Anne L. Cohen

Marine calcifiers exhibit mixed responses to CO2-induced ocean acidification

Anthropogenic elevation of atmospheric carbon dioxide ( p CO2) is making the oceans more acidic, thereby reducing their degree of saturation with respect to calcium carbonate (CaCO3). There is mounting concern over the impact that future CO2-induced reductions in the CaCO3 saturation state of seawater will have on marine organisms that construct their shells and skeletons from this mineral. Here, we present the results of 60 d laboratory experiments in which we investigated the effects of CO2-induced ocean acidification on calcification in 18 benthic marine organisms. Species were selected to span a broad taxonomic range (crustacea, cnidaria, echinoidea, rhodophyta, chlorophyta, gastropoda, bivalvia, annelida) and included organisms producing aragonite, low-Mg calcite, and high-Mg calcite forms of CaCO3. We show that 10 of the 18 species studied exhibited reduced rates of net calcification and, in some cases, net dissolution under elevated p CO2. However, in seven species, net calcification increased under the intermediate and/or highest levels of p CO2, and one species showed no response at all. These varied responses may reflect differences amongst organisms in their ability to regulate pH at the site of calcification, in the extent to which their outer shell layer is protected by an organic covering, in the solubility of their shell or skeletal mineral, and in the extent to which they utilize photosynthesis. Whatever the specific mechanism(s) involved, our results suggest that the impact of elevated atmospheric p CO2 on marine calcification is more varied than previously thought.

Projecting coral reef futures under global warming and ocean acidification

Many physiological responses in present-day coral reefs to climate change are interpreted as consistent with the imminent disappearance of modern reefs globally because of annual mass bleaching events, carbonate dissolution, and insufficient time for substantial evolutionary responses. Emerging evidence for variability in the coral calcification response to acidification, geographical variation in bleaching susceptibility and recovery, responses to past climate change, and potential rates of adaptation to rapid warming supports an alternative scenario in which reef degradation occurs with greater temporal and spatial heterogeneity than current projections suggest. Reducing uncertainty in projecting coral reef futures requires improved understanding of past responses to rapid climate change; physiological responses to interacting factors, such as temperature, acidification, and nutrients; and the costs and constraints imposed by acclimation and adaptation.

Geochemical Perspectives on Coral Mineralization

Corals open an exceptional window into many phenomena of geological, geochemical, climatic, and paleontological interest. From the Paleozoic to the present, corals provide some of the finest high-resolution archives of marine conditions. Corals are likewise exceptional for chronometric purposes, and even the terrestrial 14C timescale has now been calibrated against coral 230Th/234U. Corals also represent a testing ground for basic ideas about mineralogy and geochemistry. The shapes, sizes, and organization of skeletal crystals have been attributed to factors as diverse as mineral supersaturation levels and organic mediation of crystal growth. The coupling between calcification and photosynthesis in symbiotic corals is likewise attributed to everything from photosynthetic alkalinization of the water, to efforts by the coral to prevent photosynthetic alkalinization. Corals also leave a significant geochemical imprint on the oceans. Their aragonite skeletons accept about 10 times more strontium than does calcite, hence the proportion of marine aragonite precipitation affects the oceanic chemical balance. Biological carbonates represent the biosphere’s largest carbon reservoir, hence calcareous organisms affect the ocean’s pH, CO2 content, and ultimately global temperatures through the greenhouse gas connection. Finally, corals present some geochemical puzzles for ecology and conservation. How do symbiotic corals obtain nutrients in some of the most nutrient deficient parts of the planet? Are global geochemical changes partially responsible for the widespread declines in coral reefs during recent decades? We will address many of these issues, but will concentrate on coral skeletal structure and calcification mechanism. These topics bear most directly on the biomineralization process and generally affect the choice of skeletal materials and analytical techniques used in geochemical investigations. The coral reef is probably the planet’s most spectacular biomineralization product. These grand and complex ecosystems build on the accumulated skeletal debris of countless generations of organisms, especially calcareous algae and symbiotic …

Ocean warming slows coral growth in the central Red Sea.

Red Sea Coral Decline Large, rapid sea surface temperature rises of 1°C or greater typically cause bleaching of corals. Cantin et al. (p. 322) show that smaller temperature increases also have detrimental effects on corals, dramatically reducing their rates of calcification and skeletal extension. Corals in the Red Sea, where water temperatures have risen by 0.4 to 1°C since the mid-1970s, have declined in skeletal extension by about 30%, and decreased in calcification rates by around 18% since 1998. This finding suggests that we may see an end to coral growth in the Red Sea this century. Rising summertime sea surface temperatures are slowing the rate of growth of healthy corals in the Red Sea. Sea surface temperature (SST) across much of the tropics has increased by 0.4° to 1°C since the mid-1970s. A parallel increase in the frequency and extent of coral bleaching and mortality has fueled concern that climate change poses a major threat to the survival of coral reef ecosystems worldwide. Here we show that steadily rising SSTs, not ocean acidification, are already driving dramatic changes in the growth of an important reef-building coral in the central Red Sea. Three-dimensional computed tomography analyses of the massive coral Diploastrea heliopora reveal that skeletal growth of apparently healthy colonies has declined by 30% since 1998. The same corals responded to a short-lived warm event in 1941/1942, but recovered within 3 years as the ocean cooled. Combining our data with climate model simulations by the Intergovernmental Panel on Climate Change, we predict that should the current warming trend continue, this coral could cease growing altogether by 2070.

Kinetic control of skeletal Sr/Ca in a symbiotic coral: Implications for the paleotemperature proxy

Modeling of past climates is critically dependent on estimates of past sea surface temperatures (SSTs), for which one of the principal techniques used is the measurement of Sr/Ca ratios in corals [Guilderson et al., 1994; McCulloch et al., 1999; Hughen et al., 1999]. The link between coral Sr/Ca and SST is not well-understood and there have been a number of discrepant observations [de Villiers et al., 1995; Alibert, 1998]. Corals with symbiotic zooxanthellae are known to show large diurnal fluctuations in calcification rate associated with the photosynthetic activity of their symbionts. Using detailed measurements with the ion microprobe, we compared the Sr/Ca content of discrete daytime and nighttime skeletal structures in the massive hermatypic coral Porties lutea over the course of 1 year and a seasonal temperature range of 4°C. The Sr/Ca content of daytime skeleton is always lower than that of adjacent nighttime skeleton. While the slope of the nighttime Sr/Ca-SST correlation is close to that seen in inorganic aragonite precipitates, that of the daytime correlation is >4 times as steep. We attribute these differences to the role of photosynthesis in calcification and conclude that bulk Sr/Ca is related principally to daytime calcification rate rather than directly to SST. More reliable estimates of past SST may be arrived at through selective analysis of nighttime skeleton.

An ion probe study of annual cycles of Sr/Ca and other trace elements in corals

Abstract Corals show great promise for preserving century-long records of ocean chemistry and temperature with weekly time resolution. Allison and Tudhope (1992) showed that direct microscale analysis of coral skeletons was possible with ion microprobe techniques. We show here that analysis of B, F, Mg, Sr, and Ba (relative to Ca) can be rapidly achieved on Porites skeleton at 50 μm (subweekly) spatial scales with precisions of 0.3–3%. The B, F, and Mg concentrations show large well-behaved annual variations of 44–57%, well-correlated with Sr/Ca variations of 12%, in the 1967–1969 bands from a live Porites from Two-Mile Reef, South Africa. The Sr/Ca ratio correlates well with the δ 18 O record but shows a larger annual temperature amplitude, with additional numerous sub-weekly temperature spikes. Ba/Ca in recent bands of Porites shows both a large (factor of 5) annual cycle and a large yearly spike in the late summer; the annual variation is not observed in 30-year-old bands, though the annual spikes are still sharp and clear. Barium thus appears to be unstable in Porites during aging and is probably not skeletally-bound. Deep-sea (nonzooxanthellate) corals show uncorrelated variations of these same trace elements, with amplitudes similar to the Porites . These variations are likely biologically-mediated, as the thermal forcing function in the deep sea is nil. This suggests that the annual variations seen in Porites may also be driven by vital effects and not directly by temperature. Ion microprobe techniques are shown to provide rapid, precise, and high-resolution trace-element records in corals. Full exploitation of trace element paleotemperature methods in corals will require a more sophisticated understanding of how corals accrete their skeletal components. Ion probe analysis can contribute significantly to this understanding.

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

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.

A Holocene marine climate record in mollusc shells from the Southwest African coast

Abstract Details of short-term climatic variability are often lost from marine sediments through bioturbation in the upper, aerobic sediment layers. Alternatively, a high-resolution and dated record of climatic events may be obtained using material preserved in archaeological deposits. The Holocene history of the southern Benguela upwelling regime has been constructed from the oxygen isotope and mineral analysis of midden shells. Three discrete episodes of significant isotope enrichment corresponded to periods of glacial expansion in the northern hemisphere. Significant changes in shell mineralogy, which is a response to sea-surface temperatures, were also recorded. The timing and duration of these changes approximated those in the isotope record and may provide a link between events affecting the subcontinent and global temperature changes of the late Quaternary.

Diverse coral communities in naturally acidified waters of a Western Pacific reef

Anthropogenic carbon dioxide emissions are acidifying the oceans, reducing the concentration of carbonate ions ([CO32−]) that calcifying organisms need to build and cement coral reefs. To date, studies of a handful of naturally acidified reef systems reveal depauperate communities, sometimes with reduced coral cover and calcification rates, consistent with results of laboratory-based studies. Here we report the existence of highly diverse, coral-dominated reef communities under chronically low pH and aragonite saturation state (Ωar). Biological and hydrographic processes change the chemistry of the seawater moving across the barrier reefs and into Palaus Rock Island bays, where levels of acidification approach those projected for the western tropical Pacific open ocean by 2100. Nevertheless, coral diversity, cover, and calcification rates are maintained across this natural acidification gradient. Identifying the combination of biological and environmental factors that enable these communities to persist could provide important insights into the future of coral reefs under anthropogenic acidification.

Compositional variability in a cold‐water scleractinian, Lophelia pertusa: New insights into “vital effects”

We analyzed Sr/Ca and Mg/Ca ratios in the thecal wall of Lophelia pertusa, a cold-water coral, using SIMS ion microprobe techniques. The wall grows by simultaneous upward extension and outward thickening. Compositional variability displays similar trends along the upward and outward growth axes. Sr/Ca and Mg/Ca ratios oscillate systematically and inversely. The sensitivity of Lophelia Sr/Ca ratios to the annual temperature cycle (−0.18 mmol · mol−1/°C) is twice as strong as that exhibited by tropical reef corals, and four times as strong as the temperature dependence of Sr/Ca ratios of abiogenic aragonites precipitated experimentally from seawater. A comparison of the skeletal composition of Lophelia with results from precipitation calculations carried out using experimentally determined partition coefficients suggests that both temperature-dependent element partitioning and seasonal changes in the mass fraction of aragonite precipitated from the calcifying fluid influence the composition of Lophelia skeleton. Results from calculations that combine these effects reproduce both the exaggerated amplitude of the Sr/Ca and Mg/Ca oscillations and the inverse relationship between Sr/Ca and Mg/Ca ratios.