Limited microbial community responses of marine macroalgae to artificial light at night and moderate warming conditions

Multiple stressors such as Artificial Light at Night (ALAN) and warming are increasingly common in marine systems and can interact in complex ways. Microbial communities play critical roles in the functioning of coastal habitat-forming species such as seaweeds, however the effects of ALAN on seaweed-associated microbial communities remain unknown. 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, ALAN increased the relative abundance of two potentially light-responsive taxa: Dokdonia sp000212355 and an unidentified ASV from Pseudomonadales, whereas warming had the opposite effect. Warming increased microbial community dispersion in Ecklonia and resulted in non-significant increases in relative abundance of putative pathogenic and agarolytic taxa (microbes capable of degrading algal polysaccharides). However, further analyses using metagenomics are needed to confirm functional roles. In contrast, neither ALAN nor warming affected dominant taxa associated with Sargassum. Contrary to expectations, cyanobacteria relative abundance was unaffected by ALAN in either seaweed host, despite their photosynthetic capacity. We found limited evidence for interactive effects of ALAN and warming, and community composition remained unchanged in both seaweed species. Our findings highlight the importance of considering species-specific microbial responses to ALAN and warming, with implications for coastal management.