Vitamin B 12 conveys a protective advantage to phycosphere-associated bacteria at high temperatures.

Many marine microbes require vitamin B 12 (cobalamin) but are unable to synthesize it, necessitating reliance on other B 12 -producing microbes. Thus, phytoplankton and bacterioplankton community dynamics can partially depend on the production and release of a limiting resource by members of the same community. We tested the impact of temperature and B 12 availability on the growth of two bacterial taxa commonly associated with phytoplankton: Ruegeria pomeroyi, which produces B 12 and fulfills the B 12 requirements of some phytoplankton, and Alteromonas macleodii, which does not produce B 12 but also does not strictly require it for growth. For B 12 -producing R. pomeroyi, we further tested how temperature influences B 12 production and release. Access to B 12 significantly increased growth rates of both species at the highest temperatures tested (38 °C for R. pomeroyi, 40 °C for A. macleodii) and A. macleodii biomass was significantly reduced when grown at high temperatures without B 12 , indicating that B 12 is protective at high temperatures. Moreover, R. pomeroyi produced more B 12 at warmer temperatures but did not release detectable amounts of B 12 at any temperature tested. Results imply that increasing temperatures and more frequent marine heatwaves with climate change will influence microbial B 12 dynamics and could interrupt symbiotic resource sharing.