Seawater carbonate chemistry and coverage and dry weight of Ecklonia radiata during experiments, 2010

Predictions about the ecological consequences of oceanic uptake of CO2 have been preoccupied with the effects of ocean acidification on calcifying organisms, particularly those critical to the formation of habitats (e.g. coral reefs) or their maintenance (e.g. grazing echinoderms). This focus overlooks the direct effects of CO2 on non-calcareous taxa, particularly those that play critical roles in ecosystem shifts. We used two experiments to investigate whether increased CO2 could exacerbate kelp loss by facilitating non-calcareous algae that, we hypothesized, (i) inhibit the recovery of kelp forests on an urbanized coast, and (ii) form more extensive covers and greater biomass under moderate future CO2 and associated temperature increases. Our experimental removal of turfs from a phase-shifted system (i.e. kelp- to turf-dominated) revealed that the number of kelp recruits increased, thereby indicating that turfs can inhibit kelp recruitment. Future CO2 and temperature interacted synergistically to have a positive effect on the abundance of algal turfs, whereby they had twice the biomass and occupied over four times more available space than under current conditions. We suggest that the current preoccupation with the negative effects of ocean acidification on marine calcifiers overlooks potentially profound effects of increasing CO2 and temperature on non-calcifying organisms.

Seawater carbonate chemistry and calcification rates of Porites rus and Hydrolithon onkodes in experiments of Moorea

Central to evaluating the effects of ocean acidification (OA) on coral reefs is understanding how calcification is affected by the dissolution of CO2 in sea water, which causes declines in carbonate ion concentration [CO3]2- and increases in bicarbonate ion concentration [HCO3]-. To address this topic, we manipulated [CO3]2- and [HCO3]- to test the effects on calcification of the coral Porites rus and the alga Hydrolithon onkodes, measured from the start to the end of a 15-day incubation, as well as in the day and night. [CO3]2- played a significant role in light and dark calcification of P. rus, whereas [HCO3]- mainly affected calcification in the light. Both [CO3]2- and [HCO3]- had a significant effect on the calcification of H. onkodes, but the strongest relationship was found with [CO3]2-. Our results show that the negative effect of declining [CO3]2- on the calcification of corals and algae can be partly mitigated by the use of [HCO3]- for calcification and perhaps photosynthesis. These results add empirical support to two conceptual models that can form a template for further research to account for the calcification response of corals and crustose coralline algae to OA.

Fast coral reef calcifiers are more sensitive to ocean acidification in short-term laboratory incubations

To identify the properties of taxa sensitive and resistant to ocean acidification (OA), we tested the hypothesis that coral reef calcifiers differ in their sensitivity to OA as predictable outcomes of functional group alliances determined by conspicuous traits. We contrasted functional groups of eight corals and eight calcifying algae defined by morphology in corals and algae, skeletal structure in corals, spatial location of calcification in algae, and growth rate in corals and algae. The responses of calcification to OA were unrelated to morphology and skeletal structure in corals; they were, however, affected by growth rate in corals and algae (fast calcifiers were more sensitive than slow calcifiers), and by the site of calcification and morphology in algae. Species assemblages characterized by fast growth, and for algae, also cell-wall calcification, are likely to be ecological losers in the future ocean. This shift in relative success will affect the relative and absolute species abundances as well as the goods and services provided by coral reefs.

Seawater carbonate chemistry and biological processes of Porites panamensis during experiments, 2011

Survival of coral planulae, and the successful settlement and healthy growth of primary polyps are critical for the dispersal of scleractinian corals and hence the recovery of degraded coral reefs. It is therefore important to explore how the warmer and more acidic oceanic conditions predicted for the future could affect these processes. This study used controlled culture to investigate the effects of a 1 °C increase in temperature and a 0.2-0.25 unit decrease in pH on the settlement and survival of planulae and the growth of primary polyps in the Tropical Eastern Pacific coral Porites panamensis. We found that primary polyp growth was reduced only marginally by more acidic seawater but the combined effect of high temperature and lowered pH caused a significant reduction in growth of primary polyps by almost a third. Elevated temperature was found to significantly reduce the amount of zooxanthellae in primary polyps, and when combined with lowered pH resulted in a significant reduction in biomass of primary polyps. However, survival and settlement of planula larvae were unaffected by increased temperature, lowered acidity or the combination of both. These results indicate that in future scenarios of increased temperature and oceanic acidity coral planulae will be able to disperse and settle successfully but primary polyp growth may be hampered. The recovery of reefs may therefore be impeded by global change even if local stressors are curbed and sufficient sources of planulae are available.

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.

Secondary calcification and dissolution respond differently to future ocean conditions

Climate change threatens both the accretion and erosion processes that sustain coral reefs. Secondary calcification, bioerosion, and reef dissolution are integral to the structural complexity and long-term persistence of coral reefs, yet these processes have received less research attention than reef accretion by corals. In this study, we use climate scenarios from RCP 8.5 to examine the combined effects of rising ocean acidity and sea surface temperature (SST) on both secondary calcification and dissolution rates of a natural coral rubble community using a flow-through aquarium system. We found that secondary reef calcification and dissolution responded differently to the combined effect of pCO2 and temperature. Calcification had a non-linear response to the combined effect of pCO2 and temperature: the highest calcification rate occurred slightly above ambient conditions and the lowest calcification rate was in the highest temperature-pCO2 condition. In contrast, dissolution increased linearly with temperature-pCO2 . The rubble community switched from net calcification to net dissolution at +271 µatm pCO2 and 0.75 °C above ambient conditions, suggesting that rubble reefs may shift from net calcification to net dissolution before the end of the century. Our results indicate that (i) dissolution may be more sensitive to climate change than calcification and (ii) that calcification and dissolution have different functional responses to climate stressors; this highlights the need to study the effects of climate stressors on both calcification and dissolution to predict future changes in coral reefs.

Molecular fingerprinting and amplicon sequencing of the bacterial biofilm on settlement tiles deployed along natural pH gradients in Papua New Guinea

Bacterial biofilms provide cues for the settlement of marine invertebrates such as coral larvae, and are therefore important for the resilience and recovery of coral reefs. This study aimed to better understand how ocean acidification may affect the community composition and diversity of bacterial biofilms on surfaces under naturally reduced pH conditions. Settlement tiles were deployed at coral reefs in Papua New Guinea along pH gradients created by two CO2 seeps, and upper and lower tiles surfaces were sampled 5 and 13 months after deployment. Automated Ribosomal Intergenic Spacer Analysis were used to characterize more than 200 separate bacterial communities, complemented by amplicon sequencing of the bacterial 16S rRNA gene of 16 samples. The bacterial biofilm consisted predominantly of Alpha-, Gamma- and Deltaproteobacteria, as well as Cyanobacteria, Flavobacteriia and Cytophaga, whereas putative settlement-inducing taxa only accounted for a small fraction of the community. Bacterial biofilm composition was heterogeneous with approximately 25% shared operational taxonomic units between samples. Among the observed environmental parameters, pH only had a weak effect on community composition (R² ~ 1%) and did not affect community richness and evenness. In contrast, there were strong differences between upper and lower surfaces (contrasting in light exposure and grazing intensity). There also appeared to be a strong interaction between bacterial biofilm composition and the macroscopic components of the tile community. Our results suggest that on mature settlement surfaces in situ, pH does not have a strong impact on the composition of bacterial biofilms. Other abiotic and biotic factors such as light exposure and interactions with other organisms may be more important in shaping bacterial biofilms than changes in seawater pH.

Effects of light and elevated pCO2 on the growth and photochemical efficiency of Acropora cervicornis

The effects of light and elevated pCO2 on the growth and photochemical efficiency of the critically endangered staghorn coral, Acropora cervicornis, were examined experimentally. Corals were subjected to high and low treatments of CO2 and light in a fully crossed design and monitored using 3D scanning and buoyant weight methodologies. Calcification rates, linear extension, as well as colony surface area and volume of A. cervicornis were highly dependent on light intensity. At pCO2 levels projected to occur by the end of the century from ocean acidification (OA), A. cervicornis exhibited depressed calcification, but no change in linear extension. Photochemical efficiency (F v /F m ) was higher at low light, but unaffected by CO2. Amelioration of OA-depressed calcification under high-light treatments was not observed, and we suggest that the high-light intensity necessary to reach saturation of photosynthesis and calcification in A. cervicornis may limit the effectiveness of this potentially protective mechanism in this species. High CO2 causes depressed skeletal density, but not linear extension, illustrating that the measurement of extension by itself is inadequate to detect CO2 impacts. The skeletal integrity of A. cervicornis will be impaired by OA, which may further reduce the resilience of the already diminished populations of this endangered species.

Ocean acidification has no effect on thermal bleaching in the coral Seriatopora caliendrum

The objective of this study was to test whether elevated pCO2 predicted for the year 2100 (85.1 Pa) affects bleaching in the coral Seriatopora caliendrum (Ehrenberg 1834) either independently or interactively with high temperature (30.5 °C). Response variables detected the sequence of events associated with the onset of bleaching: reduction in the photosynthetic performance of symbionts as measured by maximum photochemical efficiency (F v/F m) and effective photochemical efficiency (delta F/F m') of PSII, declines in net photosynthesis (P net) and photosynthetic efficiency (alpha), and finally, reduced chlorophyll a and symbiont concentrations. S. caliendrum was collected from Nanwan Bay, Taiwan, and subjected to combinations of temperature (27.7 vs. 30.5 °C) and pCO2 (45.1 vs. 85.1 Pa) for 14 days. High temperature reduced values of all dependent variables (i.e., bleaching occurred), but high pCO2 did not affect Symbiodinium photophysiology or productivity, and did not cause bleaching. These results suggest that short-term exposure to 81.5 Pa pCO2, alone and in combination with elevated temperature, does not cause or affect coral bleaching.

Seawater carbonate chemistry and encrusting algal communities duirng a mesocosm experiment, 2007

Owing to anthropogenic emissions, atmospheric concentrations of carbon dioxide could almost double between 2006 and 2100 according to business-as-usual carbon dioxide emission scenarios. Because the ocean absorbs carbon dioxide from the atmosphere, increasing atmospheric carbon dioxide concentrations will lead to increasing dissolved inorganic carbon and carbon dioxide in surface ocean waters, and hence acidification and lower carbonate saturation states. As a consequence, it has been suggested that marine calcifying organisms, for example corals, coralline algae, molluscs and foraminifera, will have difficulties producing their skeletons and shells at current rates, with potentially severe implications for marine ecosystems, including coral reefs. Here we report a seven-week experiment exploring the effects of ocean acidification on crustose coralline algae, a cosmopolitan group of calcifying algae that is ecologically important in most shallowwater habitats. Six outdoor mesocosms were continuously supplied with sea water from the adjacent reef and manipulated to simulate conditions of either ambient or elevated seawater carbon dioxide concentrations. The recruitment rate and growth of crustose coralline algae were severely inhibited in the elevated carbon dioxide mesocosms. Our findings suggest that ocean acidification due to human activities could cause significant change to benthic community structure in shallow-warm-water carbonate ecosystems.

Differential response of two Mediterranean cold-water coral species to ocean acidification

Cold-water coral (CWC) reefs constitute one of the most complex deep-sea habitats harboring a vast diversity of associated species. Like other tropical or temperate framework builders, these systems are facing an uncertain future due to several threats, such as global warming and ocean acidification. In the case of Mediterranean CWC communities, the effect may be exacerbated due to the greater capacity of these waters to absorb atmospheric CO2 compared to the global ocean. Calcification in these organisms is an energy-demanding process, and it is expected that energy requirements will be greater as seawater pH and the availability of carbonate ions decrease. Therefore, studies assessing the effect of a pH decrease in skeletal growth, and metabolic balance are critical to fully understand the potential responses of these organisms under a changing scenario. In this context, the present work aims to investigate the medium- to long-term effect of a low pH scenario on calcification and the biochemical composition of two CWCs from the Mediterranean, Dendrophyllia cornigera and Desmophyllum dianthus. After 314 d of exposure to acidified conditions, a significant decrease of 70 % was observed in Desmophyllum dianthus skeletal growth rate, while Dendrophyllia cornigera showed no differences between treatments. Instead, only subtle differences between treatments were observed in the organic matter amount, lipid content, skeletal microdensity, or porosity in both species, although due to the high variability of the results, these differences were not statistically significant. Our results also confirmed a heterogeneous effect of low pH on the skeletal growth rate of the organisms depending on their initial weight, suggesting that those specimens with high calcification rates may be the most susceptible to the negative effects of acidification.

Responses of the metabolism of the larvae of Pocillopora damicornis to ocean acidification and warming

Ocean acidification and warming are expected to threaten the persistence of tropical coral reef ecosystems. As coral reefs face multiple stressors, the distribution and abundance of corals will depend on the successful dispersal and settlement of coral larvae under changing environmental conditions. To explore this scenario, we used metabolic rate, at holobiont and molecular levels, as an index for assessing the physiological plasticity of Pocillopora damicornis larvae from this site to conditions of ocean acidity and warming. Larvae were incubated for 6 hours in seawater containing combinations of CO2 concentration (450 and 950 µatm) and temperature (28 and 30°C). Rates of larval oxygen consumption were higher at elevated temperatures. In contrast, high CO2 levels elicited depressed metabolic rates, especially for larvae released later in the spawning period. Rates of citrate synthase, a rate-limiting enzyme in aerobic metabolism, suggested a biochemical limit for increasing oxidative capacity in coral larvae in a warming, acidifying ocean. Biological responses were also compared between larvae released from adult colonies on the same day (cohorts). The metabolic physiology of Pocillopora damicornis larvae varied significantly by day of release. Additionally, we used environmental data collected on a reef in Moorea, French Polynesia to provide information about what adult corals and larvae may currently experience in the field. An autonomous pH sensor provided a continuous time series of pH on the natal fringing reef. In February/March, 2011, pH values averaged 8.075±0.023. Our results suggest that without adaptation or acclimatization, only a portion of naïve Pocillopora damicornis larvae may have suitable metabolic phenotypes for maintaining function and fitness in an end-of-the century ocean.

Seawater carbonate chemistry and protein and chlorophyll a content per nubbin of Acropora muricata during observations of 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.