The global warming debate has sparked an unprecedented interest in temperature effects on coccolithophores. We show that sub-optimal growth temperatures lead to an increase in malformed coccoliths in a strain-specific fashion and the inorganic / organic carbon has a minimum at optimum growth temperature. Global warming might cause a decline in coccoliths' inorganic carbon contribution to the "rain ratio", as well as improved fitness in some genotypes by reducing coccolith malformation.