Seawater carbonate chemistry and community calcification near Lizar Island, 2011

It is predicted that surface ocean pH will reach 7.9, possibly 7.8 by the end of this century due to increased carbon dioxide (CO2) in the atmosphere and in the surface ocean. While aragonite-rich sediments don't begin to dissolve until a threshold pH of ~ 7.8 is reached, dissolution from high-Mg calcites is evident with any drop in pH. Indeed, it is high-Mg calcite that dominates the reaction of carbonate sediments with increased CO2, which undergoes a rapid neomorphism process to a more stable, low-Mg calcite. This has major implications for the future of the high-Mg calcite producing organisms within coral reef ecosystems. In order to understand any potential buffering system offered by the dissolution of carbonate sediments under a lower oceanic pH, this process of high-Mg calcite dissolution in the reef environment must be further elucidated.

Seawater carbonate chemistry, length, mass and otholith development of spiny damselfish Acanthochromis polyacanthus during experiments, 2011

Determining which marine species are sensitive to elevated CO2 and reduced pH, and which species tolerate these changes, is critical for predicting the impacts of ocean acidification on marine biodiversity and ecosystem function. Although adult fish are thought to be relatively tolerant to higher levels of environmental CO2, very little is known about the sensitivity of juvenile stages, which are usually much more vulnerable to environmental change. We tested the effects of elevated environmental CO2 on the growth, survival, skeletal development and otolith (ear bone) calcification of a common coral reef fish, the spiny damselfish Acanthochromis polyacanthus. Newly hatched juveniles were reared for 3 wk at 4 different levels of PCO2(seawater) spanning concentrations already experienced in near-reef waters (450 µatm CO2) to those predicted to occur over the next 50 to 100 yr in the IPCC A2 emission scenario (600, 725, 850 µatm CO2). Elevated PCO2 had no effect on juvenile growth or survival. Similarly, there was no consistent variation in the size of 29 different skeletal elements that could be attributed to CO2 treatments. Finally, otolith size, shape and symmetry (between left and right side of the body) were not affected by exposure to elevated PCO2, despite the fact that otoliths are composed of aragonite. This is the first comprehensive assessment of the likely effects of ocean acidification on the early life history development of a marine fish. Our results suggest that juvenile A. polyacanthus are tolerant of moderate increases in environmental CO2 and that further acidification of the ocean will not, in isolation, have a significant effect on the early life history development of this species, and perhaps other tropical reef fishes

Great Barrier Reef coral element/Ca and stable isotope data and U-Th ages from IODP Expedition 325

Tropical south-western Pacific temperatures are of vital importance to the Great Barrier Reef (GBR), but the role of sea surface temperatures (SSTs) in the growth of the GBR since the Last Glacial Maximum remains largely unknown. Here we present records of Sr/Ca and d18O for Last Glacial Maximum and deglacial corals that show a considerably steeper meridional SST gradient than the present day in the central GBR. We find a 1-2 °C larger temperature decrease between 17° and 20°S about 20,000 to 13,000 years ago. The result is best explained by the northward expansion of cooler subtropical waters due to a weakening of the South Pacific gyre and East Australian Current. Our findings indicate that the GBR experienced substantial meridional temperature change during the last deglaciation, and serve to explain anomalous deglacial drying of northeastern Australia. Overall, the GBR developed through significant SST change and may be more resilient than previously thought.

Seawater carbonate chemistry and biological processes of coral Acropora muricata during experiments and observations in La Saline fringing reef, La Reunion Island, western Indian Ocean, 2011

The effect of decreasing aragonite saturation state (Omega Arag) of seawater (elevated pCO2) on calcification rates of Acropora muricata was studied using nubbins prepared from parent colonies located at two sites of La Saline reef (La Réunion Island, western Indian Ocean): a back-reef site (BR) affected by nutrient-enriched groundwater discharge (mainly nitrate), and a reef flat site (RF) with low terrigenous inputs. Protein and chlorophyll a content of the nubbins, as well as zooxanthellae abundance, were lower at RF than BR. Nubbins were incubated at ~27°C over 2 h under sunlight, in filtered seawater manipulated to get differing initial pCO2 (1,440-340 µatm), Omega Arag (1.4-4.0), and dissolved inorganic carbon (DIC) concentrations (2,100-1,850 µmol/kg). Increasing DIC concentrations at constant total alkalinity (AT) resulted in a decrease in Omega Arag and an increase in pCO2. AT at the beginning of the incubations was kept at a natural level of 2,193 ± 6 µmol/kg (mean ± SD). Net photosynthesis (NP) and calcification were calculated from changes in pH and AT during the incubations. Calcification decrease in response to doubling pCO2 relative to preindustrial level was 22% for RF nubbins. When normalized to surface area of the nubbins, (1) NP and calcification were higher at BR than RF, (2) NP increased in high pCO2 treatments at BR compared to low pCO2 treatments, and (3) calcification was not related to Omega Arag at BR. When normalized to NP, calcification was linearly related to Omega Arag at both sites, and the slopes of the relationships were not significantly different. The increase in NP at BR in the high pCO2 treatments may have increased calcification and thus masked the negative effect of low Omega Arag on calcification. Removing the effect of NP variations at BR showed that calcification declined in a similar manner with decreased Omega Arag (increased pCO2) whatever the nutrient loading.

Major cellular and physiological impacts of ocean acidification on a reef building coral

As atmospheric levels of CO2 increase, reef-building corals are under greater stress from both increased sea surface temperatures and declining sea water pH. To date, most studies have focused on either coral bleaching due to warming oceans or declining calcification due to decreasing oceanic carbonate ion concentrations. Here, through the use of physiology measurements and cDNA microarrays, we show that changes in pH and ocean chemistry consistent with two scenarios put forward by the Intergovernmental Panel on Climate Change (IPCC) drive major changes in gene expression, respiration, photosynthesis and symbiosis of the coral, Acropora millepora, before affects on biomineralisation are apparent at the phenotype level. Under high CO2 conditions corals at the phenotype level lost over half their Symbiodinium populations, and had a decrease in both photosynthesis and respiration. Changes in gene expression were consistent with metabolic suppression, an increase in oxidative stress, apoptosis and symbiont loss. Other expression patterns demonstrate upregulation of membrane transporters, as well as the regulation of genes involved in membrane cytoskeletal interactions and cytoskeletal remodeling. These widespread changes in gene expression emphasize the need to expand future studies of ocean acidification to include a wider spectrum of cellular processes, many of which may occur before impacts on calcification.

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 (Omega ar). Biological and hydrographic processes change the chemistry of the seawater moving across the barrier reefs and into Palau's 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.

Seawater carbonate chemistry and expression of hsp70, hsp90 and hsf1 in the reef coral Acropora digitifera during experiments, 2012

Ocean acidification is an ongoing threat for marine organisms due to the increasing atmospheric CO2 concentration. Seawater acidification has a serious impact on physiologic processes in marine organisms at all life stages. On the other hand, potential tolerance to external pH changes has been reported in coral larvae. Information about the possible mechanisms underlying such tolerance responses, however, is scarce. In the present study, we examined the effects of acidified seawater on the larvae of Acropora digitifera at the molecular level. We targeted two heat shock proteins, Hsp70 and Hsp90, and a heat shock transcription factor, Hsf1, because of their importance in stress responses and in early life developmental stages. Coral larvae were maintained under the ambient and elevated CO2 conditions that are expected to occur within next 100 years, and then we evaluated the expression of hsps and hsf1 by quantitative real-time polymerase chain reaction (PCR). Expression levels of these molecules significantly differed among target genes, but they did not change significantly between CO2conditions. These findings indicate that the expression of hsps is not changed due to external pH changes, and suggest that tolerance to acidified seawater in coral larvae may not be related to hsp expression.

Effects of ocean acidification and global warming on reef bioerosion-lessons from a clionaid sponge

Coral reefs are under threat, exerted by a number of interacting effects inherent to the present climate change, including ocean acidification and global warming. Bioerosion drives reef degradation by recycling carbonate skeletal material and is an important but understudied factor in this context. Twelve different combinations of pCO2 and temperature were applied to elucidate the consequences of ocean acidification and global warming on the physiological response and bioerosion rates of the zooxanthellate sponge Cliona orientalis-one of the most abundant and effective bioeroders on the Great Barrier Reef, Australia. Our results confirm a significant amplification of the sponges' bioerosion capacity with increasing pCO2, which is expressed by more carbonate being chemically dissolved by etching. The health of the sponges and their photosymbionts was not affected by changes in pCO2, in contrast to temperature, which had significant negative impacts at higher levels. However, we could not conclusively explain the relationship between temperature and bioerosion rates, which were slightly reduced at both colder as well as warmer temperatures than ambient. The present findings on the effects of ocean acidification on chemical bioerosion, however, will have significant implications for predicting future reef carbonate budgets, as sponges often contribute the lion's share of internal bioerosion on coral reefs.

Threatened Caribbean coral is able to mitigate the adverse effects of ocean acidification on calcification by increasing feeding rate

Global climate change threatens coral growth and reef ecosystem health via ocean warming and ocean acidification (OA). Whereas the negative impacts of these stressors are increasingly well-documented, studies identifying pathways to resilience are still poorly understood. Heterotrophy has been shown to help corals experiencing decreases in growth due to either thermal or OA stress; however, the mechanism by which it mitigates these decreases remains unclear. This study tested the ability of coral heterotrophy to mitigate reductions in growth due to climate change stress in the critically endangered Caribbean coral Acropora cervicornis via changes in feeding rate and lipid content. Corals were either fed or unfed and exposed to elevated temperature (30°C), enriched pCO2 (800 ppm), or both (30°C/800 ppm) as compared to a control (26°C/390 ppm) for 8 weeks. Feeding rate and lipid content both increased in corals experiencing OA vs. present-day conditions, and were significantly correlated. Fed corals were able to maintain ambient growth rates at both elevated temperature and elevated CO2, while unfed corals experienced significant decreases in growth with respect to fed conspecifics. Our results show for the first time that a threatened coral species can buffer OA-reduced calcification by increasing feeding rates and lipid content.

Radiocarbon age determinations of coral and reef cores from the Philippines off North Bohol and Olango

Nine holes were drilled with a submersible hydraulic drill into the slopes and reef flats of the Caubyan and Calituban reefs as well as of Olango Flat. The maximum depth of core penetration was 11 m. 14C ages showed that the Caubyan and Calituban reefs were formed within the last 6,000 years. Corals settled on a pre-existing relief parallel to the island of Bohol, building a framework for other carbonate-producing organisms. The reef flat south of Olango has a different structure. Formation took place during a Pleistocene high sea level, e.g. 125,000 years ago.

Stable oxygen isotope record from a high-latitude reef off Western Australia

A core from a coral colony of Porites lutea was analysed for stable oxygen isotopic composition*. A 200-year proxy record of sea surface temperatures from the Houtman Abrolhos Islands off west Australia was obtained from coral delta18O. At 29°S, the Houtman Abrolhos are the southernmost major reef complex of the Indian Ocean. They are located on the path of the Leeuwin Current, a southward flow of warm, tropical water, which is coupled to Indonesian throughflow. Coral delta18O primarily reflects local oceanographic and climatic variability, which is largely determined by spatial variability of the Leeuwin Current. However, coherence between coral delta18O and the current strength itself is relatively weak. Evolutionary spectral and singular spectrum analyses of coral delta18O demonstrate a high variability in spectral composition through time. Oscillations in the 5-7-y, 14-15-y, and quasi-biennial bands reflect teleconnections of local sea surface temperature (SST) to tropical Pacific climate variability. Deviations between local (coral-based) and regional (instrument) SST contain a cyclic component with a period of 15 y. Coral delta18O suggests a rise in SST by 0.6°C since AD 1944, consistent with available instrumental SST records. A long-term warming by 1.4°C since AD 1795 is inferred from the coral record.