Above and below-ground bacterial communities shift in seagrass beds with warming temperatures Provisionally Accepted

Luke D. Walker^1* Paul Gribben² Tim Glasby³ Ezequiel M. Marzinelli⁴ Deepa Varkey⁵ Katherine Dafforn¹

• ¹Macquarie University, Australia
• ²University of New South Wales, Australia
• ³Division of Fisheries, New South Wales Department of Primary Industries, Australia
• ⁴The University of Sydney, Australia
• ⁵Australian Institute of Marine Science, Australia

Current rates of ocean warming are predicted to exacerbate ongoing declines in seagrass populations.Above-ground responses of seagrass to increasing temperatures have been studied from a direct physiological perspective while indirect effects, including changes to microbially-mediated belowground processes, remain poorly understood. To test potential effects of increased temperature on seagrass growth and associated microbial communities, we sampled seagrass beds experiencing ambient and elevated water temperatures at Lake Macquarie, Australia. Sites with warmer water were associated with a plume from a power station discharge channel with temperatures analogous to conditions predicted by 2100 under current rates of ocean warming (+3 °C). The microbial community composition in both sediments and leaf tissues varied significantly between warm and ambient water temperatures with higher relative abundances of putative sulphate-reducing bacteria such as Desulfocapsaceae, Desulfobulbaceae and Desulfosarcinaceae in sedimentary communities in warm water. Above-ground biomass and seagrass growth rates were greater at warm sites while below-ground biomass and detrital decomposition rates showed no difference suggesting potential buffering of temperature effects below-ground. These findings suggest a 3 °C rise in temperate regions is unlikely to induce mortality in seagrass however, it may shift microbial communities towards more homogenous structure and composition.