| Summary: | Emiliania huxleyi is an important calcifier and primary producer, playing key roles in global carbon cycling. It is thought to be vulnerable to the effects that rising atmospheric pCO2 may have on ocean chemistry. However, past studies show inconsistent phenotypic responses to elevated pCO2 conditions, suggesting an underlying genetic basis for such variation. E. huxleyi isolates from different geographic locations were genotyped according to the coccolith morphology motif (CMM), found in the 3’ untranslated region of a gene coding for a protein thought to be involved in calcification. Isolates were exposed to short-term ambient and elevated pCO2 conditions in order to determine the relative extent that genotype or environment may play on phenotypic plasticity. CMM genotype was associated with morphological and primary productivity response to elevated pCO2 while the calcification response correlated with environmental history; namely sea surface temperature and associated seawater chemistry. We suggest that the regulation of the CMM region is affected by acidification, altering morphology and thus the efficiency of photosynthesis in E. huxleyi. Furthermore, we conclude that increased calcification response of high-latitude E. huxleyi isolates represents the result of adaptive evolution acting to select physiology which increases fitness in environments already indicative of a future higher-pCO2 world. In collaboration with The Marine Biological Association, Devon, UK
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