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.

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Coral reef productivity depends on fast nutrient cycling, mediated largely by enzymatic breakdown of organic matter. Alkaline phosphatases hydrolyse phosphomonoesters and are one of the key enzymes involved in marine phosphorus cycling. They are expressed by a plethora of marine organisms including both planktonic microbes and metazoans such as corals, often in response to phosphate limitation, and are potentially important for coral P nutrition and reef biogeochemical cycling. However, most alkaline phosphatase activity (APA) data are from open-ocean environments, and the rates and drivers of APA in coastal waters are not well understood. Here, we measured APA both in the water column and associated with three coral species at reefs in Singapore, where the monsoonal ocean current reversal creates strong seasonal changes in dissolved nutrient availability. Water column APA was consistently high, averaging 9 ± 10 nmol l-1 h-1, but was not correlated with dissolved phosphate or other biogeochemical parameters. Experimental phosphate addition did not reduce seawater APA but addition of labile organic carbon did increase seawater APA, indicating that the increase in APA was driven by heterotrophic activity rather than phosphate stress. Coral APA ranged from 12–163 µmol m-2 h-1 depending on species and was equivalent to the APA in several meters of overlying water. While most coral APA was associated with the coral holobiont rather than the coral mucus, corals released 12 – 55 µmol h-1 per m2 of mucus-associated APA into the water column, which is potentially significant for water column DOP cycling.

M. (2022). Monsoon-driven biogeochemical dynamics in an equatorial shelf sea: time-series observations in the Singapore Strait.