Dityrosine formation via reactive oxygen consumption yields increasingly recalcitrant humic‐like fluorescent organic matter in the ocean

Marine dissolved organic matter (DOM) is a massive elemental pool on Earth and is thought to consist of a chemically complex mixture of molecules. Part of marine DOM is fluorescent (FDOM) and includes humic‐like compounds. The chemical composition of, and biochemical pathways that yield, autochthonous humic‐like FDOM in the ocean is largely unknown. Inspired by medical and biochemical research detailing the formation of colored and fluorescent dityrosine via peroxidase mediated reactions, we used fluorometry and spectroscopy and found dityrosine exhibits humic‐like fluorescence similar to that of marine FDOM. Investigating its recalcitrance, we conducted short‐term nutrient‐addition and light exposure experiments with dityrosine and found it is resistant to respiration or assimilation by marine microbial communities, yet readily photodegraded. The formation and characteristics of dityrosine newly points to polymerization via reactive oxygen quenching, under aphotic and putatively photic conditions, as an explanation for autochthonous, humic‐like, (semi‐)recalcitrant DOM in the ocean.