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Ocean acidification is elicited by anthropogenic carbon dioxide emissions and resulting oceanic uptake of excess C[O.sub.2] and might constitute an abiotic stressor powerful enough to alter marine ecosystem structures. For surface waters in gas-exchange equilibrium with the atmosphere, models suggest increases in C[O.sub.2] partial pressure (pC[O.sub.2]) from current values of ca. 390 µatm to ca. 700-1,000 µatm by the end of the century. However, in typically unequilibrated coastal hypoxic regions, much higher pC[O.sub.2] values can be expected, as heterotrophic degradation of organic material is necessarily related to the production of C[O.sub.2] (i.e., dissolved inorganic carbon). Here, we provide data and estimates that, even under current conditions, maximum pC[O.sub.2] values of 1,700-3,200 µatm can easily be reached when all oxygen is consumed at salinities between 35 and 20, respectively. Due to the nonlinear nature of the carbonate system, the approximate doubling of seawater pC[O.sub.2] in surface waters due to ocean acidification will most strongly affect coastal hypoxic zones as pC[O.sub.2] during hypoxia will increase proportionally: we calculate maximum pC[O.sub.2] values of ca. 4,500 µatm at a salinity of 20 (T = 10°C) and ca. 3,400 µatm at a salinity of 35 (T = 10°C) when all oxygen is consumed. Upwelling processes can bring these C[O.sub.2]-enriched waters in contact with shallow water ecosystems and may then affect species performance there as well. We conclude that (1) combined stressor experiments (pC[O.sub.2] and p[O.sub.2]) are largely missing at the moment and that (2) coastal ocean acidification experimental designs need to be closely adjusted to carbonate system variability within the specific habitat. In general, the worldwide spread of coastal hypoxic zones also simultaneously is a spread of C[O.sub.2]-enriched zones. The magnitude of expected changes in pC[O.sub.2] in these regions indicates that coastal systems may be more endangered by future global climate change than previously thought., Introduction Currently, large research efforts are directed toward studying anthropogenic climate change effects on marine ecosystems. Global warming, primarily caused by increasing C[O.sub.2] emissions, has the potential to restructure marine [...]

Marine organisms have to cope with increasing C[O.sub.2] partial pressures and decreasing pH in the oceans. We elucidated the impacts of an 8-week acclimation period to four seawater pC[O.sub.2] treatments (39, 113, 243 and 405 Pa/385, 1,120, 2,400 and 4,000 µatm) on mantle gene expression patterns in the blue mussel Mytilus edulis from the Baltic Sea. Based on the M. edulis mantle tissue transcriptome, the expression of several genes involved in metabolism, calcification and stress responses was assessed in the outer (marginal and pallial zone) and the inner mantle tissues (central zone) using quantitative real-time PCR. The expression of genes involved in energy and protein metabolism (F-ATPase, hexokinase and elongation factor alpha) was strongly affected by acclimation to moderately elevated C[O.sub.2] partial pressures. Expression of a chitinase, potentially important for the calcification process, was strongly depressed (maximum ninefold), correlating with a linear decrease in shell growth observed in the experimental animals. Interestingly, shell matrix protein candidate genes were less affected by C[O.sub.2] in both tissues. A compensatory process toward enhanced shell protection is indicated by a massive increase in the expression of tyrosinase, a gene involved in periostracum formation (maximum 220-fold). Using correlation matrices and a force-directed layout network graph, we were able to uncover possible underlying regulatory networks and the connections between different pathways, thereby providing a molecular basis of observed changes in animal physiology in response to ocean acidification., Introduction Ongoing C[O.sub.2]-induced ocean acidification threatens marine ecosystems (Doney et al. 2009). In the eutrophic Western Baltic Sea, ocean acidification is accompanied by seasonal upwelling of C[O.sub.2]-enriched water masses, resulting [...]

The Baltic Sea has a salinity gradient decreasing from fully marine (> 25) in the West to below 7 in the Central Baltic Proper. Reef forming mytilid mussels exhibit decreasing growth when salinity <11, however the mechanisms underlying reduced calcification rates in dilute seawater are not fully understood. In fact, both [HCO3- ] and [Ca2+] also decrease with salinity, challenging calcifying organisms through CaCO3 undersaturation (Ω < 1) and unfavourable ratios of calcification substrate (Ca2+ and HCO3-) to inhibitor (H+). In this study we assessed the impact of isolated individual factors (salinity, [Ca2+], [HCO3"] and pH) on calcification and growth of mytilid mussel populations along the Baltic salinity gradient. Laboratory experiments rearing juvenile Baltic Mytilus at a range of salinities (6, 11 and 16), HCO3" concentrations (300-2100 μmol kg-1) and Ca2+ concentrations (0.5-4 mmol kg-1) were coupled with field monitoring in three Baltic mussel reefs. Results reveal that as individual factors, low [HCO3-], pH and salinity cannot explain low calcification rates in the Baltic Sea. Calcification rates are impeded when Ω^ω^ <1 or the substrate inhibitor ratio < 0.7, primarily due to [Ca2+] limitation which corresponds to a salinity of ca. 11. Increased food availability may be able to mask these negative impacts, but not when seawater conditions are permanently adverse, as observed in two Baltic reefs at salinities < 11. Future climatic models predict rapid desalination of the southwest and Central Baltic and potentially a reduction in [Ca2+] which may lead to a westward distribution shift of marine calcifiers. It is therefore vital to understand the mechanisms by which the ionic composition of seawater impacts bivalve calcification for better predicting the future of benthic Baltic ecosystems. [ABSTRACT FROM AUTHOR]