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

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 pH-sensitive 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.

钙化的海胆幼虫对海水碳酸盐化学变化的响应表现为生长减缓与发育延迟。迄今为止，关于海洋酸化如何影响棘皮动物幼虫pH稳态的研究尚未见报道。理解酸碱调节能力至关重要，因为碳酸钙骨骼的细胞内形成与维持依赖于pH稳态。利用氢离子选择性微电极（H(+)-selective microelectrodes）与pH敏感荧光染料BCECF，我们对棘皮动物幼虫的细胞外及细胞内pH（pH(e)与pH(i)）进行了在体测量。我们将长腕幼虫（pluteus larvae）暴露于不同海水CO₂浓度条件下，结果表明，在海水pCO₂升高时，钙化初级间充质细胞（calcifying primary mesenchyme cells, PMCs）周围的细胞外区域pH与周围海水pH保持一致。通过使用异硫氰酸荧光素葡聚糖结合物（FITC dextran conjugates），我们发现海胆幼虫的体被具有通透性。因此，每当海水pH发生变化时，PMCs与骨针会直接暴露于剧烈的细胞外pH（pH(e)）变化中。然而，细胞内pH（pH(i)）测量结果显示，PMCs能够完全代偿诱导的细胞内酸中毒。这一过程高度依赖于钠离子（Na(+)）与碳酸氢根离子（HCO₃⁻），提示存在一种涉及继发性主动钠依赖膜转运蛋白的碳酸氢盐缓冲机制。我们认为，在海洋酸化条件下，尽管细胞外pH（pH(e)）降低，维持细胞内pH（pH(i)）仍能使钙化过程继续进行。然而，这可能会增加能量消耗。钙化或细胞稳态维持所需能量消耗的增加，可能是导致能量分配改变的主要因素之一，进而影响生长，并最终导致浮游生活阶段的海胆幼虫死亡率上升。