Abstract
Coralline algae are a significant component of the benthic ecosystem. Their ability to withstand physical stresses in high energy environments relies on their skeletal structure which is composed of high Mg-calcite. High Mg-calcite is, however, the most soluble form of calcium carbonate and therefore potentially vulnerable to the change in carbonate chemistry resulting from the absorption of anthropogenic CO2 by the ocean. We examine the geochemistry of the cold water coralline alga Lithothamnion glaciale grown under predicted future (year 2050) high pCO2 (589 μatm) using Electron microprobe and NanoSIMS analysis. In the natural and control material, higher Mg calcite forms clear concentric bands around the algal cells. As expected, summer growth has a higher Mg content compared to the winter growth. In contrast, under elevated CO2 no banding of Mg is recognisable and overall Mg concentrations are lower. This reduction in Mg in the carbonate undermines the accuracy of the Mg/Ca ratio as proxy for past temperatures in time intervals with significantly different carbonate chemistry. Fundamentally, the loss of Mg in the calcite may reduce elasticity thereby changing the structural properties, which may affect the ability of L. glaciale to efficiently function as a habitat former in the future ocean.
Highlights
Coralline algae are a major contributor to the marine ecosystems within the photic zone from the cold high latitudes to the tropics[1,2]
Bulk X-ray diffraction (XRD) confirms that the natural sample is composed of high-Mg calcite (Fig. S1) and that dolomite, as reported in other coralline algae[22,23], was not present in our specimens at the beginning of the experiment
Electron microprobe transects document the expected cyclicity in Mg concentration in response to its temperature-dependent Mg incorporation[10,24,25] of the pre-cultured material reflecting summer and winter growth (Fig. 1)
Summary
Coralline algae are a major contributor to the marine ecosystems within the photic zone from the cold high latitudes to the tropics[1,2]. We assess changes in the geochemistry and crystallography of the non-geniculated cold water coralline alga Lithothamnion glaciale, one of the three main maerl bed-forming species in the northern latitudes, grown under natural conditions (summer and winter), in the laboratory under controlled ambient (the control) and at high CO2 conditions (589 μ atm)[15]. We use Electron Microprobe and nanoscale secondary ion mass spectrometry (NanoSIMS) analyses to generate elemental maps of Mg and Sr at very high spatial resolution on both natural and laboratory-cultured specimens exposed to high CO2 These data allow us to evaluate if changes in chemistry might provide an acclamatory response for these species to maintain their structural integrity
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