Kelp forest collapse alters the reef microbiome and associated metabolome

In many temperate regions experiencing rapid ocean warming, kelp forests are being replaced by low-lying turf algae. Yet, whether this change in biogenic habitat alters reef-level microbial structure and function, including carbon and nutrient cycling, remains largely unknown. Here, we integrated shotgun metagenomics and nontargeted metabolomics to reveal that kelp forest loss alters the composition of the reef microbial community and its associated biochemical machinery, resulting in distinct metabolomes and microbially driven elemental use/transformations on kelp- vs. turf-dominated reefs. Our results therefore suggest that microbes play a key role in shaping kelp forest ecosystem functioning. Further, they demonstrate that human-induced ocean warming has cascading effects on microbially mediated chemistry, with implications for coastal carbon storage and nutrient regeneration.