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Researchers have unveiled a surprising new way that soil microbes can use sunlight energy—even after the lights go out. A team from Kunming University of Science and Technology and the University of Massachusetts Amherst has developed a "bio-photovoltage soil-microbe battery" that can capture, store, and release solar energy to power the breakdown of antibiotic pollutants in the dark.

The study, in Environmental and Biogeochemical Processes, shows that common soil bacteria known as Bacillus megaterium can partner with iron minerals to form a living biofilm that behaves like a rechargeable geochemical capacitor.

When exposed to light, the iron-bacteria film absorbs photons and stores the resulting electrons. Later, in the absence of light, it releases these stored charges to trigger chemical reactions that degrade antibiotics such as tetracycline and chloramphenicol.

"Our findings reveal that soil microorganisms and minerals can together function like tiny natural batteries," said co-corresponding author Professor Bo Pan of Kunming University of Science and Technology. "This system can capture sunlight during the day and use that energy at night to remove pollutants."

In laboratory experiments, the Fe₂O₃–B. megaterium composite generated a total accumulated charge of 8.06 microcoulombs per square centimeter during light–dark cycles. After one hour of light exposure, the system degraded up to 22% of antibiotics in complete darkness, a performance up to 67% higher than with shorter light exposure.

The mechanism relies on the cycling of iron between its Fe(II) and Fe(III) forms, aided by bacterial metabolism. This redox relay enables electron storage and gradual release, creating a stable power source for dark-phase reactions. The team's electrochemical analyses confirmed that this mineral–microbe interface enhances charge transfer and reduces energy losses, forming a biological pseudocapacitor.

"This discovery opens a new window into how solar energy can drive biogeochemical processes even below the soil surface where sunlight cannot reach," said Professor Baoshan Xing of the University of Massachusetts Amherst, a co-corresponding author.

"It also suggests an environmentally sustainable way to remediate contaminated soils and groundwater."

The researchers believe that similar mineral–microbe systems may play a hidden but vital role in natural energy cycles and pollution control across ecosystems.