Emiliania huxleyi -Bacteria Interactions under Increasing CO 2 Concentrations.

The interactions established between marine microbes, namely phytoplankton-bacteria, are key to the balance of organic matter export to depth and recycling in the surface ocean. Still, their role in the response of phytoplankton to rising CO 2 concentrations is poorly understood. Here, we show that the response of the cosmopolitan Emiliania huxleyi ( E. huxleyi ) to increasing CO 2 is affected by the coexistence with bacteria. Specifically, decreased growth rate of E. huxleyi at enhanced CO 2 concentrations was amplified in the bloom phase (potentially also related to nutrient concentrations) and with the coexistence with Idiomarina abyssalis ( I. abyssalis ) and Brachybacterium sp. In addition, enhanced CO 2 concentrations also affected E. huxleyi 's cellular content estimates, increasing organic and decreasing inorganic carbon, in the presence of I. abyssalis , but not Brachybacterium sp. At the same time, the bacterial isolates only survived in coexistence with E. huxleyi , but exclusively I. abyssalis at present CO 2 concentrations. Bacterial species or group-specific responses to the projected CO 2 rise, together with the concomitant effect on E. huxleyi , might impact the balance between the microbial loop and the export of organic matter, with consequences for atmospheric carbon dioxide.