Seawater carbonate chemistry, gross photosynthesis and metabolically induced rate of pH change during experiments with macroalgae, 2012

Ocean acidification (OA) is a reduction in oceanic pH due to increased absorption of anthropogenically produced CO2. This change alters the seawater concentrations of inorganic carbon species that are utilized by macroalgae for photosynthesis and calcification: CO2 and HCO3 increase; CO32 decreases. Two common methods of experimentally reducing seawater pH differentially alter other aspects of carbonate chemistry: the addition of CO2 gas mimics changes predicted due to OA, while the addition of HCl results in a comparatively lower [HCO3]. We measured the short-term photosynthetic responses of five macroalgal species with various carbon-use strategies in one of three seawater pH treatments: pH 7.5 lowered by bubbling CO2 gas, pH 7.5 lowered by HCl, and ambient pH 7.9. There was no difference in photosynthetic rates between the CO2, HCl, or pH 7.9 treatments for any of the species examined. However, the ability of macroalgae to raise the pH of the surrounding seawater through carbon uptake was greatest in the pH 7.5 treatments. Modeling of pH change due to carbon assimilation indicated that macroalgal species that could utilize HCO3 increased their use of CO2 in the pH 7.5 treatments compared to pH 7.9 treatments. Species only capable of using CO2 did so exclusively in all treatments. Although CO2 is not likely to be limiting for photosynthesis for the macroalgal species examined, the diffusive uptake of CO2 is less energetically expensive than active HCO3 uptake, and so HCO3-using macroalgae may benefit in future seawater with elevated CO2.

Seawater carbonate chemistry and buffer capacity of the coelomic fluid in echinoderms in a laboratory experiment

The increase in atmospheric CO2 due to anthropogenic activity results in an acidification of the surface waters of the oceans. The impact of these chemical changes depends on the considered organisms. In particular, it depends on the ability of the organism to control the pH of its inner fluids. Among echinoderms, this ability seems to differ significantly according to species or taxa. In the present paper, we investigated the buffer capacity of the coelomic fluid in different echinoderm taxa as well as factors modifying this capacity. Euechinoidea (sea urchins except Cidaroidea) present a very high buffer capacity of the coelomic fluid (from 0.8 to 1.8 mmol/kg SW above that of seawater), while Cidaroidea (other sea urchins), starfish and holothurians have a significantly lower one (from -0.1 to 0.4 mmol/kg SW compared to seawater). We hypothesize that this is linked to the more efficient gas exchange structures present in the three last taxa, whereas Euechinoidea evolved specific buffer systems to compensate lower gas exchange abilities. The constituents of the buffer capacity and the factors influencing it were investigated in the sea urchin Paracentrotus lividus and the starfish Asterias rubens. Buffer capacity is primarily due to the bicarbonate buffer system of seawater (representing about 63% for sea urchins and 92% for starfish). It is also partly due to coelomocytes present in the coelomic fluid (around 8% for both) and, in P. lividus only, a compound of an apparent size larger than 3 kDa is involved (about 15%). Feeding increased the buffer capacity in P. lividus (to a difference with seawater of about 2.3 mmol/kg SW compared to unfed ones who showed a difference of about 0.5 mmol/kg SW) but not in A. rubens (difference with seawater of about 0.2 for both conditions). In P. lividus, decreased seawater pH induced an increase of the buffer capacity of individuals maintained at pH 7.7 to about twice that of the control individuals and, for those at pH 7.4, about three times. This allowed a partial compensation of the coelomic fluid pH for individuals maintained at pH 7.7 but not for those at pH 7.4.

Seawater carbonate chemistry in Hog reef and calcification rate in the 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 maximum growth rates of Skeletonema marinoi and Alexandrium ostenfeldii, toxin composition of Alexandrium ostenfeldii in a laboratory experiment

Phytoplankton populations can display high levels of genetic diversity that, when reflected by phenotypic variability, may stabilize a species response to environmental changes. We studied the effects of increased temperature and CO2 availability as predicted consequences of global change, on 16 genetically different isolates of the diatom Skeletonema marinoi from the Adriatic Sea and the Skagerrak (North Sea), and on eight strains of the PST (paralytic shellfish toxin)-producing dinoflagellate Alexandrium ostenfeldii from the Baltic Sea. Maximum growth rates were estimated in batch cultures of acclimated isolates grown for five to 10 generations in a factorial design at 20 and 24 °C, and present day and next century applied atmospheric pCO2, respectively. In both species, individual strains were affected in different ways by increased temperature and pCO2. The strongest response variability, buffering overall effects, was detected among Adriatic S. marinoi strains. Skagerrak strains showed a more uniform response, particularly to increased temperature, with an overall positive effect on growth. Increased temperature also caused a general growth stimulation in A. ostenfeldii, despite notable variability in strain-specific response patterns. Our data revealed a significant relationship between strain-specific growth rates and the impact of pCO2 on growth-slow growing cultures were generally positively affected, while fast growing cultures showed no or negative responses to increased pCO2. Toxin composition of A. ostenfeldii was consistently altered by elevated temperature and increased CO2 supply in the tested strains, resulting in overall promotion of saxitoxin production by both treatments. Our findings suggest that phenotypic variability within populations plays an important role in the adaptation of phytoplankton to changing environments, potentially attenuating short-term effects and forming the basis for selection. In particular, A. ostenfeldii blooms may expand and increase in toxicity under increased water temperature and atmospheric pCO2 conditions, with potentially severe consequences for the coastal ecosystem.

Growth, respiration and photophysiology of coral massive Porites spp. in the experiment of Moorea

I tested the hypothesis that high pCO2 (76.6 Pa and 87.2 Pa vs. 42.9 Pa) has no effect on the metabolism of juvenile massive Porites spp. after 11 days at 28 °C and 545 µmol quanta/m**2/s. The response was assessed as aerobic dark respiration, skeletal weight (i.e., calcification), biomass, and chlorophyll fluorescence. Corals were collected from the shallow (3-4 m) back reef of Moorea, French Polynesia (17°28.614'S, 149°48.917'W), and experiments conducted during April and May 2011. An increase in pCO2 to 76.6 Pa had no effect on any dependent variable, but 87.2 Pa pCO2 reduced area-normalized (but not biomass-normalized) respiration 36 %, as well as maximum photochemical efficiency (Fv/Fm) of open RCIIs and effective photochemical efficiency of RCIIs in actinic light (Delta F/F'm ); neither biomass, calcification, nor the energy expenditure coincident with calcification (J/g) was effected. These results do not support the hypothesis that high pCO2 reduces coral calcification through increased metabolic costs and, instead, suggest that high pCO2 causes metabolic depression and photochemical impairment similar to that associated with bleaching. Evidence of a pCO2 threshold between 76.6 and 87.2 Pa for inhibitory effects on respiration and photochemistry deserves further attention as it might signal the presence of unpredictable effects of rising pCO2.

Experiment: Influence of CO2 and nitrogen limitation on the coccolith volume of Emiliania huxleyi (Haptophyta)

Coccolithophores, a key phytoplankton group, are one of the most studied organisms regarding their physiological response to ocean acidification/carbonation. The biogenic production of calcareous coccoliths has made coccolithophores a promising group for paleoceanographic research aiming to reconstruct past environmental conditions. Recently, geochemical and morphological analyses of fossil coccoliths have gained increased interest in regard to changes in seawater carbonate chemistry. The cosmopolitan coccolithophore Emiliania huxleyi (Lohm.) Hay and Mohler was cultured over a range of pCO2 levels in controlled laboratory experiments under nutrient replete and nitrogen limited conditions. Measurements of photosynthesis and calcification revealed, as previously published, an increase in particulate organic carbon production and a moderate decrease in calcification from ambient to elevated pCO2. The enhancement in particulate organic carbon production was accompanied by an increase in cell diameter. Changes in coccolith volume were best correlated with the coccosphere/cell diameter and no significant correlation was found between the coccolith volume and the particulate inorganic carbon production. The conducted experiments revealed that the coccolith volume of E. huxleyi is variable with aquatic CO2 concentration but its sensitivity is rather small in comparison with its sensitivity to nitrogen limitation. Comparing coccolith morphological and geometrical parameters like volume, mass and size to physiological parameters under controlled laboratory conditions is an important step to understand variations in fossil coccolith geometry.

Seawater carbonate chemistry and calcification during tropical reef studies at Moorea (French Polynesia), 1997

Community metabolism and air-sea carbon dioxide (CO2) fluxes were investigated in July 1992 on a fringing reef at Moorea (French Polynesia). The benthic community was dominated by macroalgae (85% substratum cover) and comprised of Phaeophyceae Padina tenuis (Bory), Turbinaria ornata (Turner) J. Agardh, and Hydroclathrus clathratus Bory (Howe); Chlorophyta Halimeda incrassata f. ovata J. Agardh (Howe); and Ventricaria ventricosa J. Agardh (Olsen et West), as well as several Rhodophyta (Actinotrichia fragilis Forskál (Børgesen) and several species of encrusting coralline algae). Algal biomass was 171 g dry weight/m**2. Community gross production (Pg), respiration (R), and net calcification (G) were measured in an open-top enclosure. Pg and R were respectively 248 and 240 mmol Co2/m**2/d, and there was a slight net dissolution of CaCO3 (0.8 mmol/m**2/d). This site was a sink for atmospheric CO2 (10 ± 4 mmol CO2/m**2/d), and the analysis of data from the literature suggests that this is a general feature of algal-dominated reefs. Measurement of air-sea CO2 fluxes in open water close to the enclosure demonstrated that changes in small-scale hydrodynamics can lead to misleading conclusions. Net CO2 evasion to the atmosphere was measured on the fringing reef due to changes in the current pattern that drove water from the barrier reef (a C02 source) to the study site.

Seawater carbonate chemistry and community calcification during Shiraha coral reef (Ishigaki Island, Japan) studies, 1995

Only about half of all the CO_2 that has been produced by the burning of fossil fuels now remains in the atmosphere. The CO_2 "missing" from the atmosphere is the subject of an important debate. It was thought that the great majority of the missing CO_2 has invaded the ocean, for this system naturally acts as a giant chemical regulator of the atmosphere. Although it is clear that ocean processes have a major role in the regulation of the carbon dioxide content of the atmosphere through air-sea exchange processes, recent studies of the oceanic carbon cycle and air-sea interaction indicate that oceanic carbon is in a quasi-steady state via the system of biological and physical processes in the ocean interior. It is difficult to determine whether the ocean has the capacity to take up the increasing air-born CO_2 released by human activities over the past five or six decades. To understand this enigma, we need a better understanding of the natural variability of the oceanic carbon cycle.

Seawater carbonate chemistry and calcification during a study of barrier reef flat in Moorea, French Polynesia, 1998

The relative contribution of soft bottoms to the community metabolism (primary production, respiration and net calcification) of a barrier reef flat has been investigated at Moorea (French Polynesia). Community metabolism of the sedimentary area was estimated using in situ incubations in perspex chambers, and compared with estimates of community metabolism of the whole reef flat obtained using a Lagrangian technique (Gattuso et al., 1996. Carbon flux in coral reefs. 1. Lagrangian measurement of community metabolism and resulting air-sea CO2 disequilibrium. Mar. Ecol. Prog. Ser. 145, 109-121). Net organic carbon production (E), respiration (R) and net calcification (G) of sediments were measured by seven incubations performed in triplicate at different irradiance. Respiration and environmental parameters were also measured at four randomly selected additional stations. A model of Photosynthesis-irradiance allowed to calculate oxygen (O2), organic carbon (CO2) and calcium carbonate (CaCO3) evolution from surface irradiance during a diel cycle. As chlorophyll a content of the sediment was not significantly different between stations, primary production of the sediment was considered as homogeneous for the whole lagoon. Thus, carbon production at the test station can be modelled from surface light irradiance. The modelled respiration was two times higher at the test station than the mean respiration of the barrier reef, and thus underestimated sediment contribution to excess production. Sediments cover 40-60% of the surface and accounted for 2.8-4.1% of organic carbon excess production estimated with the modelled R and 21-32% when mean R value was considered. The sedimentary CaCO3 budget was a very minor component of the whole reef budget.

Boron isotopes, trace elements and pH calculation for Norwegian deep sea coral specimen Madrepora o. (1968-2007), 2012

d11B and trace results obtained for a deep sea coral specimen Madrepora oculata collected from the Norwegian Sea (67°N, 9°E, 340 m) during the RV Polarstern ARK/II/Ia cruise (2007). Such coral specimen grew during the last four decades (1968-2007) and geochemical results highligh a seawater pH decrease with an order of magnitude in good agreement with an ocean acidification rate today known. This pH record is strongly impacted by inter-decadal change of ocean dynamic (NAO) and productivity. pHT calculation parameters (Hönisch et al., 2007): a=5; a=0.9804, d11B=39.5 PER MIL, Li/Mg temperature, salinity=35.1, pKB from Dickson (1990).

Seawater carbonate chemistry and biological processes during experiments with temperate coral Cladocora caespitosa, 2010

Atmospheric CO2 partial pressure (pCO2) is expected to increase to 700 µatm or more by the end of the present century. Anthropogenic CO2 is absorbed by the oceans, leading to decreases in pH and the CaCO3 saturation state of the seawater. Elevated pCO2 was shown to drastically decrease calcification rates in tropical zooxanthellate corals. Here we show, using the Mediterranean zooxanthellate coral Cladocora caespitosa, that an increase in pCO2, in the range predicted for 2100, does not reduce its calcification rate. Therefore, the conventional belief that calcification rates will be affected by ocean acidification may not be widespread in temperate corals. Seasonal change in temperature is the predominant factor controlling photosynthesis, respiration, calcification and symbiont density. An increase in pCO2, alone or in combination with elevated temperature, had no significant effect on photosynthesis, photosynthetic efficiency and calcification. The lack of sensitivity C. caespitosa to elevated pCO2 might be due to its slow growth rates, which seem to be more dependent on temperature than on the saturation state of calcium carbonate in the range projected for the end of the century.