Combined effect of pCO2 and temperature levels on the thermal niche in the early benthic ontogeny of a keystone species

We evaluated the effects of projected, near future ocean acidification (OA) and extreme events of temperature (warming or cooling) on the thermal tolerance of Concholepas concholepas, a coastal benthic keystone species. Three separate trials of an experiment were conducted by exposing juvenile C. concholepas for 1 month to one of two contrasting pCO(2) levels (similar to 500 and similar to 1200 mu atm). In addition, each pCO(2) level was combined with one of four temperature treatments. The control was 15 degrees C, whilst the other temperatures were 10 degrees C (Trial 1), 20 degrees C (Trial 2) and 25 degrees C (Trial 3). At the end of each trial, we assessed Critical Thermal maximum (CTmax) and minimum (CTmin) via self-righting success, calculated partial thermal tolerance polygons, measured somatic growth, determined transcription of Heat Shock Proteins 70 (HSP70) and measured oxygen consumption rates. Regardless of pCO(2) level, HSP70 transcript levels were significantly higher in juveniles after exposure to extreme temperatures (10 degrees C and 25 degrees C) indicating physiological stress. Oxygen consumption rates increased with increasing temperature from 10 degrees C to 20 degrees C though showed a decrease at 25 degrees C. This rate was not affected by pCO(2) or the interaction between temperature and pCO(2). Juveniles exposed to present-day and near future pCO(2) levels at 20 degrees C showed similar thermal tolerance polygonal areas; whilst changes in both CTmin and CTmax at 25 degrees C and 10 degrees C caused narrower and broader areas, respectively. Temperature affected growth, oxygen consumption and HSP70 transcription in small juvenile C. concholepas. Exposure to elevated pCO(2) did not affect thermal tolerance, growth or oxygen consumption at temperatures within the thermal range normally experienced by this species in northern Chile (15-20 degrees C). At elevated pCO(2) conditions, however, exposure to warmer (25 degrees C) or colder (10 degrees C) temperatures reduced or increased the thermal area, respectively. This study demonstrates the importance of examining the thermal-tolerance edges to better understand how OA and temperature will combine to physiologically challenge inter-tidal organisms. (C) 2020 Published by Elsevier B.V.