Amino acids could sustain fungal life in the energy-limited anaerobic sediments below the seafloor.

Unlabelled: Deep-sea sediments harbor abundant microbial communities extending vertically up to ~2.5 km below the seafloor. Despite their prevalence, the reasons for their large community sizes and low energy fluxes remain unclear. Particularly, the reliance of fungi, the predominant eukaryotic group, on amino acids in these energy-limited, anaerobic conditions is poorly understood. We investigated the role of amino acids in the growth and development of the fungus 20R-7-F01, isolated from anaerobic sub-seafloor sediments. The fungus efficiently used all amino acids as carbon sources, and some as nitrogen sources, with specific amino acids influencing sexual reproduction and fruit-body formation. Notably, amino acids with hydrocarbon chains or methyl groups appeared crucial for fruit-body production. The upregulation of genes, metabolites, and pathways related to amino acid metabolism in the fungus under anaerobic conditions underscores the significance of amino acids as energy and nutrient sources in such environments. Amino acids not only served as carbon/nitrogen sources but also contributed to fungal fruit-body formation under low oxygen conditions, vital for long-term fungal survival in the energy-limited deep biosphere. This study sheds light on the crucial role of amino acids in fungal growth and reproduction in energy-limited anaerobic conditions.

Importance: In the depths beneath the ocean floor, where darkness, anaerobic conditions, and energy scarcity prevail, life persists against all odds. This study illuminates the pivotal role of amino acids, the fundamental building blocks of life, as a vital energy for deep subseafloor fungi. Our research uncovers how these fungi not only rely on amino acids for survival but also utilize them to reproduce, forming fruit bodies in environments deprived of oxygen and energy. This revelation not only elucidates the mechanisms enabling fungal survival in extreme conditions but also hints at the essentiality of amino acids as nutrients for other deep-sea microbes. By unraveling these mysteries of the hidden biosphere, our study opens new frontiers in understanding the resilience and adaptation of life in the most inhospitable environments on our planet.