Resource allocation and extracellular acid–base status in the sea urchin Strongylocentrotus droebachiensis in response to CO2 induced seawater acidification

Anthropogenic CO2 emission will lead to an increase in seawater pCO2 of up to 80–100 Pa (800–1000 μatm) within this century and to an acidification of the oceans. Green sea urchins (Strongylocentrotus droebachiensis) occurring in Kattegat experience seasonal hypercapnic and hypoxic conditions already today. Thus, anthropogenic CO2 emissions will add up to existing values and will lead to even higher pCO2 values >200 Pa (>2000 μatm). To estimate the green sea urchins’ potential to acclimate to acidified seawater, we calculated an energy budget and determined the extracellular acid base status of adult S. droebachiensis exposed to moderately (102–145 Pa, 1007–1431 μatm) and highly (284–385 Pa, 2800–3800 μatm) elevated seawater pCO2 for 10 and 45 days.A 45-day exposure to elevated pCO2 resulted in a shift in energy budgets, leading to reduced somatic and reproductive growth. Metabolic rates were not significantly affected, but ammonium excretion increased in response to elevated pCO2. This led to decreased O:N ratios. These findings suggest that protein metabolism is possibly enhanced under elevated pCO2 in order to support ion homeostasis by increasing net acid extrusion. The perivisceral coelomic fluid acid–base status revealed that S. droebachiensis is able to fully (intermediate pCO2) or partially (high pCO2) compensate extracellular pH (pHe) changes by accumulation of bicarbonate (maximum increases 2.5 mM), albeit at a slower rate than typically observed in other taxa (10-day duration for full pHe compensation). At intermediate pCO2, sea urchins were able to maintain fully compensated pHe for 45 days. Sea urchins from the higher pCO2 treatment could be divided into two groups following medium-term acclimation: one group of experimental animals (29%) contained remnants of food in their digestive system and maintained partially compensated pHe (+2.3 mM HCO3−), while the other group (71%) exhibited an empty digestive system and a severe metabolic acidosis (−0.5 pH units, −2.4 mM HCO3−). There was no difference in mortality between the three pCO2 treatments.The results of this study suggest that S. droebachiensis occurring in the Kattegat might be pre-adapted to hypercapnia due to natural variability in pCO2 in its habitat. We show for the first time that some echinoderm species can actively compensate extracellular pH. Seawater pCO2 values of >200 Pa, which will occur in the Kattegat within this century during seasonal hypoxic events, can possibly only be endured for a short time period of a few weeks. Increases in anthropogenic CO2 emissions and leakages from potential sub-seabed CO2 storage (CCS) sites thus impose a threat to the ecologically and economically important species S. droebachiensis.

► Animals tolerated a pCO2 up to 145 Pa, their health status deteriorated at 284 Pa.
► Exposure to elevated pCO2 reduced somatic and reproductive growth.
► N excretion increased, feeding rate decreased leading to a shift in energy budgets.
► First report of extracellular pH compensation by HCO3-accumulation in echinoderms.
► At high pCO2 71% of animals ceased feeding and revealed a strong metabolic acidosis.