO3 about 100 times faster than it could otherwise form. It is this rapid and efficient production of CaCO3 crystals that enables corals to build coral reefs. Ocean acidification reduces the pH and thus the abundance of carbonate ions in seawater. Corals living in acidified seawater continue to produce CaCO3 and expend as much energy as their counterparts in normal seawater to raise the pH of the calcifying fluid. However, in acidified seawater, corals are unable to elevate the concentration of carbonate ions to the level required for normal skeletal growth. In several experiments, we found that boosting the energetic status of corals by enhanced heterotrophic feeding or moderate increases in inorganic nutrients helped to offset the negative impact of ocean acidification. However, this built-in defense is unlikely to benefit corals as levels of CO2 in the atmosphere continue to rise. Most climate models predict that the availability of inorganic nutrients and plankton in the surface waters where corals live will decrease as a consequence of global warming. Thus, corals and coral reefs may be significantly more vulnerable to ocean acidification than previously thought.Anne L. Cohen acknowledges support from the WHOI Directorate for our Marine Calcification and Culture Labs, from WHOI’s Ocean Life and Tropical Research Institutes, and from NSF CO-0648157. Michael Holcomb’s graduate research was supported in part by an NSF graduate student fellowship, an MIT Presidential Award, and an International Coral Reef Society fellowship

An investigation of the calcification response of the scleractinian coral Astrangia poculata to elevated pCO2 and the effects of nutrients, zooxanthellae and gender

© The Author(s), 2012. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Biogeosciences 9 (2012): 29-39, doi:10.5194/bg-9-29-2012.The effects of nutrients and pCO2 on zooxanthellate and azooxanthellate colonies of the temperate scleractinian coral Astrangia poculata (Ellis and Solander, 1786) were investigated at two different temperatures (16 °C and 24 °C). Corals exposed to elevated pCO2 tended to have lower relative calcification rates, as estimated from changes in buoyant weights. Experimental nutrient enrichments had no significant effect nor did there appear to be any interaction between pCO2 and nutrients. Elevated pCO2 appeared to have a similar effect on coral calcification whether zooxanthellae were present or absent at 16 °C. However, at 24 °C, the interpretation of the results is complicated by a significant interaction between gender and pCO2 for spawning corals. At 16 °C, gamete release was not observed, and no gender differences in calcification rates were observed – female and male corals showed similar reductions in calcification rates in response to elevated CO2 (15% and 19% respectively). Corals grown at 24 °C spawned repeatedly and male and female corals exhibited two different growth rate patterns – female corals grown at 24 °C and exposed to CO2 had calcification rates 39% lower than females grown at ambient CO2, while males showed a non-significant decline of 5% under elevated CO2. The increased sensitivity of females to elevated pCO2 may reflect a greater investment of energy in reproduction (egg production) relative to males (sperm production). These results suggest that both gender and spawning are important factors in determining the sensitivity of corals to ocean acidification, and considering these factors in future research may be critical to predicting how the population structures of marine calcifiers will change in response to ocean acidification.This material is based upon work supported under a National Science Foundation Graduate Research Fellowship, the WHOI Ocean Life Institute, NSF OCE-1041106, and an International Society for Reef Studies/Ocean Conservancy Fellowship

Comment on “Equatorial Pacific coral geochemical records show recent weakening of the Walker circulation” by J. Carilli et al.

This article is a comment on Carilli et al. [2014] doi:10.1002/2014PA0026832015-11-1

Mitigation of coral reef warming across the central Pacific by the Equatorial Undercurrent : a past and future divide

© The Author(s), 2016. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Scientific Reports 6 (2016): 21213, doi:10.1038/srep21213.Global climate models (GCMs) predict enhanced warming and nutrient decline across the central tropical Pacific as trade winds weaken with global warming. Concurrent changes in circulation, however, have potential to mitigate these effects for equatorial islands. The implications for densely populated island nations, whose livelihoods depend on ecosystem services, are significant. A unique suite of in situ measurements coupled with state-of-the-art GCM simulations enables us to quantify the mitigation potential of the projected circulation change for three coral reef ecosystems under two future scenarios. Estimated historical trends indicate that over 100% of the large-scale warming to date has been offset locally by changes in circulation, while future simulations predict a warming mitigation effect of only 5–10% depending on the island. The pace and extent to which GCM projections overwhelm historical trends will play a key role in defining the fate of marine ecosystems and island communities across the tropical Pacific.Support provided by NSF OCE-1031971 (ALC and KBK). KBK also acknowledges support from NSF OCE-1233282, the DoD Strategic Environmental Research and Development Program (SERDP), the WHOI Ocean and Climate Change Institute Moltz Fellowship, and the Alfred P. Sloan Foundation. Funding for the Pacific Reef Assessment and Monitoring Program, was provided by NOAA’s Coral Reef Conservation Program

Low and variable ecosystem calcification in a coral reef lagoon under natural acidification

© The Author(s), 2017. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Limnology and Oceanography 63 (2018): 714–730, doi:10.1002/lno.10662.Laboratory‐based CO2 experiments and studies of naturally low pH coral reef ecosystems reveal negative impacts of ocean acidification on the calcifying communities that build coral reefs. Conversely, in Palau's low pH lagoons, coral cover is high, coral communities are diverse, and calcification rates of two reef‐building corals exhibit no apparent sensitivity to the strong natural gradient in pH and aragonite saturation state (Ωar). We developed two methods to quantify rates of Net Ecosystem Calcification (NEC), the ecosystem‐level balance between calcification and dissolution, in Risong Lagoon, where average daily pH is ∼ 7.9 and Ωar ∼ 2.7. While coral cover in the lagoon is within the range of other Pacific reefs (∼ 26%), NEC rates were among the lowest measured, averaging 25.9 ± 13.7 mmol m−2 d−1 over two 4 d study periods. NEC rates were highly variable, ranging from a low of 13.7 mmol m−2 d−1 in March 2012 to a high of 40.3 mmol m−2 d−1 in November 2013, despite no significant changes in temperature, salinity, inorganic nutrients, Ωar, or pH. Our results indicate that the coral reef community of Risong Lagoon produces just enough calcium carbonate to maintain net positive calcification but comes dangerously close to net zero or negative NEC (net dissolution). Identifying the factors responsible for low NEC rates as well as the drivers of NEC variability in naturally low pH reef systems are key to predicting their futures under 21st century climate change.This work was supported by NSF award 1220529 to A.L.C., S.J.L., and K.E.F.S. and a Woods Hole Oceanographic Institution Postdoctoral Scholarship to K.E.F.S

Relationship between water and aragonite barium concentrations in aquaria reared juvenile corals

This paper is not subject to U.S. copyright. The definitive version was published in Geochimica et Cosmochimica Acta 209 (2017): 123-134, doi:10.1016/j.gca.2017.04.006.Coral barium to calcium (Ba/Ca) ratios have been used to reconstruct records of upwelling, river and groundwater discharge, and sediment and dust input to the coastal ocean. However, this proxy has not yet been explicitly tested to determine if Ba inclusion in the coral skeleton is directly proportional to seawater Ba concentration and to further determine how additional factors such as temperature and calcification rate control coral Ba/Ca ratios. We measured the inclusion of Ba within aquaria reared juvenile corals (Favia fragum) at three temperatures (∼27.7, 24.6 and 22.5 °C) and three seawater Ba concentrations (73, 230 and 450 nmol kg−1). Coral polyps were settled on tiles conditioned with encrusting coralline algae, which complicated chemical analysis of the coral skeletal material grown during the aquaria experiments. We utilized Sr/Ca ratios of encrusting coralline algae (as low as 3.4 mmol mol−1) to correct coral Ba/Ca for this contamination, which was determined to be 26 ± 11% using a two end member mixing model. Notably, there was a large range in Ba/Ca across all treatments, however, we found that Ba inclusion was linear across the full concentration range. The temperature sensitivity of the distribution coefficient is within the range of previously reported values. Finally, calcification rate, which displayed large variability, was not correlated to the distribution coefficient. The observed temperature dependence predicts a change in coral Ba/Ca ratios of 1.1 μmol mol−1 from 20 to 28 °C for typical coastal ocean Ba concentrations of 50 nmol kg−1. Given the linear uptake of Ba by corals observed in this study, coral proxy records that demonstrate peaks of 10–25 μmol mol−1 would require coastal seawater Ba of between 60 and 145 nmol kg−1. Further validation of the coral Ba/Ca proxy requires evaluation of changes in seawater chemistry associated with the environmental perturbation recorded by the coral as well as verification of these results for Porites species, which are widely used in paleo reconstructions.M.E.G. was supported by a NDSEG graduate fellowship. Funding for this research came from the NSF Chemical Oceanography program (OCE-0751525) and the Coastal Ocean Institute, the Ocean and Climate Change Institute and the Ocean Ventures Fund at Woods Hole Oceanographic Institution

Early exposure of bay scallops (Argopecten irradians) to high CO2 causes a decrease in larval shell growth

© The Author(s), 2013. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in PLoS ONE 8 (2013): e61065, doi:10.1371/journal.pone.0061065.Ocean acidification, characterized by elevated pCO2 and the associated decreases in seawater pH and calcium carbonate saturation state (Ω), has a variable impact on the growth and survival of marine invertebrates. Larval stages are thought to be particularly vulnerable to environmental stressors, and negative impacts of ocean acidification have been seen on fertilization as well as on embryonic, larval, and juvenile development and growth of bivalve molluscs. We investigated the effects of high CO2 exposure (resulting in pH = 7.39, Ωar = 0.74) on the larvae of the bay scallop Argopecten irradians from 12 h to 7 d old, including a switch from high CO2 to ambient CO2 conditions (pH = 7.93, Ωar = 2.26) after 3 d, to assess the possibility of persistent effects of early exposure. The survival of larvae in the high CO2 treatment was consistently lower than the survival of larvae in ambient conditions, and was already significantly lower at 1 d. Likewise, the shell length of larvae in the high CO2 treatment was significantly smaller than larvae in the ambient conditions throughout the experiment and by 7 d, was reduced by 11.5%. This study also demonstrates that the size effects of short-term exposure to high CO2 are still detectable after 7 d of larval development; the shells of larvae exposed to high CO2 for the first 3 d of development and subsequently exposed to ambient CO2 were not significantly different in size at 3 and 7 d than the shells of larvae exposed to high CO2 throughout the experiment.This work was funded by a Woods Hole Oceanographic Institution Interdisciplinary Award to Mullineaux & McCorkle; and awards to Mullineaux & White, to McCorkle, and to Cohen & McCorkle through NOAA (National Oceanic and Admosphereic Administration) Sea Grant #NA10OAR4170083. White was funded through a National Defense Science and Engineering Graduate Fellowship through the American Society for Engineering Education

Reduced calcification and lack of acclimatization by coral colonies growing in areas of persistent natural acidification

Author Posting. © The Author(s), 2013. This is the author's version of the work. It is posted here by permission of National Academy of Sciences for personal use, not for redistribution. The definitive version was published in Proceedings of the National Academy of Sciences of the United States of America 110 (2013):11044-11049, doi:10.1073/pnas.1301589110.As the surface ocean equilibrates with rising atmospheric CO2, the pH of surface seawater is decreasing with potentially negative impacts on coral calcification. A critical question is whether corals will be able to adapt or acclimate to these changes in seawater chemistry. We use high precision CT scanning of skeletal cores of Porites astreoides, an important Caribbean reef-building coral, to show that calcification rates decrease significantly along a natural gradient in pH and aragonite saturation (Ωarag). This decrease is accompanied by an increase in skeletal erosion and predation by boring organisms. The degree of sensitivity to reduced Ωarag measured on our field corals is consistent with that exhibited by the same species in laboratory CO2 manipulation experiments. We conclude that the Porites corals at our field site were not able to acclimatize enough to prevent the impacts of local ocean acidification on their skeletal growth and development, despite spending their entire lifespan in low pH, low Ωarag seawater.This research was funded by Na¬tional Science Foundation (NSF) OCE-1040952, a University of California Institute for Mexico and the United States (UC-Mexus) grant (to A.P.), and NSF OCE-1041106 (to A.L.C.). E.D.C. was funded through NSF-GFR and a EPA-STAR fellowships.2013-12-1

Record of Little Ice Age sea surface temperatures at Bermuda using a growth-dependent calibration of coral Sr/Ca

Author Posting. © American Geophysical Union, 2005. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Paleoceanography 20 (2005): PA4016, doi:10.1029/2005PA001140.Strontium to calcium ratios (Sr/Ca) are reported for a massive brain coral Diploria labyrinthiformis collected from the south shore of Bermuda and are strongly correlated with both sea surface temperature (SST) and mean annual skeletal growth rate. High Sr/Ca ratios correspond with cold SSTs and slow skeletal growth rate and vice versa. We provide a quantitative calibration of Sr/Ca to extension rate and SST along the axis of maximum growth and derive a growth-dependent Sr/Ca–SST calibration equation to reconstruct western subtropical North Atlantic SSTs for the past 223 years. When the influence of growth rate is excluded from the calibration, Sr/Ca ratios yield SSTs that are too cold during cool anomalies and too warm during warm anomalies. Toward the end of the Little Ice Age (∼1850), SST changes derived using a calibration that is not growth-dependent are exaggerated by a factor of 2 relative to those from the growth-corrected model that yields SSTs ∼1.5°C cooler than today. Our results indicate that incorporation of growth rate effects into coral Sr/Ca calibrations may improve the accuracy of SSTs derived from living and fossil corals.A Stanley Watson Foundation Fellowship (N.F.G.), and grants from NSF (OCE-0402728) and WHOI (K.A.H., A.L.C., and M.S.M.) supported this work

Tidal modulation of Sr/Ca ratios in a Pacific reef coral

Author Posting. © American Geophysical Union, 2004. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 31 (2004): L16310, doi:10.1029/2004GL020600.The strontium-to-calcium ratio (Sr/Ca) of reef coral skeleton is an important tool for reconstructing past sea surface temperatures (SSTs). However, the accuracy of paleoSSTs derived from fossil coral Sr/Ca is challenged by evidence that physiological processes influence skeletal chemistry. Here we show that water level variations from tidal forcing are correlated with changes in coral Sr/Ca that cannot be accounted for by changes in SST. Ion microprobe measurements of Sr/Ca ratios in a Pacific Porites lutea reveal high-frequency variations at periods of ~6, ~10, and ~25 days. The relationship between Sr/Ca and temperature on these short timescales does not follow trends observed at longer periods, indicating that an additional forcing is required to explain our observations. We demonstrate that Sr/Ca is correlated with both tidal water level variations and SST, and that their contributions to the Sr/Ca content of the skeleton vary as a function of period. We propose that water level influences Sr/Ca indirectly via modulation of photosynthetically-active radiation (PAR) that drives large changes in zooxanthellate photosynthesis.This research was supported by WHOI Ocean Life Institute grant 25051316 to ALC; NSF grants EAR-9628749 and EAR-9904400 to the WHOI Northeast National Ion Microprobe Facility; DAMD 17-93-J-3052 supported ALC’s fieldwork on JA