The Xiamen Symposium on Marine Environmental Sciences (XMAS 2025)

Dissolved organic carbon (DOC) is a key part of the ocean’s carbon cycle, and understanding the processes and rates at which DOC is remineralized to CO2 is therefore important. The two main processes that degrade DOC are photochemical and microbial remineralization. Quantifying the rates of each process in isolation is fairly straightforward. However, it has long been known that the two processes interact: photochemical degradation can sometimes make DOC more bioavailable, and sometimes make it less bioavailable, or simply compete with microbial degradation. Quantifying the rate of DOC remineralization that will result from these interactions in the environment is very difficult, and most research to date has studied these interactions in experiments where the photochemical and microbial degradation are performed successively. In contrast, in the natural environment both processes occur and can interact simultaneously. We will present a novel experimental design in which photochemical and microbial DOC degradation can take place simultaneously, and in which the light dose absorbed by the DOC can be quantified and maintained at environmentally realistic levels. We have applied this technique to study the remineralization of terrestrial DOC from tropical peatlands, and we show that interactive degradation can account for the majority of terrestrial DOC remineralization in coastal waters. Moreover, the interactive degradation rate is a highly non-linear, asymptotic function of the rate at which sunlight is absorbed. This means that low light doses are particularly effective at stimulating microbial DOC remineralization by converting refractory to labile DOC, but that higher light doses increasingly have competitive or inhibitive effects. From our data, we have developed a simple equation to represent the remineralization of terrestrial DOC as a function of the absorbed ultraviolet solar radiation in a regional ocean biogeochemical model for Southeast Asia, which shows good agreement compared to field data.

Dissolved organic matter,Photodegradation,Terrestrial organic carbon,Carbon cycle modelling