Biogeography and Functional Traits of Fungi in the Ocean

Marine fungi are increasingly recognized as hidden but pivotal drivers of the oceans carbon cycle. Despite their relatively low abundance compared to bacteria, fungi possess unique enzymatic repertoires capable of decomposing complex, recalcitrant organic matter such as chitin, lignin-like polymers, and macroaggregate matrices. By breaking down these otherwise persistent carbon pools, fungi accelerate organic matter turnover, reshape carbon residence times, and influence the balance between carbon sequestration and remineralization in the deep sea. However, the true ecological significance of fungi remains vastly underestimated. Their diversity, activity, and interactions with other microorganisms are still poorly resolved, representing one of the largest blind spots in marine biogeochemistry. Understanding the biogeography of marine fungi is therefore essential. Their distributions are shaped by environmental gradients such as temperature, oxygen, and particle flux, but current global datasets are fragmented and taxonomically inconsistent. Without a clear picture of where different fungal lineages occur, how they respond to shifting climate regimes, and what ecological roles they play across depth layers (epipelagic, mesopelagic, bathypelagic), we cannot integrate fungi into predictive models of future ocean carbon cycling. Mapping their global diversity and environmental associations will reveal whether key functional guilds, degraders, parasites, symbionts, particle colonizers are expanding, contracting, or shifting with ongoing climate change. To resolve these knowledge gaps, a multi-omics framework is indispensable. Metagenomics provides genomic potential and expands the fungal tree of life with thousands of previously unknown environmental lineages. Metatranscriptomics and metaproteomics reveal which pathways are actively expressed in situ, uncovering their real-time roles in organic matter degradation. Integrating these layers enables us to move beyond presence/absence toward a mechanistic understanding of fungal function, metabolic flexibility, and cross-kingdom interactions with bacteria and other plankton. Only through such an integrated approach can we quantitatively assess the contribution of fungi to carbon fluxes and identify the environmental drivers governing their distribution and activity. Together, these efforts will transform our understanding of the oceans microbial engine. By uncovering the diversity, function, and ecological networks of marine fungi, we can finally begin to incorporate this overlooked kingdom into global carbon models and predict how their roles may shift in a warming, increasingly stratified ocean.