Iron concentration induces a biphasic response of glucose biodegradation by marine fungi

Iron serves as a pivotal biogeochemical nexus linking microbial activity and the carbon cycle, yet its concentration-dependent regulation of dissolved organic matter biodegradation remains poorly understood. Here, we demonstrate a biphasic response of glucose biodegradation by the marine fungus Rhodotorula mucilaginosa to iron concentration through systematic methods, including transcriptomic and stable carbon isotope analyses, combined with morphological characterization. A high glucose biodegradation rate (0.25 d−1) was observed under 200 ppm ferric chloride, supported by transcriptomic evidence of upregulated alcohol dehydrogenase and enhanced glycolytic nicotinamide adenine dinucleotide regeneration. Conversely, 2000 ppm ferric chloride induced reactive oxygen species stress and substantially reduced the degradation rate (0.085 d−1). Iron speciation shifted toward solid iron phosphate and aqueous ferric iron via both abiotic and biotic processes. These findings suggest a mechanistic perspective on how iron concentration may govern dissolved organic matter fate—alternating between biodegradation and preservation, with potential implications for marine redox and carbon cycling. In the marine fungus Rhodotorula mucilaginosa, low iron levels enhance glucose biodegradation, while high levels induce cytotoxicity, according to fungal glucose biodegradation assays.