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 for tDOC biodegradation across peat‐draining rivers and the coastal waters of Sarawak, Borneo, and report experimental measurements of microbial tDOC remineralization rates from this region. We first show that phenol oxidase assays in aquatic samples are problematic because of the rapid, pH‐dependent autoxidation of the assay substrate. Our field measurements of phenol oxidase activity detected only substrate autoxidation, 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 colored dissolved organic matter) during several 56‐day incubation experiments at an 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.

Extensive Remineralization of Peatland‐Derived Dissolved Organic Carbon and Ocean Acidification in the Sunda Shelf Sea, Southeast Asia

“…Although the leucine and glucose added as labile carbon to one treatment was rapidly consumed, no additional remineralization of DOC was induced in this treatment. Our results, consistent with the low biodegradability of tDOC from the Maludam River (Nichols & Martin, 2021), suggest that direct microbial remineralization is not a main driver of tDOC remineralization. Whether the observed biodegradation represents loss of tDOC at all or only of autochthonous DOC is unclear.…”

“…For the δ 13 C of the DIC produced from remineralization, microbial respiration causes little carbon isotopic fractionation, while results from our experiments (Section 3.6) and previous studies (Opsahl & Zepp, 2001; Osburn et al., 2001; Spencer et al., 2009) indicated that photodegradation (and combined photo‐bio‐degradation) of tDOC can cause fractionation in δ 13 C of −1.4 to −5.8‰ between the produced DIC and the initial DOC (Table ). Because of the low bio‐degradability (Nichols & Martin, 2021) but high photo‐degradability of Southeast Asian peatland tDOC (Section 3.6), we infer that a major part of the remineralization of tDOC in this region might be via photodegradation, and therefore entails a carbon isotopic fractionation. We adopted a fractionation of −3‰, and thus used −32‰ as the δ 13 C of the DIC produced from tDOC remineralization.…”

“…Our data showed at most low biodegradability of the tDOC collected from the Singapore Strait, even when amended with labile carbon to stimulate a potential priming effect (Bianchi, 2011; Guenet et al., 2010; van Nugteren et al., 2009). Similarly, freshly collected tDOC from the peatland‐draining Maludam River showed at most very limited biodegradability over 56 days, even when amended with dissolved nutrients or mixed into coastal seawater (Nichols & Martin, 2021). In contrast, we found that up to about 74% of peatland‐derived tDOC can be photochemically remineralized.…”

“…The peatland‐draining rivers in Southeast Asia are characterized by very high DOC concentrations (1,000–5,000 µmol L −1 ) and high colored dissolved organic matter (CDOM a 350 , 50–200 m −1 ) (Alkhatib et al., 2007; Martin et al., 2018; Moore et al., 2011; Rixen et al., 2008; Wit et al., 2015, 2018), which are among the highest values found in rivers globally (Meybeck, 1982). However, these peat‐draining rivers only show moderate‐to‐low CO 2 efflux relative to their high DOC concentrations (Müller et al., 2015; Müller‐Dum et al., 2019; Wit et al., 2015), and recent experimental data suggest that Southeast Asian peatland DOC is relatively refractory to microbial decomposition (Nichols & Martin, 2021). In addition, field surveys have typically revealed conservative mixing of DOC across peatland‐draining river estuaries (Alkhatib et al., 2007; Martin et al., 2018; Rixen et al., 2008; Wit et al., 2015; Zhou et al., 2019).…”

Rising dissolved organic carbon concentrations in coastal waters of northwestern Borneo related to tropical peatland conversion

“…The environmental implications of rising tDOC in coastal waters depend on its biogeochemical fate. Peat-derived tDOC in Southeast Asia often mixes conservatively across estuaries ( 16 , 38 , 54 ) and appears to be relatively refractory to direct microbial remineralization, but is labile to photooxidation ( 12 , 55 ). Research off Sumatra suggests that the majority of peatland tDOC that reaches coastal waters is remineralized within the Sunda Shelf Sea over time scales of 1 to 2 years, resulting first in ocean acidification before gradually degassing to the atmosphere ( 12 , 56 ).…”