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.