Effects of artificial light at night and warming on seaweed-associated microbial communities

Marine ecosystems are rapidly changing due to multiple stressors, which often co-occur and can interact in complex ways. Artificial Light at Night (ALAN) is a global stressor that can interact with ocean warming, disrupting ecosystems directly or indirectly, e.g. through changes to ecological interactions involving habitat-forming species. Microbes play critical roles in the functioning of coastal habitat-forming species such as seaweeds. Disruptions to seaweed-microbe interactions can therefore have significant consequences for coastal habitats, but we have little understanding of the independent and interactive effects of ALAN on seaweed-associated microbial communities. We tested the independent and combined effects of ALAN and warming on microbial communities associated with the habitat-forming seaweeds Ecklonia radiata and Sargassum sp. In Ecklonia, while ALAN increased the relative abundance of two abundant, potentially light-responsive taxa, Dokdonia sp000212355 and an unidentified ASV from Order Pseudomonadales, warming had the opposite effect. Warming also increased microbial community dispersion and was associated with trends of higher relative abundance of putative pathogenic and agarolytic taxa (microbes capable of degrading algal polysaccharides). In contrast, neither ALAN nor warming affected the dominant taxa on Sargassum, or overall community composition and cyanobacteria relative abundance in either species, despite expectations that cyanobacteria would respond to ALAN due to their photosynthetic capacity. While ALAN had seaweed-specific effects on some abundant microbial taxa, our results suggest that microbial communities were resilient overall, with no interactive effects with warming. Our findings highlight the importance of considering species-specific microbial responses to ALAN and warming, with implications for coastal management.