Acute extracellular acid–base disturbance in the burrowing sea urchin Brissopsis lyrifera during exposure to a simulated CO2 release

We tested the hypothesis that as infaunal organisms are regularly exposed to elevated CO2, burrowing sea urchins will demonstrate a lower sensitivity to massive CO2 release than has previously been recorded for epifaunal organisms. Infaunal urchins Brissopsis lyrifera were exposed to CO2 acidified sea water (nominal pH 7.8 (control), 7.3, 6.5 and 5.9; T = 10 °C, S = 34) for 12 h and aspects of their extracellular acid–base balance measured every 2 h. In common with epifaunal urchins B. lyrifera exhibited an uncompensated respiratory acidosis in its extracellular fluid, but was more sensitive to CO2 acidification than epifaunal urchins. The lower extracellular pH of B. lyrifera may indicate a higher metabolism than epifaunal urchins and this could explain the heightened sensitivity of this species to elevated CO2. Thus, the results of this present study do not support our original hypothesis. Instead we suggest an alternative hypothesis that as infaunal organisms are exposed naturally to high levels of CO2, they may already be closer to the limits of their physiological performance. Thus any further CO2 increase could compromise their function. As a result of this sensitivity, infaunal urchins may be more at risk from an accidental release of CO2 from geological sub-seabed storage sites, or from the deliberate injection of CO2 into deep water masses, than their epifaunal counterparts.

► Sea urchins Brissopsis lyrifera exposed to high CO2/low pH showed an uncompensated extracellular respiratory acidosis.
► Infaunal B. lyrifera appears more sensitive to CO2 acidification than epifaunal urchins.
► Infaunal organisms exposed naturally to high CO2/low pH may already be close to their physiological capacity.
► Infaunal sea urchins may be more at risk from accidental CO2 leakage from CCS sites than epifaunal urchins.