Impact of high CO2 on the geochemistry of the coralline algae Lithothamnion glaciale

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.

Figure 1. Two Mg/Ca (mmol/mol) transects for each sample were measured using EMPA.

The data is represented as a 30 point moving average. Error bars in the top right corner of each graph indicate the uncertainty based on the counting statistics. The grey line demarks the Alizarin staining at the beginning of the experiment. Data left of the line represents growth under laboratory conditions while data right of the graph shows growth in the natural environment. Reconstructed temperature (°C) is shown on the secondary vertical axis using L. glaciale temperature equation from Kamenos et al. 2008. The yellow horizontal band indicates the culture temperature of the aquaria (7 ± 0.5 °C), the thin lines indicate the seasonal temperate range at the collection site.

Figure 2

NanoSIMS ratio images of Mg/Ca (left) and Sr/Ca (right) of natural growth in the summer (a,b) and winter (c,d). The field of view is 20 × 20 um. Note the pronounced banding in both Mg and Sr in the summer samples (top) versus the lower concentrations and less pronounced banding in the winter sample (bottom). The colour scheme represents the ratio of Mg and Sr to Ca, respectively. Blue represents a ratio of 0.0001 Mg/Ca and magenta a ratio of 0.1 for Mg, and 0.0001 and 0.0085 for Sr.

Figure 3

NanoSIMS ratio images of Mg/Ca (right) and Sr/Ca (left) ratios of material deposited during the culturing experiment under (a,b) control CO₂ (422 μatm) conditions (top) and (c,d) acidified conditions (589 μatm, bottom). Note the similarity of the banding in the control experiment and natural summer growth (Fig. 4 top) and the distinct loss of banding and overall lower concentrations in the material grown under acidified conditions. The field of view is 20 × 20 um. The colour scale is the same as Fig. 3.

Figure 4

Structural comparison of L. glaciale cell wall grown under natural conditions (top) and cultured under high CO₂ conditions (bottom) using secondary electron microscopy (SEM, middle) (b,c) and transmitted electron microscopy (d) (TEM, right). The scale model (a) of L. glaciale on the far left is a modification of Ragazzola et al. (2012). Note the TEM images are not at the same scale. The higher CO₂ growth results in thinner walls and larger cells due to the CO_s fertilisation of the photosynthesis (a,b). The control sample exhibits a narrow central channel structure, with low porosity and small crystallites with little alignment. In contrast the acidified sample shows strongly oriented, elongate crystals filling the central interstitial zone approximately parallel to the wall surface.

Figure 5

(a) SEM image of polished cross section of L. glaciale showing material grown under natural conditions (below the red line) and cultured under high CO₂ conditions (589 μatm, above the red line). Red line indicates the Alizarin staining. Darker areas represent smaller cells and winter growth while lighter areas represent the larger cells grown during the summer. (b) Location of the Electron microprobe analysis (EMPA, in colour) and lift outs for TEM and NanoSIMS (summer, natural environment and 596 μatm CO₂, laboratory experiments). The sample shows clear seasonal growth with lower Mg concentrations (blue colours) in wider bands and higher Mg concentrations (yellow and red colours) in the summer than in the winter.