Low biodegradability of dissolved organic matter from Southeast Asian
peat-draining rivers
Abstract
Southeast Asia’s extensive tropical peatlands account for a significant
proportion of the global riverine dissolved organic carbon (DOC) flux to
the ocean. Peat-derived DOC is rich in polyphenolic compounds, the
microbial degradation of which is thought to rely on extracellular
phenol oxidases. Despite substantial interest in the biogeochemical fate
of terrigenous DOC (tDOC), few studies have quantified phenol oxidase
activity in aquatic environments, and microbial remineralization rates
of tDOC have never been measured in Southeast Asia. Here, we assess the
potential for using phenol oxidase assays as a proxy of tDOC
biodegradation across peat-draining rivers and coastal waters of
Sarawak, Borneo, and report experimental measurements of microbial tDOC
remineralization rates from this region. We show first that phenol
oxidase assays in aquatic samples are problematic because of the rapid,
pH-dependent auto-oxidation of the assay substrate. Our field
measurements of phenol oxidase activity detected only substrate
auto-oxidation, suggesting that real phenol oxidase activity was low or
absent. Second, we report that peatland tDOC, collected from one of the
few remaining intact peatlands on Borneo, showed at most very limited
biodegradation (0–6% loss of DOC, and 0–12% loss of coloured
dissolved organic matter) during several 56-day incubation experiments
at in-situ temperature of ~30°C, even when diluted with
seawater or amended with nutrients. Our results suggest that direct
microbial respiration is perhaps not a major pathway for peatland tDOC
remineralization in Southeast Asia, and that photo-oxidation is more
likely to control the fate of this carbon.