Bivalve shell formation in a naturally CO2-enriched habitat: Unraveling the resilience mechanisms from elemental signatures

Abstract

Marine bivalves inhabiting naturally pCO₂-enriched habitats can likely tolerate high levels of acidification. Consequently, elucidating the mechanisms behind such resilience can help to predict the fate of this economically and ecologically important group under near-future scenarios of CO₂-driven ocean acidification. Here, we assess the effects of four environmentally realistic pCO₂ levels (900, 1500, 2900 and 6600 μatm) on the shell production rate of Mya arenaria juveniles originating from a periodically pCO₂-enriched habitat (Kiel Fjord, Western Baltic Sea). We find a significant decline in the rate of shell growth as pCO₂ increases, but also observe unchanged shell formation rates at moderate pCO₂ levels of 1500 and 2900 μatm, the latter illustrating the capacity of the juveniles to partially mitigate the impact of high pCO₂. Using recently developed geochemical tracers we show that M. arenaria exposed to a natural pCO₂ gradient from 900 to 2900 μatm can likely concentrate HCO₃^- in the calcifying fluid through the exchange of HCO₃^-/Cl^- and simultaneously maintain the pH homeostasis through active removal of protons, thereby being able to sustain the rate of shell formation to a certain extent. However, with increasing pCO₂ beyond natural maximum the bivalves may have limited capacity to compensate for changes in the calcifying fluid chemistry, showing significant shell growth reduction. Findings of the present study may pave the way for elucidating the underlying mechanisms by which marine bivalves acclimate and adapt to high seawater pCO₂.

Keywords: Acclimation and adaption; Geochemical tracers; Marine bivalve mollusks; Ocean acidification; Shell growth rate.