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 CO(2), burrowing sea urchins will demonstrate a lower sensitivity to massive CO(2) release than has previously been recorded for epifaunal organisms. Infaunal urchins Brissopsis lyrifera were exposed to CO(2) 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 2h. In common with epifaunal urchins B. lyrifera exhibited an uncompensated respiratory acidosis in its extracellular fluid, but was more sensitive to CO(2) 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 CO(2). 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 CO(2), they may already be closer to the limits of their physiological performance. Thus any further CO(2) increase could compromise their function. As a result of this sensitivity, infaunal urchins may be more at risk from an accidental release of CO(2) from geological sub-seabed storage sites, or from the deliberate injection of CO(2) into deep water masses, than their epifaunal counterparts.