Coccolith Sr/Ca is a robust temperature and growth rate indicator that withstands dynamic microbial interactions

Coccolithophores are a diverse group of calcifying microalgae that have left a prominent fossil record on Earth. Various coccolithophore relics, both organic and inorganic, serve as proxies for reconstruction of past oceanic conditions. Emiliania huxleyi is the most widely distributed representative of the coccolithophores in modern oceans and is known to engage in dynamic interactions with bacteria. Algal–bacterial interactions influence various aspects of algal physiology and alter algal alkenone unsaturation (UK’37), a frequently used organic coccolithophore-derived paleo-temperature proxy. Whether algal–bacterial interactions influence inorganic coccolithophore-derived paleo-proxies is yet unknown. A commonly used inorganic proxy for past productivity and sea surface temperature is the Sr/Ca ratio of the coccolith calcite. Interestingly, during interactions between bacteria and a population of calcifying algae, bacteria were shown to physically attach only to non-calcified algal cells, suggesting an influence on algal calcification. In this study, we explore the effects of algal–bacterial interactions on calcification and coccolith Sr/Ca ratios. We find that while bacteria attach only to non-calcified algal cells, coccolith cell coverage and overall calcite production in algal populations with and without bacteria is similar. Furthermore, we find that Sr/Ca values are impacted only by water temperature and algal growth rate, regardless of bacterial influences on algal physiology. Our observations reinforce the robustness of coccolith Sr/Ca ratios as a paleo-proxy independent of microbial interactions and highlight a fundamental difference between organic and inorganic paleo-proxies.