Dolomite-rich coralline algae in reefs resist dissolution in acidified conditions

Abstract

Coral reef ecosystems develop best in high-flow environments but their fragile frameworks are also vulnerable to high wave energy. Wave-resistant algal rims, predominantly made up of the crustose coralline algae (CCA) Porolithon onkodes and P. pachydermum 1,2 , are therefore critical structural elements for the survival of many shallow coral reefs. Concerns are growing about the susceptibility of CCA to ocean acidification because CCA Mg-calcite skeletons are more susceptible to dissolution under low pH conditions than coral aragonite skeletons 3 . However, the recent discovery 4 of dolomite (Mg 0.5 Ca 0.5 (CO 3 )), a stable carbonate 5 , in P. onkodes cells necessitates a reappraisal of the impacts of ocean acidification on these CCA. Here we show, using a dissolution experiment, that dried dolomite-rich CCA have 6-10 times lower rates of dissolution than predominantly Mg-calcite CCA in both high-CO 2 (∼700 ppm) and control (∼380 ppm) environments, respectively. We reveal this stabilizing mechanism to be a combination of reduced porosity due to dolomite infilling and selective dissolution of other carbonate minerals. Physical break-up proceeds by dissolution of Mg-calcite walls until the dolomitized cell eventually drops out intact. Dolomite-rich CCA frameworks are common in shallow coral reefs globally and our results suggest that it is likely that they will continue to provide protection and stability for coral reef frameworks as CO 2 rises.Coralline algae form extensive carbonate structures on the highenergy windward side of many tropical coral reefs. For example, the algal rim on the fringing reef of Rodrigues Island (Indian Ocean) is ∼11 km long, 4 m thick and in parts protrudes ∼1m above the reef flat 6 , providing substantial protection for island communities from high-energy waves. Only the surface veneer (the top few millimetres) of CCA is living 7 and the dense carbonate underneath the algal rim is predominantly in situ CCA skeleton and overlapping layers of coral branches cemented together by CCA crusts 6 . Development of these reef structures is dependent on preservation of the dead CCA skeleton post-mortem. Thus, understanding how declining seawater pH will affect this skeletal preservation is of paramount importance if we are to understand the changes to coral reef structural stability in a high-CO 2 world.In the 1950s-1970s the mineral composition of coralline algae skeletons was determined to be ∼12-18 mol% Mg-calcite [8][9][10] . However, many bulk chemical analyses of tropical coralline algae showed a surplus of magnesium compared with those determined by X-ray diffraction (XRD) using Mg-calcite peak position 8,9 . Recently, we discovered this discrepancy was due to the presence of previously undetected dolomite and magnesite (MgCO 3 ) forming within the cell spaces 4 . Past determinations of thermodynamic solubility for coralline algae were based on the assumption that they were composed of Mg-calcite 10-12 . As dolomite was not believed to form in the modern marine environment, determining d...

Persistent natural acidification drives major distribution shifts in marine benthic ecosystems

“…They also challenge the widespread concern over the global fate of calcified algae in the near future, as there are species such as P. rosa-marina showing a moderate resilience to OA [36]. The recent discovery of calcium-magnesium carbonate (dolomite) in crustose coralline algae [37] and its lower solubility in comparison to magnesium calcite [38] suggest that some calcifying algae will not be doomed under moderate decreases in pH. This highlights the need of further research on the carbonate composition of these organisms.…”

Pacific-wide contrast highlights resistance of reef calcifiers to ocean acidification

“…Aside from growth rates as a means to explain varied responses to OA, differential sensitivities to OA among sites in the calcified algae might reflect different mineral compositions of the skeletons. For example, a varying proportion of magnesium, or the presence of dolomite and magnesite that are less sensitive to dissolution, could underlie differential responses of algae to OA [55].…”

Ocean acidification effects on calcifying macroalgae

“…The combination of decreasing skeletal solubility by decreasing the Mg:Ca ratio and speeding up calcification rates may be an attempt by coralline algae to compensate for high dissolution that may be occurring under low pH conditions. Nevertheless, high-Mg calcite de positing species seem to be more heavily impacted by elevated CO 2 than aragonite-depositing macroalgae, with the exception of dolomite-depositing coralline algae (Nash et al 2012. Porolithon spp.…”

“…However, when the calcium (Ca 2+ ) ions are replaced by magnesium (Mg 2+ ) ions and the ratio of Mg:Ca becomes greater than 0.04, the CaCO 3 mineral is considered high-Mg calcite, which is the most soluble CaCO 3 crystal in seawater. Another form of calcite, dolomite (Mg 0.5 Ca 0.5 [CO 3 ]), is deposited along the cell rims in some tropical calcifying macroalgae and is less soluble in seawater than high-Mg calcite (Nash et al 2012. Aragonite crystals have an orthorhombic shape, and are more soluble in seawater than calcite, but less soluble than high-Mg calcite.…”