Terrestrial dissolved organic carbon (tDOC) plays an important role in the global carbon cycle with an annual flux of 0.25 Pg C delivered from land to ocean (Hedges et al., 1997; Benner, 2004; Ciais et al., 2013; Raymond & Spencer, 2015). Extensive remineralization of tDOC has been widely reported along the land–ocean continuum, such as on the Louisiana shelf (~ 40%), on the Eurasian shelves (~ 50%), in the North Sea (~ 100%) and in the Sunda Shelf Sea (60–70%) (Fichot & Benner, 2014; Kaiser et al., 2017; Painter et al., 2018; Wit et al., 2018; Zhou et al., 2021). The spatial variations in the remineralization of tDOC are dependent on sources and intrinsic characteristics of tDOC, as well as on the rates of photo-, bio- and photo-bio interactive degradation (Graeber et al., 2012; Cory et al., 2014; Ward et al., 2017).
However, the relative contributions of these three degradation processes have been estimated only in a limited number of regions (Fichot & Benner, 2014; Cory et al., 2014; Reader & Miller, 2012; Osburn et al., 2009; Li et al., 2019). It is striking that direct photodegradation typically does not contribute substantially to tDOC remineralization, despite its generally high photochemical reactivity, with contributions of < 1% in the Arctic Shelf (Osburn et al., 2009), 8% in the Louisiana Shelf Sea (Fichot & Benner, 2014), ~ 20% in Arctic freshwaters (Cory et al., 2014), ~ 3% in coastal estuaries of the South Atlantic Bight (Reader & Miller, 2012) and 4% in the Pearl River estuary (Li et al., 2019). In contrast, biodegradation tends to account for the majority of tDOC remineralization, with 60% in the Louisiana Shelf Sea (Fichot & Benner, 2014), 80% in the Baltic Sea (Fransner et al., 2016; 2019), and 5–34% in Arctic freshwaters and estuaries (Cory et al., 2014; Clark et al., 2022).
Notably, solar irradiation can cause not only complete photodegradation of tDOC to CO2, but also partial photodegradation in which tDOC is converted to other organic molecules, rendering the tDOC either more or less biodegradable (Cory et al., 2014; Ward et al., 2017; Cory & Kling, 2018; Bowen et al., 2020). Although tDOC can be photo-altered to less labile compounds (Amado et al., 2007; Ward et al., 2017), it is more commonly observed that relatively high-molecular-weight aromatic tDOC is cleaved into smaller, aliphatic molecules that are more labile, such that the tDOC biodegradation rate is enhanced by prior irradiation (Gonsior et al., 2014; Cory & Kling, 2018; Bowen et al., 2020; Kragh et al., 2022; Wagner & Jaffé, 2015). For example, photo-enhanced biodegradation has been observed for tDOC in coastal Georgia (Moran et al., 2000), the Arctic rivers (Cory et al., 2014), permafrost in the Arctic (Ward et al., 2017) and lakes in Mississippi (Sankar et al., 2019). Therefore, the importance of interactions between photo- and bio- degradation can be high but variable, contributing 11–74% in these environmental settings (Aarnos et al., 2012; Fichot & Benner, 2014; Cory et al., 2014).
To date, most studies of tDOC remineralization have focused on the Arctic, or temperate and subtropical latitudes, especially in North America. Southeast Asia is also a globally relevant hotspot for land–ocean tDOC flux due to its extensive peatlands, which deliver around 10% of global land-to-ocean tDOC flux (Baum et al., 2007; Moore et al., 2011; Page et al., 2022). Moreover, previous studies have inferred from mass balance budgets that 60–70% of the tDOC exported from Southeast Asian peatlands must be remineralized within the Sunda Shelf Sea (Wit et al., 2018; Zhou et al., 2021). Two recent studies suggested that photodegradation may control the fate of tDOC in the Sunda Shelf Sea due to its low biodegradability and high photochemical reactivity (Nichols & Martin, 2021; Zhou et al., 2021). However, although direct photodegradation was confirmed in a modelling study to be more important in this region compared to high latitudes (Zhou et al., 2023), it is still insufficient to account for the observed extent of remineralization. Although microbial remineralization is slow (Nichols & Martin, 2021), over sufficiently long time scales low biodegradation rates are still important, as shown in the Baltic Sea (Fransner et al., 2016; 2019). Given that the water residence time in the Sunda Shelf Sea might be up to 2 years (Mayer et al., 2015), but previous experiments on microbial degradation of tDOC were only conducted for < 60 days (Nichols & Martin, 2021), longer incubation experiments are warranted to quantify microbial remineralization rates better. Besides, an assessment of possible interaction between photo- and bio-degradation is also needed to better understand the tDOC remineralization in Southeast Asia.
We conducted a series of experiments with different water samples to estimate the degradability and decay rates of tDOC in the Sunda Shelf Sea and to investigate the effect of photo-bio interactive processes. Photodegradation experiments with simulated solar irradiation were performed to quantify the photodegradability and obtain the efficiency of photodegradation, presented as the apparent quantum yield. Biodegradation experiments lasting for 2 months to 1.5 years with coastal marine microbes were conducted to determine decay rates for both tDOC and terrestrial chromophoric dissolved organic matter (tCDOM). We then extended the previous model of pure photodegradation (Zhou et al., 2023) to incorporate biodegradation to understand their relative importance in tDOC remineralization. Photo-influenced biodegradation experiments were conducted to assess the interaction between photochemical and microbial processes, to provide a more complete understanding of tDOC remineralization in this region.