Seawater carbonate chemistry and processes during experiments with amphipod Gammarus locusta, 2009

We report an investigation of the effects of increases in pCO2 on the survival, growth and molecular physiology of the neritic amphipod Gammarus locusta which has a cosmopolitan distribution in estuaries. Amphipods were reared from juvenile to mature adult in laboratory microcosms at three different levels of pH in nominal range 8.1-7.6. Growth rate was estimated from weekly measures of body length. At sexual maturity the amphipods were sacrificed and assayed for changes in the expression of genes coding for a heat shock protein (hsp70 gene) and the metabolic enzyme glyceraldehyde-3-phosphate dehydrogenase (gapdh gene). The data show that the growth and survival of this species is not significantly impacted by a decrease in sea water pH of up to 0.5 units. Quantitative real-time PCR analysis indicated that there was no significant effect of growth in acidified sea water on the sustained expression of the hsp70 gene. There was a consistent and significant increase in the expression of the gapdh gene at a pH of ~7.5 which, when combined with observations from other workers, suggests that metabolic changes may occur in response to acidification. It is concluded that sensitive assays of tissue physiology and molecular biology should be routinely employed in future studies of the impacts of sea water acidification as subtle effects on the physiology and metabolism of coastal marine species may be overlooked in conventional gross "end-point" studies of organism growth or mortality.

Seawater carbonate chemistry and biological parameters during experiments with white sea bass Atractoscion nobilis, 2009

A large fraction of the carbon dioxide added to the atmosphere by human activity enters the sea, causing ocean acidification. We show that otoliths (aragonite ear bones) of young fish grown under high CO2 (low pH) conditions are larger than normal, contrary to expectation. We hypothesize that CO2 moves freely through the epithelium around the otoliths in young fish, accelerating otolith growth while the local pH is controlled. This is the converse of the effect commonly reported for structural biominerals.

Seawater carbonate chemistry and processes during experiments with cyanobacterium Trichodesmium (IMS101), 2009

We investigated carbon acquisition by the N2-fixing cyanobacterium Trichodesmium IMS101 in response to CO2 levels of 15.1, 37.5, and 101.3 Pa (equivalent to 150, 370, and 1000 ppm). In these acclimations, growth rates as well as cellular C and N contents were measured. In vivo activities of carbonic anhydrase (CA), photosynthetic O2 evolution, and CO2 and HCO3- fluxes were measured using membrane inlet mass spectrometry and the 14C disequilibrium technique. While no differences in growth rates were observed, elevated CO2 levels caused higher C and N quotas and stimulated photosynthesis and N2 fixation. Minimal extracellular CA (eCA) activity was observed, indicating a minor role in carbon acquisition. Rates of CO2 uptake were small relative to total inorganic carbon (Ci) fixation, whereas HCO{3 contributed more than 90% and varied only slightly over the light period and between CO2 treatments. The low eCA activity and preference for HCO3- were verified by the 14C disequilibrium technique. Regarding apparent affinities, half-saturation concentrations (K1/2) for photosynthetic O2 evolution and HCO3- uptake changed markedly over the day and with CO2 concentration. Leakage (CO2 efflux : Ci uptake) showed pronounced diurnal changes. Our findings do not support a direct CO2 effect on the carboxylation efficiency of ribulose-1,5-bisphosphate carboxylase/oxygenase (RubisCO) but point to a shift in resource allocation among photosynthesis, carbon acquisition, and N2 fixation under elevated CO2 levels. The observed increase in photosynthesis and N2fixation could have potential biogeochemical implications, as it may stimulate productivity in N-limited oligotrophic regions and thus provide a negative feedback in rising atmospheric CO2 levels.

Seawater carbonate chemistry and processes during experiments with Emiliania huxleyi (PML B93/11A)

The formation of calcareous skeletons by marine planktonic organisms and their subsequent sinking to depth generates a continuous rain of calcium carbonate to the deep ocean and underlying sediments. This is important in regulating marine carbon cycling and ocean-atmosphere CO2 exchange. The present rise in atmospheric CO2 levels causes significant changes in surface ocean pH and carbonate chemistry. Such changes have been shown to slow down calcification in corals and coralline macroalgae, but the majority of marine calcification occurs in planktonic organisms. Here we report reduced calcite production at increased CO2 concentrations in monospecific cultures of two dominant marine calcifying phytoplankton species, the coccolithophorids Emiliania huxleyi and Gephyrocapsa oceanica . This was accompanied by an increased proportion of malformed coccoliths and incomplete coccospheres. Diminished calcification led to a reduction in the ratio of calcite precipitation to organic matter production. Similar results were obtained in incubations of natural plankton assemblages from the north Pacific ocean when exposed to experimentally elevated CO2 levels. We suggest that the progressive increase in atmospheric CO2 concentrations may therefore slow down the production of calcium carbonate in the surface ocean. As the process of calcification releases CO2 to the atmosphere, the response observed here could potentially act as a negative feedback on atmospheric CO2 levels.

Seawater carbonate chemistry, cell numbers and growth rate during experiments with dinoflagellates, 2007

The effects of dissolved inorganic carbon (DIC) on the growth of 3 red-tide dinoflagellates (Ceratium lineatum, Heterocapsa triquetra and Prorocentrum minimum) were studied at pH 8.0 and at higher pH levels, depending upon the pH tolerance of the individual species. The higher pH levels chosen for experiments were 8.55 for C. lineatum and 9.2 for the other 2 species. At pH 8.0, which approximates the pH found in the open sea, the maximum growth in all species was maintained until the total DIC concentration was reduced below ~0.4 and 0.2 mM for C. lineatum and the other 2 species, respectively. Growth compensation points (concentration of inorganic carbon needed for maintenance of cells) were reached at ~0.18 and 0.05 mM DIC for C. lineatum and the other 2 species, respectively. At higher pH levels, maximum growth rates were lower compared to growth at pH 8, even at very high DIC concentrations, indicating a direct pH effect on growth. Moreover, the concentration of bio-available inorganic carbon (CO2 + HCO3-) required for maintenance as well as the half-saturation constants were increased considerably at high pH compared to pH 8.0. Experiments with pH-drift were carried out at initial concentrations of 2.4 and 1.2 mM DIC to test whether pH or DIC was the main limiting factor at a natural range of DIC. Independent of the initial DIC concentrations, growth rates were similar in both incubations until pH had increased considerably. The results of this study demonstrated that growth of the 3 species was mainly limited by pH, while inorganic carbon limitation played a minor role only at very high pH levels and low initial DIC concentrations.

Seawater carbonate chemistry, nutrients and growth rate during experiments with coral Astrangia poculata and field observations, 2010

Zooxanthellate colonies of the scleractinian coral Astrangia poculata were grown under combinations of ambient and elevated nutrients (5 µM NO, 0.3 µM PO4, and 2nM Fe) and CO2 (780 ppmv) treatments for a period of 6 months. Coral calcification rates, estimated from buoyant weights, were not significantly affected by moderately elevated nutrients at ambient CO2 and were negatively affected by elevated CO2 at ambient nutrient levels. However, calcification by corals reared under elevated nutrients combined with elevated CO2 was not significantly different from that of corals reared under ambient conditions, suggesting that CO2 enrichment can lead to nutrient limitation in zooxanthellate corals. A conceptual model is proposed to explain how nutrients and CO2 interact to control zooxanthellate coral calcification. Nutrient limited corals are unable to utilize an increase in dissolved inorganic carbon (DIC) as nutrients are already limiting growth, thus the effect of elevated CO2 on saturation state drives the calcification response. Under nutrient replete conditions, corals may have the ability to utilize more DIC, thus the calcification response to CO2 becomes the product of a negative effect on saturation state and a positive effect on gross carbon fixation, depending upon which dominates, the calcification response can be either positive or negative. This may help explain how the range of coral responses found in different studies of ocean acidification can be obtained.

Seawater carbonate chemistry, shell linear extension and calcification during calcein staining and 45Ca experiments with pteropod Limacina helicina, 2009

Thecosome pteropods (shelled pelagic molluscs) can play an important role in the food web of various ecosystems and play a key role in the cycling of carbon and carbonate. Since they harbor an aragonitic shell, they could be very sensitive to ocean acidification driven by the increase of anthropogenic CO2 emissions. The impact of changes in the carbonate chemistry was investigated on Limacina helicina, a key species of Arctic ecosystems. Pteropods were kept in culture under controlled pH conditions corresponding to pCO2 levels of 350 and 760 µatm. Calcification was estimated using a fluorochrome and the radioisotope 45Ca. It exhibits a 28 % decrease at the pH value expected for 2100 compared to the present pH value. This result supports the concern for the future of pteropods in a high-CO2 world, as well as of those species dependent upon them as a food resource. A decline of their populations would likely cause dramatic changes to the structure, function and services of polar ecosystems.

Seawater carbonate chemistry and biological processes during experiments with four strains of Emiliania huxleyi

Four strains of the coccolithophore E. huxleyi (RCC1212, RCC1216, RCC1238, RCC1256) were grown in dilute batch culture at four CO2 levels ranging from ~200 µatm to ~1200 µatm. Growth rate, particulate organic carbon content, and particulate inorganic carbon content were measured, and organic and inorganic carbon production calculated. The four strains did not show a uniform response to carbonate chemistry changes in any of the analysed parameters and none of the four strains displayed a response pattern previously described for this species. We conclude that the sensitivity of different strains of E. huxleyi to acidification differs substantially and that this likely has a genetic basis. We propose that this can explain apparently contradictory results reported in the literature.

Seawater carbonate chemistry and biological processes during experiments with calcifiing organisms, 2009

Anthropogenic elevation of atmospheric carbon dioxide (pCO2) 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 pCO2. However, in seven species, net calcification increased under the intermediate and/or highest levels of pCO2, 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 whether they utilize photosynthesis. Whatever the specific mechanism(s) involved, our results suggest that the impact of elevated atmospheric pCO2 on marine calcification is more varied than previously thought.

Seawater carbonate chemistry and processes during experiments with Emiliania huxleyi (TW1), 2003

Precipitation of calcium carbonate by phytoplankton in the photic oceanic layer is an important process regulating the carbon cycling and the exchange of CO2 at the ocean-atmosphere interface. Previous experiments have demonstrated that, under nutrient-sufficient conditions, doubling the partial pressure of CO2 (pCO2) in seawater-a likely scenario for the end of the century-can significantly decrease both the rate of calcification by coccolithophorids and the ratio of inorganic to organic carbon production. The present work investigates the effects of high pCO2 on calcification by the coccolithophore Emiliania huxleyi (Strain TW1) grown under nitrogen-limiting conditions, a situation that can also prevail in the ocean. Nitrogen limitation was achieved in NO3-limited continuous cultures renewed at the rate of 0.5 d-1 and exposed to a saturating light level. pCO2 was increased from 400 to 700 ppm and controlled by bubbling CO2-rich or CO2-free air into the cultures. The pCO2 shift has a rapid effect on cell physiology that occurs within 2 cell divisions subsequent to the perturbation. Net calcification rate (C) decreased by 25% and, in contrast to previous studies with N-replete cultures, gross community production (GCP) and dark community respiration (DCR) also decreased. These results suggest that increasing pCO2 has no noticeable effect on the calcification/photosynthesis ratio (C/P) when cells of E. huxleyi are NO3-limited.

Seawater carbonate chemistry and processes during experiments with Emiliania huxleyi (PML B92/11), 2000

We have measured the stable carbon isotopic composition of bulk organic matter (POC), alkenones, sterols, fatty acids, and phytol in the coccolithophorid Emiliania huxleyi grown in dilute batch cultures over a wide range of CO2 concentrations (1.1-53.5 micromol L-1). The carbon isotope fractionation of POC (POC) varied by ca. 7 per mil and was positively correlated with aqueous CO2 concentration [CO2aq]. While this result confirms general trends observed for the same alga grown in nitrogen-limited chemostat cultures, considerable differences were obtained in absolute values of POC and in the slope of the relationship of POC with growth rate and [CO2aq]. Also, a significantly greater offset was obtained between the delta13C of alkenones and bulk organic matter in this study compared with previous work (5.4, cf. 3.8 per mil). This suggests that the magnitude of the isotope offset may depend on growth conditions. Relative to POC, individual fatty acids were depleted in 13C by 2.3 per mil to 4.1 per mil, phytol was depleted in 13C by 1.9 per mil, and the major sterol 24-methylcholesta-5,22E-dien-3beta-ol was depleted in 13C by 8.5 per mil. This large spread of delta13C values for different lipid classes in the same alga indicates the need for caution in organic geochemical studies when assigning different sources to lipids that might have delta13C values differing by just a few per mil. Increases in [CO2aq] led to dramatic increases in the alkenone contents per cell and as a proportion of organic carbon, but there was no systematic effect on values of U37k- used for reconstructions of paleo sea surface temperature.

Seawater carbonate chemistry and processes during experiments with Crassostrea gigas, 2007

This study demonstrated that the increased partial pressure of CO2 (pCO2) in seawater and the attendant acidification that are projected to occur by the year 2300 will severely impact the early development of the oyster Crassostrea gigas. Eggs of the oyster were artificially fertilized and incubated for 48 h in seawater acidified to pH 7.4 by equilibrating it with CO2-enriched air (CO2 group), and the larval morphology and degree of shell mineralization were compared with the control treatment (air-equilibrated seawater). Only 5% of the CO2 group developed into normal 'D-shaped' veliger larvae as compared with 68% in the control group, although no difference was observed between the groups up to the trochophore stage. Thus, during embryogenesis, the calcification process appears to be particularly affected by low pH and/or the low CaCO3 saturation state of high-CO2 seawater. Veliger larvae with fully mineralized shells accounted for 30% of the CO2-group larvae, compared with 72% in the control (p < 0.005). Shell mineralization was completely inhibited in 45% of the CO2-group larvae, but only in 16% of the control (p < 0.05). Normal D-shaped veligers of the control group exhibited increased shell length and height between 24 and 48 h after fertilization, while the few D-shaped veligers of the CO2 group showed no shell growth during the same period. Our results suggest that future ocean acidification will have deleterious impacts on the early development of marine benthic calcifying organisms.

Seawater carbonate chemistry and biological processes during experiments with spider crab Hyas araneus, 2011

The combined effects of ocean warming and acidification were compared in larvae from two populations of the cold-eurythermal spider crab Hyas araneus, from one of its southernmost populations (around Helgoland, southern North Sea, 54°N, habitat temperature 3-18°C; collection: January 2008, hatch: January-February 2008) and from one of its northernmost populations (Svalbard, North Atlantic, 79°N, habitat temperature 0-6°C; collection: July 2008, hatch: February-April 2009). Larvae were exposed to temperatures of 3, 9 and 15°C combined with present-day normocapnic (380 ppm CO2) and projected future CO2 concentrations (710 and 3,000 ppm CO2). Calcium content of whole larvae was measured in freshly hatched Zoea I and after 3, 7 and 14 days during the Megalopa stage. Significant differences between Helgoland and Svalbard Megalopae were observed at all investigated temperatures and CO2 conditions. Under 380 ppm CO2, the calcium content increased with rising temperature and age of the larvae. At 3 and 9°C, Helgoland Megalopae accumulated more calcium than Svalbard Megalopae. Elevated CO2 levels, especially 3,000 ppm, caused a reduction in larval calcium contents at 3 and 9°C in both populations. This effect set in early, at 710 ppm CO2 only in Svalbard Megalopae at 9°C. Furthermore, at 3 and 9°C Megalopae from Helgoland replenished their calcium content to normocapnic levels and more rapidly than Svalbard Megalopae. However, Svalbard Megalopae displayed higher calcium contents under 3,000 ppm CO2 at 15°C. The findings of a lower capacity for calcium incorporation in crab larvae living at the cold end of their distribution range suggests that they might be more sensitive to ocean acidification than those in temperate regions.