Effects of hypoxia and hypercapnia on thermal tolerance: an integrative assessment on the green abalone (Haliotis fulgens).

Abstract

With the rise in atmospheric concentration of greenhouse gases, most marine ecosystems are facing increasing seawater temperatures, ocean acidification and a higher frequency or intensity of extreme warming events. Moreover, rising seawater temperature is expected to interact more frequently with falling oxygen levels (hypoxia) and increased CO2 concentration (hypercapnia). Both drivers may impose constraints on physiological mechanisms that define thermal Limits thereby increasing the vulnerability towards warming in marine ectotherms. The green abalone Haliotis fulgens is an economically important marine gastropod at the Pacific Coast of Mexico. In recent years, an increased frequency and intensity of environmental extremes, such as El Nino events and upwelling of highly hypoxic or hypercapnic water, has been associated with mass mortality events, threatening natural populations. Within this framework, the present study aimed at investigating the thermal tolerance and the underlying metabolic and molecular response in multiple tissues of H. fulgens under conditions of hypoxia and hypercapnia. Juvenile abalone (25.05 A /- 2.57 mm shell length) were exposed to a temperature ramp (from 18 degree Celsius to 32 degree Celsiu 3 degree Celsius day-1) under hypoxia (50% air saturation) and hypercapnia (a 1000 I atm PCO2), both individually and in combination; the conditions are based on natural oxygen declines occurring along the Baja California Peninsula and PCO2 values predicted by the end of the century, respectively. Hypoxia constrained the whole-organism oxygen consumption at moderate temperature (27 degree Celsius) paralleled by the accumulation of anaerobic metabolites (succinate, lactate, and alanine) in gill and hepatopancreas, suggesting a limitation in the aerobic capacity and reduced thermal tolerance. On the contrary, warming under hypercapnic exposure did not constrain Oxygen consumption, but the higher Q10 in metabolic rate and the increased levels of anaerobic metabolites at the warmest temperature (32 degree Celsius) indicate some stimulatory effect on metabolism. Finally, warming under combined hypoxia and hypercapnia resulted in negative synergistic impacts with an accumulation of anaerobic metabolites at a lower temperature (24 degree Celsius), followed by a depletion of metabolites, declining whole animal oxygen consumption indicating some hypometabolic state, and finally, the onset of muscular failure and death at the warmest temperature. The integrative approach from the molecular to the systemic levels and the use of different tissues in the present thesis allowed to identify promising indicators of physiological and metabolic traits responding sensitively to environmental challenges. Therefore, the identified traits and the approaches presented here represent a powerful tool in the assessment of the sensitivity of natural populations of green abalone to climate change.