Acidified seawater impacts sea urchin larvae pH regulatory systems relevant for calcification

Abstract

Calcifying echinoid larvae respond to changes in seawater carbonate chemistry with reduced growth and developmental delay. To date, no information exists on how ocean acidification acts on pH homeostasis in echinoderm larvae. Understanding acid-base regulatory capacities is important because intracellular formation and maintenance of the calcium carbonate skeleton is dependent on pH homeostasis. Using H + -selective microelectrodes and the pHsensitive fluorescent dye BCECF, we conducted in vivo measurements of extracellular and intracellular pH (pH e and pH i ) in echinoderm larvae. We exposed pluteus larvae to a range of seawater CO 2 conditions and demonstrated that the extracellular compartment surrounding the calcifying primary mesenchyme cells (PMCs) conforms to the surrounding seawater with respect to pH during exposure to elevated seawater pCO 2 . Using FITC dextran conjugates, we demonstrate that sea urchin larvae have a leaky integument. PMCs and spicules are therefore directly exposed to strong changes in pH e whenever seawater pH changes. However, measurements of pH i demonstrated that PMCs are able to fully compensate an induced intracellular acidosis. This was highly dependent on Na + and HCO 3 − , suggesting a bicarbonate buffer mechanism involving secondary active Na + -dependent membrane transport proteins. We suggest that, under ocean acidification, maintained pH i enables calcification to proceed despite decreased pH e . However, this probably causes enhanced costs. Increased costs for calcification or cellular homeostasis can be one of the main factors leading to modifications in energy partitioning, which then impacts growth and, ultimately, results in increased mortality of echinoid larvae during the pelagic life stage. pH microelectrode | Strongylocentrotus droebachiensis | acid-base regulation | Na + -HCO 3 − transport | epithelial transport S ea urchin larvae have been shown to react with particular sensitivity to CO 2 -induced reductions in seawater pH (1-4). When larvae are chronically exposed to elevated seawater pCO 2 of >0.1 kPa, e.g., as is predicted to occur during the next century in response to anthropogenic CO 2 emissions or through upwelling of low-pH deep water, this sensitivity is reflected in reduced growth and developmental rates (5, 6). Echinoderm larvae are considered to be especially vulnerable to seawater pH reduction and to the associated changes in calcium carbonate saturation state of seawater (Ω Cal ) because their internal skeleton is composed of high magnesium calcite, a highly soluble form of CaCO 3 (7, 8). However, long-term reductions in growth and development might just as well be evoked by other physiological mechanisms that are also sensitive to hypercapnia and the related acid-base disturbances. Recent studies conducted on several marine taxa including mollusks (9) and echinoderms (10) demonstrated increased metabolic rates in response to elevated seawater pCO 2 . It was concluded that reductions in somatic growth and rate of development were caused by a sh...

Reconsidering the role of carbonate ion concentration in calcification by marine organisms

“…Most studies assume that HCO − 3 rather than CO 2− 3 is the key inorganic carbon ion (e.g., Allemand et al, 2004;Mackinder et al, 2010;Stumpp et al, 2012;Taylor et al, 2012). However, proof of this assumption on a physiological level is still missing because attempts to unequivocally determine the inorganic carbon molecule transported by molecular transport systems have not been successful so far (Pushkin and Kurtz, 2006;Mackinder et al, 2010;Lee et al, 2013;Romero et al, 2013).…”

“…This removal should be relatively easy when seawater [H + ] is low (i.e., pH is high). It could become more difficult, however, when seawater [H + ] rises and the inside-out [H + ] gradient shrinks (Cyronak et al, 2015;Jokiel, 2011b;Taylor et al, 2011;Stumpp et al, 2012;Venn et al, 2013).…”

“…(Keul et al, 2013), and larvae of the Pacific oyster Crassostrea gigas (Gazeau et al, 2011) an inhibitor (Bach et al, 2011(Bach et al, , 2013(Bach et al, , 2015Jokiel, 2011aJokiel, , b, 2013Jokiel et al, 2014 (Bach et al, 2015). Indeed, HCO − 3 uptake is a common mechanism to buffer acidosis and might be facilitated at higher HCO − 3 availability (Boron, 2004;Melzner et al, 2009;Stumpp et al, 2012).…”

CO2-induced fertilization impairment in Strongylocentrotus droebachiensis collected in the Arctic

“…-dependent HCO 3 -transport and avoiding the loss of HCO 3 -throughout the intestinal epithelium (Holtmann et al 2013). This suggests the presence of a bicarbonate buffer system that allows calcification even at environmentally inadequate pH values (Stumpp et al 2012a;Holtmann et al 2013). Yet, the plasma membrane of unfertilized eggs contains a permanent and reversible Na ?…”

“…Further studies on compensation mechanisms in the eggs have still to be done in order to obtain better conclusions, to link published results and to elucidate the role of these mechanisms in the frame of OA during fertilization. More research is also necessary to clarify which of the already described transporters involved in the pH i -regulatory machinery of larvae's PMCs (Stumpp et al 2012a) are actually playing a role in pH regulation in eggs prior to fertilization, to cellular division and to the beginning of transcription processes in the zygotes.…”