Background
The authors write that "intertidal zones are among the most stressful marine environments," as they are typically "characterized by frequent and large fluctuations of abiotic factors," including "temperature, salinity, pH, oxygen and carbon dioxide concentrations." And they add that in many instances these natural abiotic stressors are combined with anthropogenic stresses such as various types of pollution, eutrophication and "dead zones," citing Clark (1997), Jackson et al. (2001) and Howarth et al. (2011).

Continuing, the two U.S. researchers say that "survival in these environments requires efficient mechanisms of stress tolerance and involves a variety of cellular and physiological mechanisms of stress protection," as has been described by Hochachka and Somero (2002), Menge et al. (2002) and Somero (2002). And they add, in this regard, that "metabolic regulation plays a key role among those mechanisms allowing intertidal animals to maintain a positive energy balance and survive prolonged periods of extreme stress," referencing Sokolova and Portner (2001), Altieri (2006), Gracey et al. (2008) and Sokolova (2013).

What was done
In further exploring this intriguing subject, Ivanina and Sokolova examined the interactive effects of seawater pH and the presence of a pair of toxic trace metals - cadmium (Cd) and copper (Cu) at levels of 25 µM of each separately - on the mitochondrial functions of two common marine bivalves: hard clams (Mercenaria mercenaria) and eastern oysters (Crassostrea virginica).

What was learned
The two scientists say their study showed that "mitochondrial functions of the intertidal bivalves C. virginica and M. mercenaria are relatively insensitive to pH in a broad physiologically relevant range." But when they were impacted at more extreme values, they found that ocean acidification "modulates the response of their mitochondria," such that a decrease in pH was actually proven protective of the clams and oysters.

What it means
Ivanina and Sokolova conclude that "moderate acidosis (such as occurs during exposure to air, extreme salinities or elevated CO2 levels in the intertidal zone) may have a beneficial side-effect of protecting mitochondria against toxicity of metals," in that "reduced intracellular pH caused by exposure to elevated CO2 levels abolished the metal-induced generation of reactive oxygen species in isolated clam cells (Ivanina et al., 2013) consistent with a mitochondrial mechanism of the cytoprotective effects of moderate acidification," while noting that a similar mechanism had been "experimentally demonstrated for the surface proteins of unicellular algae" in the studies of Niyogi and Wood (2004), Wilde et al. (2006) and Esbaugh et al. (2013).

References
Altieri, A.H. 2006. Inducible variation in hypoxia tolerance across the intertidal-subtidal distribution of the blue mussel Mytilis edulis. Marine Ecology Progress Series 325: 295-300.

Clark, R. 1997. Marine Pollution. Clarendon Press, Oxford, United Kingdom.

Esbaugh, A.J., Mager, E.M., Brix, K.V., Santore, R. and Grosell, M. 2013. Implications of pH manipulation methods for metal toxicity: not all acidic environments are created equal. Aquatic Toxicology 130/131: 27-30.

Gracey, A.Y., Chaney, M.L., Boomhower, J.P., Tyburczy, W.R., Connor, K. and Somero, G.N. 2008. Rhythms of gene expression in a fluctuating intertidal environment. Current Biology 18: 1501-1507.

Hochachka, P.W. and Somero, G.N. 2002. Biochemical Adaptation: Mechanism and Process in Physiological Evolution. Oxford University Press, Oxford, United Kingdom.

Howarth, R., Chan, F., Conley, D.J., Garnier, J., Doney, S.C., Marino, R. and Billen, G. 2011. Coupled biogeochemical cycles: eutrophication and hypoxia in temperate estuaries and coastal marine ecosystems. Frontiers in Ecology and the Environment 9: 18-26.

Ivannia, A.V., Beniash, E., Etzkorn, M., Meyers, T.B., Ringwood, A.H. and Sokolova, I.M. 2013. Short-term acute hypercapnia affects cellular responses to trace metals in the hard clams Mercenaria mercenaria. Aquatic Toxicology 140/141: 123-133.

Menge, B.A., Olson, A.M. and Dahlhoff, E.P. 2002. Environmental stress, bottom-up effects, and community dynamics: integrating molecular-physiological and ecological approaches 1. Integrative and Comparative Biology 42: 892-908.

Niyogi, S. and Wood, C.M. 2004. Biotic ligand model, a flexible tool for developing site-specific water quality guidelines for metals. Environmental Science & Technology 38: 6177-6192.

Sokolova, I.M. 2013. Energy-limited tolerance to stress as a conceptual framework to integrate the effects of multiple stressors. Integrative and Comparative Biology 53: 597-608.

Sokolova, I.M. and Portner, H.-O. 2001. Physiological adaptations to high intertidal life involve improved water conservation abilities and metabolic rate depression in Littorina saxatilis. Marine Ecology Progress Series 224: 171-186.

Somero, G.N. 2002. Thermal physiology and vertical zonation of intertidal animals: optima, limits, and costs of living 1. Integrative and Comparative Biology 42: 780-789.

Wilde, K., Stauber, J., Markich, S., Franklin, N. and Brown, P. 2006. The effect of pH on the uptake and toxicity of copper and zinc in a tropical freshwater alga (Chlorella sp.). Archives of Environmental Contamination and Toxicology 51: 174-185.