Rock varnish is a thin coating enriched with manganese (Mn) and iron (Fe) oxides. The mineral composition and formation of rock varnish elicit considerable attention from geologists and microbiologists. However, limited research has been devoted to the semiconducting properties of these Fe/Mn oxides in varnish and relatively little attention is paid to the mineral–microbe interaction under sunlight. In this study, the mineral composition and the bacterial communities on varnish from the Gobi Desert in Xinjiang, China were analyzed. Results of principal components analysis and t-test indicated that more electroactive genera such as Acinetobacter, Staphylococcus, Dietzia, and Pseudomonas gathered on varnish bacterial communities than on substrate rock and surrounding soils. We then explored the culture of varnish, substrate and soil samples in media and the extracellular electron transfer (EET) between bacterial communities and mineral electrodes under light/dark conditions for the first time. Orthogonal electrochemical experiments demonstrated that the most remarkable photocurrent density of 6.1 ± 0.4 μA/cm² was observed between varnish electrode and varnish microflora. Finally, based on Raman and 16S rRNA gene–sequencing results, coculture system of birnessite and Pseudomonas (the major Mn oxide and a common electroactive bacterium in varnish) was established to study underlying mechanism. A steadily growing photocurrent (205 μA at 100 h) under light was observed with a stable birnessite after 110 h. However, only 47 μA was generated in the dark control and birnessite was reduced to Mn²⁺ in 13 h, suggesting that birnessite helped deliver electrons instead of serving as an electron acceptor under light. Our study demonstrated that electroactive bacterial communities were positively correlated with Fe/Mn semiconducting minerals in varnish, and diversified EET process occurred on varnish under sunlight. Overall, these phenomena may influence bacterial–community structure in natural environments over time.

Exploring the interplay between sunlight, semiconducting minerals, and microorganisms in nature has attracted great attention in recent years. Here we report for the first time the investigation of the interaction between a hematite photoelectrode and Pseudomonas aeruginosa PAO1 under visible light irradiation. Hematite is the most abundant mineral on earth, with a band gap of 2.0 eV. A hematite electrode was electrochemically deposited on fluorine-doped tin oxide (FTO). It was thoroughly characterized by environmental scanning electron microscopy (ESEM), Raman, and UV–Vis spectroscopy, and its prompt response to visible light was determined by linear sweep voltammetry (LSV). Notably, under light illumination, the hematite electrode immersed in a live cell culture was able to produce 240% more photocurrent density than that in the abiotic control of the medium, suggesting a photoenhanced extracellular electron transfer process occurring between hematite and PAO1. Different temperatures of LSV measurements showed bioelectrochemical activity in the system. Furthermore, I–t curves under various conditions demonstrated that both a direct and an indirect electron transferring process occurred between the hematite photoanode and PAO1. Moreover, the indirect electron transferring route was more dominant, which may be mainly attributed to the pyocyanin biosynthesized by PAO1. Our results have expanded our understanding in that in addition to Geobacter and Shewanella it has been shown that more microorganisms are able to perform enhanced extracellular electron transfer with semiconducting minerals under sunlight in nature.

Developing cheap electrocatalysts for cathodic oxygen reduction in neutral medium is a key factor for practical applications of microbial fuel cells (MFCs). Natural hematite was investigated as a low-cost cathode to improve the performance of microbial fuel cells (MFCs). With hematite-coated cathode, the cell current density stabilized at 330.66 ± 3.1 mA·m⁻² (with a 1000 Ω load) over 10 days under near-neutral conditions. The maximum power density of MFC with hematite cathode reached to 144.4 ± 7.5 mW·m⁻², which was 2.2 times that of with graphite cathode (64.8 ± 5.2 mW·m⁻²). X-ray diffraction (XRD), Raman, electrode potential analysis, and cyclic voltammetry (CV) revealed that hematite maintained the electrode activities due to the stable existence of Fe(II)/Fe(III) in mineral structure. Electrochemical impedance spectroscopy (EIS) results indicated that the cathodic electron transfer dynamics was significantly improved by using hematite to lower the cathodic overpotential. Therefore, this low-cost and earth-abundant natural mineral is promised as an effective cathode material with potential large-field applications of MFCs in future.