Temporal fluctuations in seawater pCO<inf>2</inf> may be as important as mean differences when determining physiological sensitivity in natural systems
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© International Council for the Exploration of the Sea 2015. All rights reserved. Most studies assessing the impactsofocean acidification (OA) onbenthic marine invertebrates have used stable mean pH/pCO 2 levelsto highlight variation in the physiological sensitivities in a range of taxa. However, many marine environments experience natural fluctuations in carbonate chemistry, and to date little attempt has been made to understand the effect of naturally fluctuating seawater pCO 2 (pCO 2sw ) on the physiological capacity of organisms to maintain acid-base homeostasis. Here, for the first time, we exposed two species of sea urchin with different acid-base tolerances, Paracentrotus lividus and Arbacia lixula, to naturally fluctuating pCO 2sw conditions at shallow water CO 2 seep systems (Vulcano, Italy) and assessed their acid-base responses. Both sea urchin species experienced fluctuations in extracellular coelomic fluid pH, pCO 2 , and [HCO - 3 ] (pH e , pCO 2e , and [HCO - 3 ] e , respectively) in line with fluctuations in pCO 2sw . The less tolerant species, P. lividus, had the greatest capacity for [HCO - 3 ] e buffering in response to acute pCO 2sw fluctuations, but it also experienced greater extracellular hypercapnia and acidification and was thus unabletofully compensate for acid-basedisturbances. Conversely, themore tolerant A.lixula reliedonnon-bicarbonate protein buffering and greater respiratory control. In the light of these findings, we discuss the possible energetic consequences of increased reliance on bicarbonate buffering activity in P. lividus compared with A. lixula and how these differing physiological responses to acute fluctuations in pCO 2sw may be as important as chronic responses to mean changes in pCO 2sw when considering how CO 2 emissions will affect survival and success of marine organisms within naturally assembled systems.
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