Effect of carbonate chemistry manipulations on calcification, respiration, and excretion of a Mediterranean pteropod

Although shelled pteropods are expected to be particularly sensitive to ocean acidification, the few available studies have mostly focused on polar species and have not allowed determining which parameter of the carbonate system controls their calcification. Specimens of the temperate Mediterranean species Creseis acicula were maintained under seven different conditions of the carbonate chemistry, obtained by manipulating pH and total alkalinity, with the goal to disentangle the effects of the pH and the saturation state with respect to aragonite (Ωa). Our results tend to show that respiration, excretion as well as rates of net and gross calcification were not directly affected by a decrease in pH but decreased significantly with a decrease in Ωa. Due to the difficulties in maintaining pteropods in the laboratory and the important variability in their abundances in our study site, long-term acclimation as well as replication of the experiment was not possible. However, we strongly believe that these results represent an important step in the mechanistic understanding of the effect of ocean acidification on pteropods physiology.

Marine bivalve shell geochemistry and ultrastructure from modern low pH environments: environmental effect versus experimental bias (update)

Bivalve shells can provide excellent archives of past environmental change but have not been used to interpret ocean acidification events. We investigated carbon, oxygen and trace element records from different shell layers in the mussels Mytilus galloprovincialis combined with detailed investigations of the shell ultrastructure. Mussels from the harbour of Ischia (Mediterranean, Italy) were transplanted and grown in water with mean pHT 7.3 and mean pHT 8.1 near CO2 vents on the east coast of the island. Most prominently, the shells recorded the shock of transplantation, both in their shell ultrastructure, textural and geochemical record. Shell calcite, precipitated subsequently under acidified seawater responded to the pH gradient by an in part disturbed ultrastructure. Geochemical data from all test sites show a strong metabolic effect that exceeds the influence of the low-pH environment. These field experiments showed that care is needed when interpreting potential ocean acidification signals because various parameters affect shell chemistry and ultrastructure. Besides metabolic processes, seawater pH, factors such as salinity, water temperature, food availability and population density all affect the biogenic carbonate shell archive.

Response of two marine bacterial isolates to high CO2 concentration

Experimental results related to the effects of ocean acidification on planktonic marine microbes are still rather inconsistent and occasionally contradictory. Moreover, laboratory or field experiments that address the effects of changes in CO₂ concentrations on heterotrophic microbes are very scarce, despite the major role of these organisms in the marine carbon cycle. We tested the direct effect of an elevated CO₂ concentration (1000 ppmv) on the biomass and metabolic rates (leucine incorporation, CO₂ fixation and respiration) of 2 isolates belonging to 2 relevant marine bacterial families, Rhodobacteraceae (strain MED165) and Flavobacteriaceae (strain MED217). Our results demonstrate that, contrary to some expectations, high pCO₂ did not negatively affect bacterial growth but increased growth efficiency in the case of MED217. The elevated partial pressure of CO₂ ( pCO₂) caused, in both cases, higher rates of CO₂ fixation in the dissolved fraction and, in the case of MED217, lower respiration rates. Both responses would tend to increase the pH of seawater acting as a negative feedback between elevated atmospheric CO₂ concentrations and ocean acidification.

Impact of ocean acidification on Mediterranean coralline algae

Coralline algae are a major calcifying component of most Mediterranean benthic coastal ecosystems. They are of particular ecological importance, inducing settlement and recruitment of numerous invertebrates and providing habitats for a high diversity of associated organisms. They are also of significant importance in the carbon and carbonate cycles of shallow coastal ecosystems, being major contributors to CO₂ fluxes through high community CaCO₃ production and dissolution. However, coralline algae are among the calcifying organisms that appear to be the most sensitive to ocean acidification due to the solubility of their high magnesium calcite skeletons. We investigated the effects of ocean acidification on coralline algae both through in situ observations in a volcanic CO₂ vent area off Ischia (Italy) and through a long-term (one-year) mesocosm experiment combining the effects of elevated pCO₂ (lowered pH) and elevated temperature.

Responses of marine benthic microalgae to elevated CO2

Increasing anthropogenic CO2 emissions to the atmosphere are causing a rise in pCO2 concentrations in the ocean surface and lowering pH. To predict the effects of these changes, we need to improve our understanding of the responses of marine primary producers since these drive biogeochemical cycles and profoundly affect the structure and function of benthic habitats. The effects of increasing CO2 levels on the colonisation of artificial substrata by microalgal assemblages (periphyton) were examined across a CO2 gradient off the volcanic island of Vulcano (NE Sicily). We show that periphyton communities altered significantly as CO2 concentrations increased. CO2 enrichment caused significant increases in chlorophyll a concentrations and in diatom abundance although we did not detect any changes in cyanobacteria. SEM analysis revealed major shifts in diatom assemblage composition as CO2 levels increased. The responses of benthic microalgae to rising anthropogenic CO2 emissions are likely to have significant ecological ramifications for coastal systems.

Marine bivalve geochemistry and shell ultrastructure from modern low pH environments

Bivalve shells can provide excellent archives of past environmental change but have not been used to interpret ocean acidification events. We investigated carbon, oxygen and trace element records from different shell layers in the mussels Mytilus galloprovincialis (from the Mediterranean) and M. edulis (from the Wadden Sea) combined with detailed investigations of the shell ultrastructure. Mussels from the harbour of Ischia (Mediterranean, Italy) were transplanted and grown in water with mean pHT 7.3 and mean pHT 8.1 near CO2 vents on the east coast of the island of Ischia. The shells of transplanted mussels were compared with M. edulis collected at pH ~8.2 from Sylt (German Wadden Sea). Most prominently, the shells recorded the shock of transplantation, both in their shell ultrastructure, textural and geochemical record. Shell calcite, precipitated subsequently under acidified seawater responded to the pH gradient by an in part disturbed ultrastructure. Geochemical data from all test sites show a strong metabolic effect that exceeds the influence of the low-pH environment. These field experiments showed that care is needed when interpreting potential ocean acidification signals because various parameters affect shell chemistry and ultrastructure. Besides metabolic processes, seawater pH, factors such as salinity, water temperature, food availability and population density all affect the biogenic carbonate shell archive.

Increased diversity of sessile epibenthos at subtidal hydrothermal vents: seven hypotheses based on observations at Milos Island, Aegean Sea

Research on subtidal hydrothermal vent ecosystems at Milos, Hellenic Volcanic Arc (Aegean Sea), suggested that vent activity increased the species richness of sessile epibenthic assemblages. Based on 303 species found in 6 sites (3 close to vents, 3 farther away), the present paper uses correspondence analysis and species/samples curves to examine the species composition and richness of these assemblages. Differences due to vent proximity were more important than those due to bottom depth and distance from the shore. Diversity was confirmed to be higher near the vents, although none of the 266 species found at the vent sites can be considered as obligate vent-associated species. Seven different, although not mutually exclusive, hypotheses are discussed to explain the pattern of increased epibenthic species diversity at the vent sites, namely: (i) vents represent an intermediate disturbance, inducing mortality by the emission of toxic fluids; (ii) higher winter temperature allows for the occurrence of warm-water species, which add to the regional background; (iii) venting disrupts the homogeneity of the water bottom layer, increasing bottom roughness and hence habitat heterogeneity; (iv) deposition of minerals and enhanced bioconstruction by Ca enrichment increment habitat provision; (v) fluid emission induces advective mechanisms that favour recruitment; (vi) vents emit CO2, nutrients and trace elements that enhance primary productivity; and (vii) bacterial chemosynthesis add to photosynthesis to provide a diversity of food sources for the fauna.

High anthropogenic carbon content in the eastern Mediterranean

This work presents data of dichlorodifluoromethane (CFC‐12), dissolved inorganic carbon and total alkalinity from a cruise to the Mediterranean Sea during October– November 2001, with the main focus on the CFC‐12 data and on the eastern basin. Using the transit time distribution method, the anthropogenic carbon concentrations in the basin were estimated. Results were cross‐checked with a back‐calculation technique. The entire water column of the Mediterranean Sea contains anthropogenic CO2, with minimum concentrations of 20.5 mmol kg−1 (error range: 16.9–27.1 mmol kg−1) in the most eastern part of the basin at intermediate depths, where the waters’ mean age is >130 yr. Column inventories of up to 154 mol m−2 (132–179 mol m−2) are found and a total inventory of 1.7 Pg (1.3–2.1 Pg) of anthropogenic carbon in the Mediterranean Sea was estimated. There is a net flux of 38 Tg yr−1 (30–47 Tg yr−1) of dissolved inorganic carbon through the Strait of Gibraltar into the Atlantic Ocean and an opposite net flux of 3.5 Tg yr−1 (−1.8–9.2 Tg yr−1) of anthropogenic carbon into the Mediterranean Sea.

Structural and geochemical alterations in the Mg calcite bryozoan Myriapora truncata under elevated seawater pCO2 simulating ocean acidification

The possible effects of ocean acidification on the calcareous skeleton of the Mediterranean bryozoan Myriapora truncata (Pallas, 1766) were studied by transplanting live and dead colonies into an area of natural volcanic CO2 vents at Ischia (Gulf of Naples, Tyrrhenian Sea), Italy. Morphology and geochemistry were compared between colonies from normal (mean pH = 8.07, min. pH 7.95), below-normal (mean pH 7.66, min. pH 7.32) and acidic (mean pH 7.43, min. pH 6.83) conditions after colonies had been exposed in situ for 45 and 128 days. Both distal (juvenile) and proximal (adult) parts of the branches were investigated. Skeletons of live colonies in acidic pH site after 45 days of exposure were less corroded than those of dead colonies, suggesting that the organic tissues enveloping the skeleton play a protective role. Colonies remained alive at the below-normal and acidic pH sites during the 45-day experiment but corrosion was very striking after 128 days, with colonies from the acidic site showing significant loss of skeleton. Compared to the control, these colonies also had lower levels of Mg (mean 8 versus 9.5 wt% Mg) within their skeletons. Electron microprobe mapping showed Mg to be higher in the outer layers of the skeletal walls in colonies from the normal pH site. Corrosion of outer layers of the walls probably explains the lower Mg level found in colonies exposed to acidic conditions. As solubility of calcite increases with Mg content, the enrichment of Mg in outer layers of the skeleton should enhance the vulnerability of Myriapora truncata to dissolution. These findings raise concerns over the survival of bryozoans with Mg calcite skeletons in the face of predicted decreases in oceanic pH levels.

Effects of ocean acidification on invertebrate settlement at volcanic CO2 vents

We present the first study of the effects of ocean acidification on settlement of benthic invertebrates and microfauna. Artificial collectors were placed for 1 month along pH gradients at CO2 vents off Ischia (Tyrrhenian Sea, Italy). Seventy-nine taxa were identified from six main taxonomic groups (foraminiferans, nematodes, polychaetes, molluscs, crustaceans and chaetognaths). Calcareous foraminiferans, serpulid polychaetes, gastropods and bivalves showed highly significant reductions in recruitment to the collectors as pCO2 rose from normal (336–341 ppm, pH 8.09–8.15) to high levels (886–5,148 ppm) causing acidified conditions near the vents (pH 7.08–7.79). Only the syllid polychaete Syllis prolifera had higher abundances at the most acidified station, although a wide range of polychaetes and small crustaceans was able to settle and survive under these conditions. A few taxa (Amphiglena mediterranea, Leptochelia dubia, Caprella acanthifera) were particularly abundant at stations acidified by intermediate amounts of CO2 (pH 7.41–7.99). These results show that increased levels of CO2 can profoundly affect the settlement of a wide range of benthic organisms.