Magnesium content, strongly correlated with temperature, has been developed as a climate archive for the late Holocene without considering anatomical controls on Mg content. In this paper we explore the ultrastructure and cellular scale Mg-content variations within four species of North Atlantic crust-forming Phymatolithon. The cell wall has radial grains of Mg-calcite whereas the interfilament (middle lamella) has grains aligned parallel to the filament axis. The proportion of interfilament and cell wall carbonate varies by tissue and species. Three distinct primary phases of Mg-calcite are identified: interfilament Mg-calcite (mean 8.9 mol% MgCO3), perithallial cell walls Mg-calcite (mean 13.4 mol% MgCO3), and hypothallium Mg-calcite (mean 17.1 mol% MgCO3). Magnesium content for the bulk crust, an average of all phases present, shows a strongly correlated (R2= 0.975) increase of 0.31 mol% MgCO3/°C. Of concern for climate reconstructions is the potential for false warming signals from undetected post-grazing wound repair carbonate that is substantially enriched in Mg, unrelated to temperature. Within a single crust, component carbonates can range from Mg content as stable as aragonite (8 mol% MgCO3), up to 150% higher (20 mol% MgCO3) a predicted unstable high magnesium calcite. It is unlikely that existing current predictions of ocean acidification impact on coralline algae, based on saturation states calculated using average Mg contents, provide an environmentally-relevant estimate.
Nash M. C., & Adey W., in press. Multiple phases of mg-calcite in crustose coralline algae suggest caution for temperature proxy and ocean acidification assessment: lessons from the ultrastructure and biomineralisation in Phymatolithon (Rhodophyta, Corallinales). Journal of Phycology. Article (subscription required).
