Chair: Gretchen Hofmann

Daniel P. Small ⁽¹⁾, Marco Milazzo ⁽²⁾, Camilla Bertolini ⁽³⁾, Helen Graham^(4,5), Chris Hauton ⁽⁶⁾_, Jason M. Hall-Spencer ⁽⁷⁾, Samuel P.S. Rastrick ^(6,8)

1 Biology Department, St. Francis Xavier University, 2320 Notre Dame Avenue, Antigonish, NS B2G 2W5, Canada. * Corresponding author: dsmall@stfx.ca.
2 Department of Earth and Marine Science, Università degli studi di Palermo, CoNISMa, Via Archirafi 20, I-90123 Palermo, Italy.
3 School of Biological Sciences, Medical Biology Centre, Queen’s University Belfast, 97 Lisburn Road, Belfast, Northern Ireland BT9 7BL, UK.
4 School of Marine Science and Technology, Ridley Building, Newcastle University, Newcastle upon Tyne, Tyne and Wear, NE1 7RU, UK.
5 Uni Research Environment, Postboks 7810, 5020, Bergen.
6 Ocean and Earth Science, National Oceanography Centre Southampton, University of Southampton Waterfront Campus, European Way, Southampton, SO14 3ZE, UK.
7 Marine Biology and Ecology Research Centre, School of Marine Science and Engineering, Plymouth University, Drake Circus, Plymouth, Devon PL4 8AA, UK.
8 Institute of Marine Research, P. O. Box 1870 Nordness, 5870 Bergen, Norway. + Presenting author: samuel.rastrick@imr.no.

Background
Most studies assessing the impacts of Ocean Acidification (OA) on benthic marine invertebrates have used stable mean pH/ pCO₂ levels to 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 pCO₂ (pCO_2sw) on the physiological capacity of organisms to maintain acid-base homeostasis.

Methods
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 CO₂ seep systems (Vulcano, Italy) and assessed their acid-base responses. Both sea urchin species experienced fluctuations in extracellular coelomic fluid pH, pCO₂, and [HCO₃^–] (pH_e, pCO_2e, and [HCO₃^–]_e, respectively) in line with fluctuations in pCO_2sw.

Findings
The less tolerant species, P. lividus, had the greatest capacity for [HCO₃^–]_e buffering in response to acute pCO_2sw fluctuations, but it also experienced greater extracellular hypercapnia and acidification and was thus unable to fully compensate for acid-base disturbances. Conversely, the more tolerant A. lixula relied on non-bicarbonate protein buffering and greater respiratory control.

Conclusions
In light of these findings, we discuss the possible energetic consequences of increased reliance on bicarbonate buffering activity in P. lividus compared to 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 CO₂ emissions will affect survival and success of marine organisms within naturally assembled systems.