Variable [Mg2+] in seawater forces mineralogy change in Spirobranchus cariniferus calcification

Abstract

Environmentally-forced mineralogical change has implications for both strength and solubility of skeletal carbonate. Three marine invertebrates from the southern hemisphere were grown for 50 days in artificial seawater with different Mg/Ca ratios (1.5, 2.5, 3.5, 4.5 and 5.3). Blue mussels (Mytilus edulis) and jingle oysters (Anomia trigonopsis) showed no changes in mineralogy. Blue tubeworms (Spirobranchus cariniferus) were able to maintain their typical mineralogical composition (calcite with 8–12 wt% MgCO3) at most seawater Mg concentrations. However, at the lowest Mg/Ca ratio (1.5), the tubeworms precipitated calcite with significantly lower amounts of MgCO3 (2–4 wt% MgCO3). Current models of passive calcification, based on a linear relationship between seawater Mg/Ca and calcite Mg content, do not accurately reflect this Mg-dependent stepwise change in minerology among organisms with an active calcification strategy. We propose a new “tipping-point” model that better describes the calcification response of blue tubeworms to changing environmental conditions. The boundary of this active/passive tipping point may be species-specific and may have consequences for preservation, alteration, and resilience of marine skeletal carbonate.