The impacts of ocean acidification on calcifying macroalgae
The ecophysiology of calcified macroalgal species of the genera Corallina (C. officinalis and C. caespitosa) and Ellisolandia (E. elongata) (Corallinales, Rhodophyta) was examined in intertidal rock pools of the NE Atlantic, to facilitate predictions of ocean-acidification and warming impacts on these ecosystem engineers. An initial phylogenetic study highlighted significant cryptic diversity within the genus Corallina, and demonstrated that C. officinalis is restricted predominantly to the North Atlantic, while the recently established C. caespitosa shows a cosmopolitan distribution. Three subsequent studies were performed across the NE Atlantic (Iceland to northern Spain) to examine (i) the production, respiration, calcification and growth of Corallina in relation to irradiance, water temperature, and carbonate chemistry; (ii) the photoacclimation and photoregulation strategies of Corallina and Ellisolandia; and (iii) the recent-past (1850 – 2010) and present-day skeletal mineralogy (Mg/Ca ratios) of Corallina and Ellisolandia and its relationship to sea surface temperature. Data demonstrated that species currently experience significant seasonal and tidal fluctuations in abiotic conditions that may be important when considering future responses to ocean-acidification and climate-change. Seasonality in production, calcification and growth were demonstrated, with decreasing growth observed with increasing latitude. Photoacclimation to allow maximal light utilisation during winter periods, and photoregulation via nonphotochemical quenching were highlighted as important in allowing Corallina and Ellisolandia to maintain maximal productivity while controlling for photo-stress. Seasonal cycles in skeletal Mg incorporation were demonstrated with strong relation to sea surface temperature, though no significant change in skeletal mineralogy was evident since pre-industrial times. Taken together, data indicated that Corallina and Ellisolandia have the potential to survive under future ocean-acidification and warming conditions, though loss of species at high latitudes and shifts in the relative abundances of species across the region is likely to be evident, with overall range contraction predicted for C. officinalis due to both warming and ocean-acidification impacts.