Sunlight-hematite promoted denitrification by Pseudomonas aeruginosa: A little-known form of nitrogen-cycling enhancement

Microorganism-mineral interactions are widely involved in material circulation and energy flow processes in the Earth's surface system and have attracted extensive attention in recent years. Here, sunlight-hematite-promoted denitrification of Pseudomonas aeruginosa PAO1 was observed. Under illumination, the nitrate removal rate reached 90.49 +/- 4.40%, which is 1.3-times higher than that in darkness (69.36 +/- 1.96%). Moreover, different concentration of P. aeruginosa PAO1 and hematite showed good synergy, suggesting that enhanced denitrifica-tion processes are common. Amperometric I-t curves indicate that P. aeruginosa PAO1 generated the highest current density (3.4 +/- 0.23 mu A/cm2) with hematite under illumination, while electrochemical impedance spectroscopy showed that the charge transfer resistance RP increased from 144 omega to 162 omega when nitrate was added, indicating that there is another electron transfer pathway in the light-hematite-P. aeruginosa PAO1 system. To better understand this phenomenon in nature, experiments were conducted with red soil in place of hematite. A similarly enhanced nitrate removal rate (89.17%) was observed, confirming that this synergistic process occurs in nature. This research demonstrates a neglected denitrification model in a sun-light-semiconducting mineral-microorganism system, which expands our understanding of nitrogen cycling in natural environments.