The transit of organic matter (OM) through the aquatic compartment of its global cycle has been intensively studied, traditionally with a focus on the processing and degradation of its dissolved fraction (DOM). Because this is so intimately related to oxidation, the notion tenaciously persists that where dioxygen (O₂) is absent, DOM turnover is markedly slowed. In this Opinion Piece, we outline how diverse processes shape, transform and degrade DOM also in anoxic aquatic environments, and we focus here on inland waters as a particular case study. A suite of biogeochemical DOM functions that have received comparatively little attention may only be expressed in anoxic conditions, and may result in enhanced biogeochemical roles of these deoxygenated habitats on a network scale.

Literature meta-analysis on the sediment release of dissolved organic matter (DOM) to anoxic fresh and marine waters, and the bacterial (secondary) production within them. We used data solely from articles that explicitly addressed anoxic environments in situ, compared oxic with anoxic conditions, or study the implications of anoxic conditions in experimental setups.