Abiotic stress destabilizes the bacterial community of sugar kelp, Saccharina latissima (Phaeophyceae).

As climate change progresses, the intensity and variability of freshwater outflow into the ocean are predicted to increase. The resulting increase in low-salinity events, paired with other abiotic stressors (including increasing temperatures), will be a source of stress for the kelp Saccharina latissima (Saccharina hereafter) and potentially Saccharina-associated bacteria. Bacteria influence host health and can facilitate or hinder host survival and acclimation to stressful abiotic conditions. Therefore, understanding how bacterial communities change under abiotic stress is critical for understanding how abiotic stress will affect kelp physiology. We investigated the effect of abiotic stress on Saccharina and associated bacteria by surveying the bacterial community associated with Saccharina across naturally occurring salinity and temperature gradients, coupled with salinity manipulation experiments. Overall, Saccharina harbored a stable core bacterial community, which decreased in relative abundance under abiotic stress. In the field, both salinity and temperature shaped the bacterial community, with temperature having higher explanatory power most of the time. In the lab, we confirmed that the patterns observed in the field could be replicated by manipulating salinity alone. Decreased relative abundance of core bacteria and increased community dissimilarity in low-salinity in the lab suggest that low-salinity alone can induce a stress response, detectable in the bacterial community of Saccharina.