Particulate organic matter triggers photoelectrotrophic denitrification: An overlooked biogeochemical reaction induced by sunlight

<p>Particulate organic matter (POM) is a major carbon pool in terrestrial and aquatic ecosystems and its biogeochemical processes significantly affect the global carbon cycle and trophic levels of ecosystems. As a natural photosensitizer, there have been no reports on photoelectron transfer from POM to microorganisms, leading to the long-term neglect of POM biogeochemical processes. Using photoelectrotrophic denitrification (PEDeN) as a model system, this study is the first to demonstrate the POM-triggered PEDeN effect; that is, photoelectrons from POM were used to drive microbial denitrification. By constructing the <i>Thiobacillus denitrificans</i>-POM system, it was found that irradiation accelerated nitrate reduction with a kinetic constant of 0.03 ± 0.007 h⁻¹. Irradiation resulted in a complete denitrification with nitrogen as the main product (56.3 ± 8.8%), compared to the dark system with nitrite as the main product. The significantly upregulated (<i>p</i> < 0.01) denitrifying genes support the idea that the conversion of nitrate to nitrogen is a microbial-mediated process. During this process, POM plays multiple roles: photosensitizing microorganisms, mediating photoelectron transfer, and acting as a sacrificial donor. The first two roles of POM have not been established previously, revealing a new electron-transfer mechanism between POM and microorganisms. The latter accelerated POM dissolution, representing a newly discovered POM photodissolution process involving microorganisms. The study highlights the previously overlooked photoelectrochemical reactions of POM with microorganism, emphasizing the need to explore the biogeochemical processes triggered by POM-microorganisms system and suggesting a strategy for promoting sunlight-driven denitrification in surface environments.</p>