Response of marine microbes to iron contained in colloids of glacial origin: a Kerguelen Island case study

The trace element iron (Fe) is a major constraint for microbially mediated processes in the Southern Ocean. The accelerated melting of glaciers could present a novel source of Fe, but whether glacial Fe is bioavailable to marine microbes is not known. We investigated the response of marine heterotrophic prokaryotes to Fe contained in colloids (20-200 nm) of glacial origin collected on Kerguelen Island (Southern Ocean). We followed prokaryotic growth in incubation experiments amended with colloids of either glacial or nonglacial origin and determined community composition and the abundance of genes involved in Fe-related processes in metagenomes and metagenome-assembled genomes (MAGs) at the final time point. Prokaryotic taxa belonging to <i>Vibrionaceae</i> and <i>Pseudomonadaceae</i> accounted together for 32% to 67% of the relative abundances in the glacial colloid-amended treatments, while <i>Rhodobacteraceae</i>, <i>Flavobacteriaceae</i>, and <i>Alteromonadaceae</i> were the dominant contributors to the communities in the incubations amended with nonglacial colloids. Metagenomic analysis revealed a higher abundance of genes involved in the biosynthesis of the siderophores pyoverdine and vibrioferrin as well as their respective transporters in the presence of glacial colloids compared to nonglacial colloids. Genes for the transport of both siderophores were present in diverse MAGs, while biosynthesis genes were detected in fewer MAGs. Our results suggest that the utilization of siderophores facilitates access to Fe from glacial colloids and points to the key role of specific prokaryotes in rendering this source of Fe available to Southern Ocean microbial communities.