Fungal contribution to marine nitrogen cycling

Fungi in terrestrial environments are known to play a key role in denitrification and heterotrophic nitrification. In contrast, the role of fungi in marine nitrogen cycling has remained underexplored and largely neglected, especially in the water column where prokaryotic organisms outnumber fungi by at least several orders of magnitude. Nitrous oxide (N₂O) is the end product of fungal denitrification due to the lack of nitrous oxide reductase in fungi. Some fungi can co-respire oxygen and nitrate under hypoxia and reduce nitrate to ammonia under anoxia. Our goal is to shed new light on well-studied nitrogen cycling processes in the marine environment from the fungal perspective.

We coupled ¹⁵N-tracer experiments with a selective inhibition method to determine the contribution of marine fungi to N₂O production in the eastern tropical North Pacific oxygen minimum zone. The highest amount of fungal N₂O production was measured at the oxic-anoxic interface of the water column, where fungi accounted for ~10% of total N₂O production. Metagenomes from the same region harbored the fungal-specific cytochrome P450 nitric oxide reductase (P450nor). We assayed the fungal diversity targeting the internal transcribed spacer region and identified the major members of the fungal community including many novel lineages. Our findings suggest that fungi play an active role in marine nitrogen cycling and highlight the necessity to consider fungi-mediated processes in future studies.