Photooxidation removes biologically recalcitrant dissolved organic carbon released by the macroalga <i>Sargassum natans</i>

Abstract Macroalgae are important primary producers in the coastal ocean, and they release a large fraction of their net primary production as dissolved organic carbon (DOC). It is assumed that much of this DOC is recalcitrant and results in the sequestration of large amounts of carbon. We lack sufficient knowledge about the bioavailability of this material and the role of other sinks such as photooxidation. We conducted dark remineralization and photooxidation experiments on DOC derived from an abundant brown macroalga and quantified the fraction of amended DOC that was remineralized by both processes. The bioavailability of the amended DOC ranged from 14% to 99% and was significantly negatively correlated with its phenolic content. Upon exposure to light, the biologically recalcitrant compounds were quickly oxidized to CO 2 , indicating that photooxidation is an important sink for recalcitrant brown macroalgal DOC. These results are especially important as macroalgae cultivation is being considered to sequester atmospheric CO 2 .