Benthic marine calcifiers coexist with CaCO₃‐undersaturated seawater worldwide

Abstract

Ocean acidification and decreasing seawater saturation state with respect to calcium carbonate (CaCO 3 ) minerals have raised concerns about the consequences to marine organisms that build CaCO 3 structures. A large proportion of benthic marine calcifiers incorporate Mg 2+ into their skeletons (Mg-calcite), which, in general, reduces mineral stability. The relative vulnerability of some marine calcifiers to ocean acidification appears linked to the relative solubility of their shell or skeletal mineralogy, although some organisms have sophisticated mechanisms for constructing and maintaining their CaCO 3 structures causing deviation from this dependence. Nevertheless, few studies consider seawater saturation state with respect to the actual Mg-calcite mineralogy (Ω Mg-x ) of a species when evaluating the effect of ocean acidification on that species. Here, a global dataset of skeletal mole % MgCO 3 of benthic calcifiers and in situ environmental conditions spanning a depth range of 0 m (subtidal/neritic) to 5600 m (abyssal) was assembled to calculate in situ Ω Mg-x . This analysis shows that 24% of the studied benthic calcifiers currently experience seawater mineral undersaturation (Ω Mg-x < 1). As a result of ongoing anthropogenic ocean acidification over the next 200 to 3000 years, the predicted decrease in seawater mineral saturation will expose approximately 57% of all studied benthic calcifying species to seawater undersaturation. These observations reveal a surprisingly high proportion of benthic marine calcifiers exposed to seawater that is undersaturated with respect to their skeletal mineralogy, underscoring the importance of using species-specific seawater mineral saturation states when investigating the impact of CO 2 -induced ocean acidification on benthic marine calcification.

Characterization of the exoskeleton of the Antarctic king crab Paralomis birsteini

“…Laboratory experiments have suggested that ocean acidification will negatively affect calcification in marine organisms, but our understanding of species’ capacity for physiological adjustment is in its infancy. The waters off the western Antarctic Peninsula are naturally undersaturated with respect to the various forms of CaCO 3 (Feely et al., ; Lebrato et al., ). In some areas, the saturation horizon occurs in depths as shallow as ~100 m (Jones et al., ).…”

“…The long‐term ability of marine calcifiers to acclimatize and adapt has not been as thoroughly investigated (Kelly & Hofmann, ), and relatively little research has examined calcification in species that are already living in environments limited in CaCO 3 (but see Hall‐Spencer et al., ; Thomsen et al., ; Thresher, Tilbrook, Fallon, Wilson, & Adkins, ; Shamberger et al., ). Globally, many calcifying species are already exposed to undersaturated seawater, in particular those inhabiting depths >1,200 m (Lebrato et al., ).…”

“…Off the western Antarctic Peninsula (WAP), the saturation horizon for aragonite is at ~100‐m depth (Jones et al., ). The waters over the continental slope and rise (~500–2,000 m) are already undersaturated with respect to the specific Mg calcite mineralogy of many species that secrete high‐Mg calcite (Lebrato et al., ). Predatory king crabs (Lithodidae) currently inhabiting the Antarctic slope (Aronson et al., ; Smith, Aronson, Steffel et al., ; Smith, Aronson, Thatje et al., ) may be negatively impacted by the limited availability of CaCO 3 resources and the cold seawater temperatures, and as a result, their exoskeletons may be weakly constructed compared with temperate and tropical crustaceans from shallow, nearshore waters.…”

Antarctica: The final frontier for marine biological invasions

“…This will have significant consequences, especially for biofouling communities: for example, a decreased proportion of encrusting (Spirorbid) worms and an increase in ascidians and sponges (Peck et al, ). The higher solubility of carbonate at low temperatures means the polar oceans may be particularly affected by undersaturation in the coming decades (Feely, Doney, & Cooley, ; Lebrato et al, ). This contributes to difficulties building external skeletons for taxa such as bivalves, gastropods and echinoderms, which are typically smaller and have thinner shells at high latitudes (McClintock et al, ; Watson et al, ) and for which shell‐building is costly (Watson et al, ).…”

Controls on the standing crop of benthic foraminifera at an oceanic scale

“…A wide variety of benthic calcifiers live in waters undersaturated with CaCO 3 , using a range of sophisticated mechanisms for constructing and maintaining their CaCO 3 structures (Lebrato et al 2016).…”