Seawater carbonate chemistry in Hog reef, Bermuda reef community, 2010

Despite the potential impact of ocean acidification on ecosystems such as coral reefs, surprisingly, there is very limited field data on the relationships between calcification and seawater carbonate chemistry. In this study, contemporaneous in situ datasets of seawater carbonate chemistry and calcification rates from the high-latitude coral reef of Bermuda over annual timescales provide a framework for investigating the present and future potential impact of rising carbon dioxide (CO2) levels and ocean acidification on coral reef ecosystems in their natural environment. A strong correlation was found between the in situ rates of calcification for the major framework building coral species Diploria labyrinthiformis and the seasonal variability of [CO32-] and aragonite saturation state omega aragonite, rather than other environmental factors such as light and temperature. These field observations provide sufficient data to hypothesize that there is a seasonal "Carbonate Chemistry Coral Reef Ecosystem Feedback" (CREF hypothesis) between the primary components of the reef ecosystem (i.e., scleractinian hard corals and macroalgae) and seawater carbonate chemistry. In early summer, strong net autotrophy from benthic components of the reef system enhance [CO32-] and omega aragonite conditions, and rates of coral calcification due to the photosynthetic uptake of CO2. In late summer, rates of coral calcification are suppressed by release of CO2 from reef metabolism during a period of strong net heterotrophy. It is likely that this seasonal CREF mechanism is present in other tropical reefs although attenuated compared to high-latitude reefs such as Bermuda. Due to lower annual mean surface seawater [CO32-] and omega aragonite in Bermuda compared to tropical regions, we anticipate that Bermuda corals will experience seasonal periods of zero net calcification within the next decade at [CO32-] and omega aragonite thresholds of ~184 micro moles kg-1 and 2.65. However, net autotrophy of the reef during winter and spring (as part of the CREF hypothesis) may delay the onset of zero NEC or decalcification going forward by enhancing [CO32-] and omega aragonite. The Bermuda coral reef is one of the first responders to the negative impacts of ocean acidification, and we estimate that calcification rates for D. labyrinthiformis have declined by >50% compared to pre-industrial times.

Seawater carbonate chemistry and Emiliania huxleyi biological processes during experiments, 2010

The calcifying phytoplankton species, coccolithophores, have their calcified coccoliths around the cells, however, their physiological roles are still unknown. Here, we hypothesized that the coccoliths may play a certain role in reducing solar UV radiation (UVR, 280-400 nm) and protect the cells from being harmed. Cells of Emiliania huxleyi with different thicknesses of the coccoliths were obtained by culturing them at different levels of dissolved inorganic carbon and their photophysiological responses to UVR were investigated. Although increased dissolved inorganic carbon decreased the specific growth rate, the increased coccolith thickness significantly ameliorated the photoinhibition of PSII photochemical efficiency caused by UVR. Increase by 91% in the coccolith thickness led to 35% increase of the PSII yield and 22% decrease of the photoinhibition of the effective quantum yield by UVR. The coccolith cover reduced more UVA (320-400 nm) than UVB (280-315 nm), leading to less inhibition per energy at the UV-A band.

Seawater carbonate chemistry, photosynthesis and calcification rate during experiments with coral Acropora muricata, 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-1). 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-1 (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.

Seawater carbonate chemistry and growth rate during experiments with phylotypes of Symbiodinium (Dinophyceae), 2011

We investigated the effect of elevated partial pressure of CO2 (pCO2) on the photosynthesis and growth of four phylotypes (ITS2 types A1, A13, A2, and B1) from the genus Symbiodinium, a diverse dinoflagellate group that is important, both free-living and in symbiosis, for the viability of cnidarians and is thus a potentially important model dinoflagellate group. The response of Symbiodinium to an elevated pCO2 was phylotype-specific. Phylotypes A1 and B1 were largely unaffected by a doubling in pCO2 in contrast, the growth rate of A13 and the photosynthetic capacity of A2 both increased by ~ 60%. In no case was there an effect of ocean acidification (OA) upon respiration (dark- or light-dependent) for any of the phylotypes examined. Our observations suggest that OA might preferentially select among free-living populations of Symbiodinium, with implications for future symbioses that rely on algal acquisition from the environment (i.e., horizontal transmission). Furthermore, the carbon environment within the host could differentially affect the physiology of different Symbiodinium phylotypes. The range of responses we observed also highlights that the choice of species is an important consideration in OA research and that further investigation across phylogenetic diversity, for both the direction of effect and the underlying mechanism(s) involved, is warranted.

Seawater carbonate chemistry and survival of mollusks: Mercenaria mercenaria, Argopecten irradians, and Crassostrea virginica during experiments, 2009

We present experiments that examined the metamorphosis, growth, and survivorship of larvae from three species of commercially and ecologically valuable shellfish (Mercenaria mercenaria, Argopecten irradians, and Crassostrea virginica) at the levels of CO2 projected to occur during the 21st century and beyond. Under CO2 concentrations estimated to occur later this century (~66 Pa, 650 ppm), M. mercenaria and A. irradians larvae exhibited dramatic declines (>50%) in survivorship as well as significantly delayed metamorphosis and significantly smaller sizes. Although C. virginica larvae also experienced lowered growth and delayed metamorphosis at ~66 Pa CO2, their survival was only diminished at ~152 Pa CO2. The extreme sensitivity of larval stages of shellfish to enhanced levels of CO2 indicates that current and future increases in pelagic CO2 concentrations may deplete or alter the composition of shellfish populations in coastal ecosystems.

Seawater carbonate chemistry, nutrients, and Calcidiscus leptoporus (strain RCC1135) growth rate and calcification rate during experiments, 2012

The coccolithophore Calcidiscus leptoporus was grown in batch culture under nitrogen (N) as well as phosphorus (P) limitation. Growth rate, particulate inorganic carbon (PIC), particulate organic carbon (POC), particulate organic nitrogen (PON), and particulate organic phosphorus (POP) production were determined and coccolith morphology was analysed. While PON production decreased by 70% under N-limitation and POP production decreased by 65% under P-limitation, growth rate decreased by 33% under N- as well as P-limitation. POC as well as PIC production (calcification rate) increased by 27% relative to the control under P-limitation, and did not change under N-limitation. Coccolith morphology did not change in response to either P or N limitation. While these findings, supported by a literature survey, suggest that coccolith morphogenesis is not hampered by either P or N limitation, calcification rate might be. The latter conclusion is in apparent contradiction to our data. We discuss the reasons for this inference.

Seawater carbonate chemistry and effects of CO2 and their modulation by light in the life-cycle stages of the coccolithophore Emiliania huxleyi strains RCC 1216 and 1217 during experiments, 2012

The effects of ocean acidification on the life-cycle stages of the coccolithophore Emiliania huxleyi and their by light were examined. Calcifying diploid and noncalcifying haploid cells (Roscoff culture collection 1216 and 1217) were acclimated to present-day and elevated CO2 partial pressures (PCO2; 38.5 vs. 101.3 Pa, ., 380 vs. 1000 matm) under low and high light (50 vs. 300 mmol photons m-2 s-1). Growth rates as well as quotas and production rates of C and N were measured. Sources of inorganic C for biomass buildup were using a 14C disequilibrium assay. Photosynthetic O2 evolution was measured as a function of dissolved inorganic C and light by means of membrane-inlet mass spectrometry. The diploid stage responded to elevated PCO2 by shunting resources from the production of particulate inorganic C toward organic C yet keeping the production of total particulate C constant. As the effect of ocean acidification was stronger under low light, the diploid stage might be less affected by increased acidity when energy availability is high. The haploid stage maintained elemental composition and production rates under elevated PCO2. Although both life-cycle stages involve different ways of dealing with elevated PCO2, the responses were generally modulated by energy availability, being typically most pronounced under low light. Additionally, PCO2 responses resembled those induced by high irradiances, indicating that ocean acidification affects the interplay between energy-generating processes (photosynthetic light reactions) and processes competing for energy (biomass buildup and calcification). A conceptual model is put forward explaining why the magnitude of single responses is determined by energy availability.

Seawater carbonate chemistry and biological processes of Emiliania huxleyi (strains RCC1256 and NZEH) during experiments, 2011

With respect to their sensitivity to ocean acidification, calcifiers such as the coccolithophore Emiliania huxleyi have received special attention, as the process of calcification seems to be particularly sensitive to changes in the marine carbonate system. For E. huxleyi, apparently conflicting results regarding its sensitivity to ocean acidification have been published (Iglesias-Rodriguez et al., 2008a; Riebesell et al., 2000). As possible causes for discrepancies, intra-specific variability and different effects of CO2 manipulation methods, i.e. the manipulation of total alkalinity (TA) or total dissolved inorganic carbon (DIC), have been discussed. While Langer et al. (2009) demonstrate a high degree of intra-specific variability between strains of E. huxleyi, the question whether different CO2 manipulation methods influence the cellular responses has not been resolved yet. In this study, closed TA as well as open and closed DIC manipulation methods were compared with respect to E. huxleyi's CO2-dependence in growth rate, POC- and PIC-production. The differences in the carbonate chemistry between TA and DIC manipulations were shown not to cause any differences in response patterns, while the latter differed between open and closed DIC manipulation. The two strains investigated showed different sensitivities to acidification of seawater, RCC1256 being more negatively affected in growth rates and PIC production than NZEH.

Seawater carbonate chemistry and reproduction of a coral reef fish in a laboratory experiment

Ocean acidification is predicted to negatively impact the reproduction of many marine species, either by reducing fertilization success or diverting energy from reproductive effort. While recent studies have demonstrated how ocean acidification will affect larval and juvenile fishes, little is known about how increasing partial pressure of carbon dioxide (pCO2) and decreasing pH might affect reproduction in adult fishes. We investigated the effects of near-future levels of pCO2 on the reproductive performance of the cinnamon anemonefish, Amphiprion melanopus, from the Great Barrier Reef, Australia. Breeding pairs were held under three CO2 treatments [Current-day Control (430 µatm), Moderate (584 µatm) and High (1032 µatm)] for a 9-month period that included the summer breeding season. Unexpectedly, increased CO2 dramatically stimulated breeding activity in this species of fish. Over twice as many pairs bred in the Moderate (67% of pairs) and High (55%) compared to the Control (27%) CO2 treatment. Pairs in the High CO2 group produced double the number of clutches per pair and 67% more eggs per clutch compared to the Moderate and Control groups. As a result, reproductive output in the High group was 82% higher than that in the Control group and 50% higher than that in the Moderate group. Despite the increase in reproductive activity, there was no difference in adult body condition among the three treatment groups. There was no significant difference in hatchling length between the treatment groups, but larvae from the High CO2 group had smaller yolks than Controls. This study provides the first evidence of the potential effects of ocean acidification on key reproductive attributes of marine fishes and, contrary to expectations, demonstrates an initially stimulatory (hormetic) effect in response to increased pCO2. However, any long-term consequences of increased reproductive effort on individuals or populations remain to be determined.

Seawater carbonate chemistry and calcification at Anse des Cuivres, NW Mediterranean, 2004

The community metabolism of a shallow infralittoral ecosystem dominated by the calcareous macroalgae Corallina elongata was investigated in Marseilles (NW Mediterranean), by monitoring hourly changes of seawater pH and total alkalinity over 6 d in February 2000. Fair weather conditions prevailed over the study period as indicated by oceanographic (temperature, salinity, and current velocity and direction) and meteorological variables, which validated the standing water hypothesis. This temperate ecosystem exhibited high community gross primary production (GPP = 519 ± 106 mmol C m-2 d-1, n = 6) and also supported high rates of community respiration (R). As a result, the system was slightly autotrophic (net community production, NCP = 20 mmol C m-2 d-1), with a GPP/R ratio of 1.06. NCP exhibited circadian variations with 2- to 3-fold changes in community respiration, both in the light and in the dark. Rates of net community calcification also exhibited circadian variations, with positive rates (up to 24 mmol CaCO3 m-2 h-1) for irradiance values >300 W m-2 (about 1380 µmol photon m-2 s-1). Below this irradiance threshold, net community dissolution prevailed. Daily net calcification (G) was on average 8 mmol CaCO3 m-2 d-1. CO2 fluxes generated by primary production, respiration, and calcification suggest that the study site was a potential atmospheric CO2 sink of 15 mmol CO2 m-2 d-1 at the time of measurement.

Seawater carbonate chemistry and growth rate during experiments with coral Acropora cervicornis, 2005

The growth rate of Acropora cervicornis branch tips maintained in the laboratory was measured before, during, and after exposure to elevated nitrate (5 and 10 µM NO3-), phosphate (2 and 4 µM P-PO43) and/or pCO2 (CO2 ~700 to 800 µatm). The effect of increased pCO2 was greater than that of nutrient enrichment alone. High concentrations of nitrate or phosphate resulted in significant decreases in growth rate, in both the presence and absence of increased pCO2. The effect of nitrate and phosphate enrichment combined was additive or antagonistic relative to nutrient concentration and pCO2 level. Growth rate recovery was greater after exposure to increased nutrients or CO2 compared to increased nutrients and CO2. If these results accurately predict coral response in the natural environment, it is reasonable to speculate that the survival and reef-building potential of this species will be significantly negatively impacted by continued coastal nutrification and projected pCO2 increases.

Surface seawater carbonate chemistry, nutrients and phytoplankton community composition on a transect between North Sea and Arctic Ocean, 2008

This data was collected during the 'ICE CHASER' cruise from the southern North Sea to the Arctic (Svalbard) in July-Aug 2008. This data consists of coccolithophore abundance, calcification and primary production rates, carbonate chemistry parameters and ancillary data of macronutrients, chlorophyll-a, average mixed layer irradiance, daily irradiance above the sea surface, euphotic and mixed layer depth, temperature and salinity.

Seawater carbonate chemistry, physiology and skeletal mineralogy of coralline alga Corallina elongata in a laboratory experiment

Marine organisms inhabiting environments where pCO2/pH varies naturally are suggested to be relatively resilient to future ocean acidification. To test this hypothesis, the effect of elevated pCO2 was investigated in the articulated coralline red alga Corallina elongata from an intertidal rock pool on the north coast of Brittany (France), where pCO2 naturally varied daily between 70 and 1000 µatm. Metabolism was measured on algae in the laboratory after they had been grown for 3 weeks at pCO2 concentrations of 380, 550, 750 and 1000 µatm. Net and gross primary production, respiration and calcification rates were assessed by measurements of oxygen and total alkalinity fluxes using incubation chambers in the light and dark. Calcite mol % Mg/Ca (mMg/Ca) was analysed in the tips, branches and basal parts of the fronds, as well as in new skeletal structures produced by the algae in the different pCO2 treatments. Respiration, gross primary production and calcification in light and dark were not significantly affected by increased pCO2. Algae grown under elevated pCO2 (550, 750 and 1000 µatm) formed fewer new structures and produced calcite with a lower mMg/Ca ratio relative to those grown under 380 µatm. This study supports the assumption that C. elongata from a tidal pool, where pCO2 fluctuates over diel and seasonal cycles, is relatively robust to elevated pCO2 compared to other recently investigated coralline algae.

Seawater carbonate chemistry and microbioerosion of coral skeletons

Biological mediation of carbonate dissolution represents a fundamental component of the destructive forces acting on coral reef ecosystems. Whereas ocean acidification can increase dissolution of carbonate substrates, the combined impact of ocean acidification and warming on the microbioerosion of coral skeletons remains unknown. Here, we exposed skeletons of the reef-building corals, Porites cylindrica and Isopora cuneata, to present-day (Control: 400 µatm - 24 °C) and future pCO2-temperature scenarios projected for the end of the century (Medium: +230 µatm - +2 °C; High: +610 µatm - +4 °C). Skeletons were also subjected to permanent darkness with initial sodium hypochlorite incubation, and natural light without sodium hypochlorite incubation to isolate the environmental effect of acidic seawater (i.e., Omega aragonite <1) from the biological effect of photosynthetic microborers. Our results indicated that skeletal dissolution is predominantly driven by photosynthetic microborers, as samples held in the dark did not decalcify. In contrast, dissolution of skeletons exposed to light increased under elevated pCO2-temperature scenarios, with P. cylindrica experiencing higher dissolution rates per month (89%) than I. cuneata (46%) in the high treatment relative to control. The effects of future pCO2-temperature scenarios on the structure of endolithic communities were only identified in P. cylindrica and were mostly associated with a higher abundance of the green algae Ostreobium spp. Enhanced skeletal dissolution was also associated with increased endolithic biomass and respiration under elevated pCO2-temperature scenarios. Our results suggest that future projections of ocean acidification and warming will lead to increased rates of microbioerosion. However, the magnitude of bioerosion responses may depend on the structural properties of coral skeletons, with a range of implications for reef carbonate losses under warmer and more acidic oceans.