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Anoxygenic photoautotrophy driven by humus and microplastics in a photosynthetic bacterium
Summary
Researchers discovered an unexpected role for microplastics in carbon cycling: when certain plastics like PLA and PET are present alongside humic substances in sunlit, oxygen-free environments, they can accelerate the growth of photosynthetic bacteria by acting as electron donors. This means microplastics are not merely passive pollutants in aquatic sediments — they may actively influence microbial communities and carbon dynamics in ways not previously understood. The finding adds a new dimension to understanding how plastic pollution reshapes the chemistry of natural ecosystems.
Humus and microplastics are recalcitrant organics in soils and aquatic systems, and their role in the geochemical cycling of elements remains elusive. Herein, we have identified a new mechanism by which humus and microplastics participate in anoxic carbon cycling. We demonstrated that the photoexcitation of 5-30 mg/l of humic acid or fulvic acid, two major fractions of humus, can drive CO2 fixation and enable the photoautotrophic growth of a photosynthetic bacterium, Rhodopseudomonas palustris. This process was enhanced by 10.69%-144.87% upon the addition of 100 mg/l of poly(lactic acid) or poly(ethylene terephthalate). Mechanistic investigations demonstrated that the microplastics act as sacrificial quenchers during humus photoexcitation, leading to their depolymerization. Transcriptomic analyses revealed high expression of genes encoding extracellular electron uptake pathways including extracellular cytochrome c and its oxidases in the photoautotrophic growth of R. palustris. This study expands our understanding of how humus and microplastics are involved in the biogeochemical cycling of carbon and sheds light on how they impact the CO2 dynamic fluxes in sunlit anoxic environments.