Skeletal mineralogy of geniculate corallines: providing context for climate change and ocean acidification research

Marine species depositing high-magnesium (Mg) calcite (>8% MgCO3) are projected to be among the first to show response to the impacts of climate change, i.e. increased sea surface temperature (SST) and ocean acidification (OA), given the increasing solubility of calcite in seawater with increasing Mg content. Temperature is a major driver of Mg incorporation into the skeletons of calcifying macroalgae, and thus climate change may induce deposition of more soluble calcite, exacerbating responses to OA. Assessment of the skeletal Mg content of 3 geniculate, calcifying species of the genera Corallina and Ellisolandia (Rhodophyta, Corallinales), C. officinalis, C. caespitosa and E. elongata, sampled during 2012-2013 in the UK intertidal, demonstrated the existence of seasonal cycles in skeletal Mg. Seasonal cycles in skeletal Mg were also observed for herbarium collections of the Natural History Museum (British Museum), London, sampled during the recent past (1850-2010). Comparative sampling across a northeastern Atlantic latitudinal transect (Iceland to northern Spain) indicated a decreasing Mg content with increasing latitude for present-day C. officinalis, and relationships between SST and Corallina Mg content (r(2) = 0.45-0.76) demonstrated the dominant influence of temperature on Corallina species skeletal mineralogy. Corallina and Ellisolandia species show lower absolute values of Mg content (0.11-0.16 mol% Mg/Ca), and smaller variation with change in SST (0.0028-0.0047 mol% Mg/Ca degrees C-1), than other temperate calcifying macroalgae studied to date. Over the period 1850-2010, no change in the magnitude of Mg incorporation by C. officinalis was detected in herbarium samples. However, the strong relationship between SST and Mg content indicates that projected increases in SST by 2100, which are far greater than temperature increases that occurred between 1850-2010, could have substantial impact on geniculate coralline algae skeletal mineralogy, and must be considered synergistically with the effects of OA.