Abstract
Climate change is driving global declines of marine habitat-forming species through physiological effects and through changes to ecological interactions, with projected trajectories for ocean warming and acidification likely to exacerbate such impacts in coming decades. Interactions between habitat-formers and their microbiomes are fundamental for host functioning and resilience, but how such relationships will change in future conditions is largely unknown. We investigated independent and interactive effects of warming and acidification on a large brown seaweed, the kelp Ecklonia radiata, and its associated microbiome in experimental mesocosms. Microbial communities were affected by warming and, during the first week, by acidification. During the second week, kelp developed disease-like symptoms previously observed in the field. The tissue of some kelp blistered, bleached and eventually degraded, particularly under the acidification treatments, affecting photosynthetic efficiency. Microbial communities differed between blistered and healthy kelp for all treatments, except for those under future conditions of warming and acidification, which after two weeks resembled assemblages associated with healthy hosts. This indicates that changes in the microbiome were not easily predictable as the severity of future climate scenarios increased. Future ocean conditions can change kelp microbiomes and may lead to host disease, with potentially cascading impacts on associated ecosystems.
Highlights
Climatic change is affecting biodiversity at a global scale [1]
We found a strong interactive effect of warming and acidification on 12 operational taxonomic units (OTUs); generally, these OTUs were found in higher abundances on kelp under the higher temperature and/or future pH treatments
Microbiomes of habitat-formers may be impacted by ocean climate change with implications for the health, persistence and resilience of entire marine ecosystems
Summary
Climatic change is affecting biodiversity at a global scale [1]. In the marine realm, ocean warming and acidification are driving regime shifts whereby dominant habitat-forming species, such as corals and large seaweeds, are being replaced by less complex and less productive habitats, affecting biodiversity [2,3,4]. Microbial communities on the surface of kelp across all treatments were sampled via swabbing each of 2 haphazardly chosen kelp individuals per mesocosm (n 1⁄4 6 per treatment) at 8, 16 and 31 days after the experiment commenced (N 1⁄4 72), ensuring independent samples of visually healthy tissue through time. Dozens to hundreds of OTUs were found to significantly differ among the treatments (see Results; electronic supplementary material, tables S2 and S3) We focused on those OTUs where the effect of warming, acidification, kelp condition and/or their combination was strongest, defined here as those for which the absolute effect size was greater than twice the standard deviation (s.d.). Residual plots were checked to ensure good model fit and the absence of overdispersion [49]
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