Reactive oxygen species produce and diversify low-molecular weight dissolved organic matter from laminarin and chitin

Reactive oxygen species (ROS) are continuously produced by abiotic and biotic processes in the worlds’ oceans. Their contribution to shaping the marine dissolved organic matter (DOM) composition is not well known, because ROS-DOM interactions likely occur stochastically and on short timescales. Long-standing research from medical and environmental engineering fields has shown that hydroxyl radicals (OH*) produced from UV/H₂O₂ treatments can depolymerize large biomolecules and partially decompose them to CO₂. We therefore investigated whether laminarin and chitin, two abundant polysaccharides in the surface ocean, can be transformed in combined UVA/H₂O₂ experiments. Overall, we found little indication of laminarin and chitin net mineralization. Instead, chitin in suspension was partially solubilized to DOM, and both substrates were depolymerized and molecularly diversified, evidenced by a net increase of sugar oligo- and monomeric molecular formulae and new, unknown compounds detectable with ultra-high resolution mass spectrometry and high-field two-dimensional nuclear magnetic resonance spectroscopy. If such depolymerization occurs in the surface ocean, it would render large biopolymers ready for immediate microbial uptake without the aid of costly enzymes, while also contributing to the structural complexity of the DOM geometabolome. Based on our study we suggest that radical reactions continuously re-work marine organic matter in the background, exerting a so-far neglected influence on its availability as a substrate for heterotrophic consumers from both an intrinsic and emergent stability perspective.