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.