Fig. 6.
Schematic model summarizing the interplay of calcification, pH regulation, and energetic costs in sea urchin larvae during environmental acidification. The primary body cavity (PBC) pH conforms to the surrounding seawater pH (pHsw). A decrease in pHe directly affects the calcifying primary mesenchyme cell (PMC) syncytia challenging the intracellular pH (pHi) regulatory machinery due to decreased proton gradients. The vesicular precipitation of amorphous calcium carbonate (ACC) within PMCs is intrinsically linked to pHi regulation. pH homeostasis is maintained by ion transporters, which either directly or indirectly depend on the consumption of energy. Increased energetic costs due to decreased proton gradients lead to a shift in the larvae’s energy budget, which decreases scope for growth and may also translate into juvenile impaired fitness. ST, stomach; putative transporters are in gray.