Are control of extracellular acid-base balance and regulation of skeleton genes linked to resistance to ocean acidification in adult sea urchins?

Abstract The changes induced in seawater chemistry by increased dissolved pCO2 in the ocean, the so- called ocean acidification (OA), impact marine metazoans but the recorded effects strongly differ according to taxa. The ability to regulate the acid-base balance of extracellular fluids has been hypothesized to be relevant for some of these differences. We tested this hypothesis in adult sea urchins by comparing two sympatric species, Paracentrotus lividus and Arbacia lixula, which both occur in and out of the plume of a CO2 vent at Vulcano Island, Italy. Acid- base physiology parameters, mechanical properties of the skeleton and the expression of four biomineralization-related genes (p19, msp130, sm50 and carbonic anhydrase) were recorded in specimens coming from an acidified (pHT seawater 7.6) and a control site (pHT seawater 7.9). The expression profiles of analyzed genes were obtained from the comparisons of individuals by performing species-specific Real-Time qPCR assays, based on mRNA sequences known and sequences here for the first time identified by in silico analysis. The P.lividus Z12.1 gene and the, here identified, A. lixula homologue, were used as reference genes in the respective assays. P. lividus whose extracellular fluids are strongly buffered by the accumulation of bicarbonate ions, did not show any difference in the studied variables between control and acidified sites, with the exception of the down-regulation of a single gene. On the contrary, A. lixula whose buffering capacity of its extracellular fluids is limited showed a reduction in mechanical resistance of its skeleton (up to 42%) as well as down-regulation of all studied biomineralization-related genes. This strongly suggests that the buffering of the extracellular fluids is indeed increasing the resistance of adult P. lividus to OA, and could account for the tolerance of this species reported in several long-term studies.