Photodegradation disproportionately impacts biodegradation of semi‐labile DOM in streams

Abstract

Exposure of dissolved organic matter (DOM) to sunlight can increase or decrease the fraction that is biodegradable (BDOM), but conceptual models fail to explain this dichotomy. We investigated the effect of sunlight exposure on BDOM, addressing three knowledge gaps: (1) how fractions of DOM overlap in their susceptibility to degradation by sunlight and microbes, (2) how the net effect of sunlight on BDOM changes with photon dose, and (3) how rates of DOM photodegradation and biodegradation compare in a stream. Stream waters were exposed to sunlight, and then fed through bioreactors designed to separate labile and semi‐labile pools within BDOM. The net effects of photodegradation on DOM biodegradability, while generally positive, represented the balance between photochemical production and removal of BDOM that was mediated by photon dose. By using sunlight exposure times representative of sunlight exposures in a headwater stream and bioreactors colonized with natural communities and scaled to whole‐stream dynamics, we were able to relate our laboratory findings to the stream. The impact of sunlight exposure on rates of DOM biodegradation in streams was calculated using rates of light absorption by chromophoric DOM, apparent quantum yields for photomineralization and photochemical alteration of BDOM, and mass transfer coefficients for labile and semi‐labile DOM. Rates of photochemical alteration of labile DOM were an order of magnitude lower than rates of biodegradation of labile DOM, but for semi‐labile DOM, these rates were similar, suggesting that sunlight plays a substantial role in the fate of semi‐labile DOM in streams.

Simultaneous attenuation of trace organics and change in organic matter composition in the hyporheic zone of urban streams

“…For DOM, sunlight can result in degradation of recalcitrant compounds to more labile molecules 39 , 40 or in alteration of labile compounds to more stable substances 41 . In a previous study, semi labile DOM was more prone to photodegradation than labile DOM 42 . We observed a decrease in molecular diversity of bioavailable DOM compound classes while less bioavailable DOM molecular diversity increased in surface water.…”

Experimental metatranscriptomics reveals the costs and benefits of dissolved organic matter photo‐alteration for freshwater microbes

“…Differential expression of specific metabolic genes also highlights the similarities between photo‐ and bio‐degraded fractions of DOM. Reduced expression of aromatic degradation genes, oxygenase genes, and decarboxylase genes in response to photo‐altered DOM indicates a substantial competition between sunlight and microbes to degrade similar types of DOM (Bowen et al ., 2019).…”

Environmental effects of stratospheric ozone depletion, UV radiation, and interactions with climate change: UNEP Environmental Effects Assessment Panel, Update 2020

“…One of the major sources of DOM involves release by melting of the permafrost in the Arctic [ 167 , 314 , 317 , 318 ]. Current research on photodegradation by UV radiation of Arctic dissolved organic matter [ 318 ] shows that formation of CO 2 is accompanied by buildup of a pool of partially photooxidised DOM that likely plays a role in photo-facilitated microbial degradation [ 167 ].…”

“…Once thawed, up to 15% of the ∼1,000 Pg of organic carbon in Arctic permafrost soils may be oxidised to CO 2 by 2100, amplifying climate change [ 317 ]. Predictions of this amplification strength are imprecise and likely are understated because they ignore the photodegradation of permafrost carbon to CO 2 in surface waters by solar radiation.…”

“…As permafrost in the Arctic thaws, large carbon and nitrogen stocks are exposed for decomposition [ 320 ]. Gaseous carbon release from Arctic soils due to permafrost thawing is substantial [ 1 ], and growing evidence suggests that Arctic soils may also be relevant sources of nitrous oxide (N 2 O) [ 317 , 320 – 324 ]. Nitrous oxide is a source gas for stratospheric nitrogen oxides and an ozone depleting substance (contributes to SDGs 13.1, 15.1); it has an almost 300 times stronger global warming potential than CO 2 on a 100-year time horizon [ 322 ].…”