Photochemical lability of dissolved organic matter in deep water in the western tropical Pacific Ocean

The production, consumption, and transformation of dissolved organic matter (DOM) in the ocean plays a key role in climate change. The majority of oceanic DOM is contained in the deep ocean and resists bacterial degradation, which allows the deep DOM to pass through the sunlit surface ocean multiple times during its lifespan by means of large-scale ocean currents. In the present study, the photochemical reactivity of DOM were determined, based on the seawater samples collected from multiple depths of the offshore South China and Philippine Seas. Photomineralization of dissolved organic carbon (DOC) and photobleaching of chromophoric and fluorescent DOM (CDOM, FDOM) were observed in all samples. Protein-like FDOM was, however, either photo-decomposed or -produced, depending on the sample’s depth. Photobleaching of CDOM and humic-like FDOM was much faster in deep than in shallow water samples while photomineralization displayed a weaker vertical zonation. Prior-irradiated deep water inoculated with surface-water bacteria showed enhanced microbial DOC removal but CDOM production. Results from this study suggest that deep-ocean CDOM and FDOM can barely survive photobleaching during one ocean mixing cycle but photochemical turnover of the biorefractory deep DOC is tens to hundreds of times longer. Photochemical processes and photochemical-microbial interactions can thus have a potential important impact on the transport, transformation, and fated of the deep-ocean DOM.