Vulnerability to ocean acidification of marine calcifying organisms cannot be predicted from the mineral type in their shells

Abstract

This essay challenges the assumption that the vulnerability of marine calcifiers to ocean acidification (OA) can be inferred from the calcium carbonate polymorph (two different polymorphs of a crystal have the same chemical composition but a different crystal structure) in their shells. Anthropogenic carbon dioxide (CO₂) emissions have led to both an increase in atmosphere/surface ocean carbon dioxide concentrations and a decrease in ocean pH, a phenomenon termed OA (Caldeira and Wickett 2005; Doney et al. 2009). The alteration of calcium carbonate (CaCO₃) chemistry of seawater (also termed carbonate chemistry, carbonate system, carbon system, and C-system) includes associated parameters, for example, the saturation state of seawater (omega) with respect to CaCO₃ minerals such as aragonite and calcite, commonly referred to as omega aragonite and omega calcite, respectively (Zeebe and Wolf-Gladrow 2001). Aragonite is a metastable polymorph of CaCO₃ and is more soluble than calcite (Morse et al. 1980). At ALOHA station Hawaii, omega aragonite decreased from 3.7 in 1990 to 3.6 in 2007, and omega calcite decreased from 5.8 in 1990 to 5.6 in 2007 (Doney et al. 2009). In 1973, omega aragonite was 4.25, and omega calcite 6.4 (Doney et al. 2009). Omega values above 1 indicate supersaturation (no dissolution of the respective mineral), and values below 1 indicate undersaturation (dissolution of the respective mineral, Zeebe and Wolf-Gladrow 2001). In the Southern Ocean, surface waters might permanently experience omega aragonite values below 1, that is, become corrosive for aragonite, by the year 2100, with winter-time undersaturation reached as early as 2030 (Orr et al. 2005; McNeil and Matear 2008).

The large majority of CaCO₃ production in the modern ocean is regulated by calcite and aragonite formation by calcifying organisms (calcifiers) (Morse and Mackenzie 1990). Many marine shell forming organisms indeed use either aragonite or calcite (or both) in their biominerals (Lowenstam 1981). These shells might dissolve in undersaturated waters with detrimental consequences for the organism. Note that in supersaturated bulk seawater significant dissolution of biogenic calcium carbonate does occur, showing that undersaturated microenvironments must exist (Ziveri et al. 2023; Sulpis et al. 2021; Dean et al. 2024; Kwon et al. 2024). Dissolution of shells in these microenvironments might impact both calcifiers and grazers.

Aragonite dissolution can create conditions that slow down or even stop calcite dissolution in marine sediments (Sulpis et al. 2022; van de Mortel et al. 2024). This observation, however, does not suffice to assess the vulnerability of aragonitic compared to calcitic calcifiers. We will unpack this statement in the following.

Gerald Langer: Conception, Writing, Patrizia Ziveri: Writing.

The authors declare no conflict of interests.