Glaciers and low-DOC ecosystems as sites of microbial carbon production and consumption

Abstract Glacier-fed waters are commonly viewed as heterotrophic ecosystems fuelled by highly bioavailable dissolved organic carbon (DOC), yet this paradigm is largely based on melt-season observations and assumes that microbial DOC dynamics are dominated by consumption. We show that net DOC production may be a widespread feature of cold, carbon-poor aquatic ecosystems. We combined 719 dissolved organic matter (DOM) measurements and 164 dark incubation bioassays across six seasons at Falljökull, Iceland, with additional bioassays from Icelandic and Alpine glacier streams, and temperate springs. More than 50% of incubations showed net DOC production rather than consumption, exceeding analytical uncertainty and exhibiting seasonal and spatial patterns. Net production was strongest during low-DOC periods at the glacier outlet, whereas downstream and summer conditions favoured net DOC consumption. To capture these bidirectional dynamics, we define microbial DOC (MDOC) as the net percentage change in DOC during incubations. Elevated dissolved iron concentrations, and iron- and sulfur-associated taxa in DOC-producing incubations suggest that chemolithoautotrophic carbon fixation may contribute to dark DOC generation under oligotrophic conditions. Our findings demonstrate that positive DOC changes in BDOC assays should not be dismissed as artifacts and may reveal a previously overlooked dimension of microbial carbon cycling in low-DOC aquatic ecosystems.