Contrasting associations between microbiota respiratory strategies and seafloor macrofauna

Microbe-macrofauna associations are central to coastal benthic ecosystems, yet the microbial functions that underpin these remain poorly understood. To address functional microbe-macrofauna associations, we analyzed 245 seafloor samples from Norwegian and Icelandic coastal zones, encompassing 1204 macrofaunal taxa, 121 543 zero-radius operational taxonomic units, and 302 high-quality metagenome-assembled genomes (MAGs). We first analyzed potential confounding factors and found that macrofauna showed the strongest association with a north-south geographic gradient, while overall there was a concordance for metadata associations. We found that macrofaunal assemblages were positively correlated with either a Bacteroidota-enriched cluster of MAGs ("BA"; n = 178 MAGs) or a cluster enriched in Actinomycetota and Pseudomonadota ("AP"; n = 124 MAGs). Functionally, the AP cluster was enriched in oxidases, cytochrome c-based respiratory processes, and fatty-acid β-oxidation. In contrast, the BA cluster showed enrichment in hydrolases, polysaccharide-degradation functions, and microaerobic/anaerobic respiration via cytochrome bd. Macrofaunal taxa associated with BA tended to have higher tolerance values for anthropogenic disturbance than those associated with AP, although this association was weak. Taken together, these results identify contrasting microbial respiratory strategies that co-vary with macrofaunal assemblages, while further experimental work is required to establish directionality and mechanism.