Biological Impact of Elevated Ocean CO2 Concentrations: Lessons from Animal Physiology and Earth History

Abstract

CO 2 currently accumulating in the atmosphere permeates into ocean surface layers, where it may impact on marine animals in addition to effects caused by global warming. At the same time, several countries are developing scenarios for the disposal of anthropogenic CO 2 in the worlds' oceans, especially the deep sea. Elevated CO 2 partial pressures (hypercapnia) will affect the physiology of water breathing animals, a phenomenon also considered in recent discussions of a role for CO 2 in mass extinction events in earth history. Our current knowledge of CO 2 effects ranges from effects of hypercapnia on acid-base regulation, calcification and growth to influences on respiration, energy turnover and mode of metabolism. The present paper attempts to evaluate critical processes and the thresholds beyond which these effects may become detrimental. CO 2 elicits acidosis not only in the water, but also in tissues and body fluids. Despite compensatory accumulation of bicarbonate, acid-base parameters (pH, bicarbonate and CO 2 levels) and ion levels reach new steady-state values, with specific, long-term effects on metabolic functions. Even though such processes may not be detrimental, they are expected to affect long-term growth and reproduction and may thus be harmful at population and species levels. Sensitivity is maximal in ommastrephid squid, which are characterized by a high metabolic rate and extremely pH-sensitive blood oxygen transport. Acute sensitivity is interpreted to be less in fish with intracellular blood pigments and higher capacities to compensate for CO 2 induced acid-base disturbances than invertebrates. Virtually nothing is known about the degree to which deep-sea fishes are affected by short or long term hypercapnia. Sensitivity to CO 2 is hypothesized to be related to the organizational level of an animal, its energy requirements and mode of life. Long-term effects expected at population and species levels are in line with recent considerations of a detrimental role of CO 2 during mass extinctions in the earth's history. Future research is needed in this area to evaluate critical effects of the various CO 2 disposal scenarios.

Effects of climate change on fish reproduction and early life history stages

“…This process, known as ocean acidification, is considered to be a serious threat to marine species, especially for calcifying species that require carbonate ions to form their shells and skeletons (Hoegh-Guldberg et al 2007;Fabry et al 2008;Smith 2009). Elevated pCO 2 can also have a direct physiological effect on aquatic species through disruption of acid-base balance and limiting oxygen supply (Pörtner et al 2004;Pörtner and Farrell 2008). The effects of increasing pCO 2 in water is probably of greater concern than reducing pH per se, because of the high permeability to biological tissue of gaseous CO 2 relative to hydrogen ions (Brauner 2009).…”

“…Experiments with red seabream (Pagrus major) demonstrate that larval fish are more sensitive to the effects of acidification with CO 2 than to the same pH achieved with mineral acids (Kikkawa et al 2004). Increased pCO 2 in tissue causes acidosis (lowering of pH and accumulation of bicarbonate), which can be detrimental to many cellular processes, including protein synthesis, enzymatic function and oxygen transport (Pörtner et al 2004). Fish compensate for acidosis by acid-base equivalent ion transport from the body to the environment, mostly across the branchial epithelium, and to a lesser extent, via the kidneys and intestine (Claiborne et al 2002).…”

How will Ocean Acidification Affect Baltic Sea Ecosystems? An Assessment of Plausible Impacts on Key Functional Groups

“…It is clear, however, that tolerance of M. edulis to ocean acidification increases with available food ration (Melzner et al 2011). Increased ocean acidification represents an additional stress on marine organisms that can cause corresponding increases in maintenance and physiological costs (Pörtner et al 2004;Pörtner 2008). This 'acidification stress' will operate in concert with other stressors that limit the distribution and function of species-particularly salinity, which is a major determinant of mussel size and distribution in the Baltic Sea (Tedengren and Kautsky 1987;Westerbom et al 2008).…”

Recent advances in understanding trans-epithelial acid-base regulation and excretion mechanisms in cephalopods

“…45,48,117 Environmental hypercapnia that alters positive diffusion gradients of CO 2 from the animal to the seawater will affect the acid-base physiology of all water breathing animals as intra-and extra-cellular CO 2 concentrations will increase as well to maintain a sufficient diffusion gradient to excrete metabolic CO 2 . 45 Among a range of marine species, some have been identified as more sensitive (e.g.…”

“…On one hand, high Bohr coefficients of the cephalopod hemocyanins were proposed to be a critical physiological characteristic that would make cephalopods particularly sensitive to acid-base disturbances. 117 On the other hand substantial acid-base regulatory abilities as found in most cephalopod species represents a common feature that was suggested to make ectothermic marine animals robust to seawater acidification. 45 However, metabolic data from the cuttlefish S. officinalis (acute exposure to 0.6 kPa pCO 2 ) and the squids Dosidicus gigas (acute exposure to 0.1 kPa pCO 2 ) and Sepiteuthis lessoniana (acute; 20 h medium term; 168 h 0.16 and 0.41 kPa pCO 2 ) indicated that these animals respond very differently toward environmental hypercapnia.…”