Bonded water is an integral component of dissolved organic matter in the ocean

Dissolved organic matter (DOM) in the ocean, as one of the largest pools of reduced carbon on Earth, plays an important role in the global carbon cycle, yet its chemical structure remains largely unknown. Here we show that high molecular weight DOM (HMW-DOM) contains approximately 10% bonded water (H2O) by weight. The bonded H2O molecules are an integral part of the DOM organic complex, likely maintained through strong intermolecular hydrogen bonds, as the H2O is not released during freeze drying and barely exchangeable with surrounding H2O in solution. When the bonded H2O was removed by mild heating (70 °C), the HMW-DOM became more bioavailable as shown by laboratory incubation experiments. These findings suggest that the bonded H2O plays a key role in regulating the stability and biogeochemical cycling of DOM in the ocean.