Interactive effects of ocean acidification and multiple stressors on physiology of marine bivalves

The continuing increase of carbon dioxide (CO2) levels in the atmosphere leads to increase in sea-surface temperature and causes ocean acidification altering seawater carbonate chemistry. Estuarine and shallow coastal areas, which are hotspots for biological productivity, are especially prone to these changes, because of low buffering capacity of brackish waters, biological CO2 production, and large fluctuations of temperature and salinity in these habitats. These additional stressors may exacerbate the acidification trend and significantly affect the physiology of marine calcifiers. Bivalves are a key group of marine calcifiers that serve as ecosystem engineers and key foundation species in estuarine and coastal environments. However, the interactive effects of elevated CO2 and other stressors, including elevated temperature and reduced salinity, are not yet fully understood in bivalves and require further investigation. This study focused on the physiological responses in two ecologically and economically important bivalve species - the eastern oyster (Crassostrea virginica) and hard shell clam (Mercenaria mercenaria). Juveniles and adults were exposed to environmentally relevant salinities at either ∼32 or ∼16 with different P CO2 levels (∼400, 800 and 1500 muatm) and temperature (22°C and 27°C), as predicted by future global climate change scenarios, for 11-21 weeks (in juveniles) and 15 weeks (in adults). Survival, metabolism and calcification were assessed. Elevated PCO2 alone and in combination with either reduced salinity or elevated temperature led to reduced survival and growth, and altered the shell mechanical properties (microhardness and fracture resistance) in juvenile and adult bivalves. Tissue energy reserves (lipid and glycogen) were reduced in juveniles under elevated P CO2 and low salinity. Standard energy metabolism (SMR) increased under the conditions of elevated PCO2 and temperature indicating higher costs of basal maintenance. In adult bivalves, elevated temperature led to the depletion of tissue energy reserves in both species indicating energy deficiency, and resulted in higher mortality in oysters but not in clams. In adult clams and oysters, elevated PCO2 alone had a small effect on the metabolism. No persistent oxidative stress (measured by total antioxidant capacity as well as oxidative markers of proteins and lipids) was seen in the bivalves after prolonged exposure to elevated temperature and PCO2. The results of this study indicate that the interactions between common abiotic stressors (salinity and temperature) and elevated PCO2 are complex, non-linear, and species-specific. Our study shows that elevated temperature and reduced salinity are predominant stressors that affect survival, metabolism, and biomineralization response of the studied bivalves. Furthermore, these stressors modulate the responses of these bivalves to ocean acidification while elevated CO2 levels have a modest impact in the absence of these stressors.