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.

Multiple phases of mg‐calcite in crustose coralline algae suggest caution for temperature proxy and ocean acidification assessment: lessons from the ultrastructure and biomineralization in Phymatolithon (Rhodophyta, Corallinales)1

“…Here, we have presented quite similar relationships for Mg content and anatomy in Phymatolithon as has been found in Lithothamnion glaciale (Ragazzola et al. ) and P. onkodes (Nash ). All species have elevated Mg in cell walls compared to interfilament.…”

Phymatolithon (Melobesioideae, Hapalidiales) in the Boreal–Subarctic Transition Zone of the North Atlantic

“…These Mg variations are in agreement with the Mg offsets obtained for the cell wall, IF, and hypothallial cells in Leptophytum laeve and Kvaleya epilaeve from Arctic Labrador (Nash and Adey, 2017a). The elevated Mg in perithallial cell walls compared to IF (called interstitial) has also been noted for Lithothamnion glaciale (Ragazzola et al, 2016). Climate archiving, using Mg concentration (and other wall chemistry) as a proxy for seawater temperature (and other variables) at the time of wall formation has become an important tool in paleoclimatology (e.g., Halfar et al, 2013).…”

Anatomical structure overrides temperature controls on magnesium uptake – calcification in the Arctic/subarctic coralline algae <i>Leptophytum laeve</i> and <i>Kvaleya epilaeve</i> (Rhodophyta; Corallinales)

“…Concentric zonations of higher Mg content have been shown, using backscatter electron imaging, in cell walls of tropical Porolithon onkodes (Nash et al, 2011). Ragazzola et al (2016), using NanoSIMS, also showed clear concentric banding of Mg within summer cell walls of Lithothamnion glaciale. These published observations together with the results in this study suggest there could be a strong organic control on Mg distribution within the cell, with this being related to the concentric fibrils.…”

“…Possibly the fibril organics enable higher Mg incorporation than the organics involved in the radial structures. Ragazzola et al (2016) further documented a decreased prominence of Mg banding in winter cells of L. glaciale and for those grown in CO 2 -enriched conditions. Results from our study offer an insight as to possible temperature or CO 2 -driven ultrastructure changes that may result in decreased Mg content.…”

“…In this paper, we rename the inner wall the cell wall and retain the terminology interfilament, noting this is equivalent to the middle lamella in higher plants (Esau, 1953); interfilament has also been referred to as interstitial (Ragazzola et al, 2016). We use the abbreviations PCW and PIF (perithallial cell wall and perithallial interfilament) and HCW and HIF (hypothallial cell wall and interfilament) to designate the carbonate wall components.…”