Temperature-induced shift in the rare microbiota of the sponge Haliclona

Ocean warming is currently leading to distributional shifts of species and an alternation of coastal communities. Vulnerable species that are most sensitive to ocean warming are able to use several acclimation mechanisms, with one of the fastest being a shift in and shuffling of their partnerships with symbiotic microorganisms. Assessing symbiosis-focused mechanisms of acclimation and adaptation in response to ocean warming is a technical challenge due to the difficulty of accurately simulating the de novo formation of coastal communities. Here, we use the Kiel Outdoor Benthocosm facility to assess which sponges species are experimentally recruited and whether they exhibit symbiosis-focused mechanisms of acclimation following selection to ocean warming. We observed one sponge species (Haliclona sp.) and found that this sponge exhibited significant shifts in the membership and composition of its associated microbiome in response to ocean warming, with much of this being attributed to the rare microbiota. Moreover, Haliclona sp. maintained the diversity and dominance of its microbiome members. Four bacteria taxa were differentially abundant at elevated temperatures, with two being a Francisella sp. that is a suspected pathogen and an uncultured Francisellaceae that is most closely related to sulfur-oxidizing endosymbionts. Changes to the Haliclona sp. microbiome are largely consistent with a limited acclimation response, which could indicate that this sponge may use microbial symbionts as part of a mechanism to acclimate and adapt to a warmer future ocean.