Photoelectrons from Minerals and Microbial World

摘要

The Earth surface is a multiple open system. Semiconducting minerals, including most metal oxides and sulfides, absorb visible light of the solar spectrum. Microorganisms evolve varied pathways to get carbon and energy sources. It is obvious that the interaction among solar light, semiconducting minerals, photoelectron/photohole, organics, inorganics, valence electrons and microorganisms occurs continuously on our planet. In a recent study, Lu et al. (2012) presented evidence demonstrating solar energy mediated by semiconducting mineral photocatalysis, acting as energy source, promoted the growth of some non-photosynthetic bacteria and revealed that the ternary system of microorganisms, minerals and solar light has played a critical role in the history of life on our planet. In simulated system, under simulated solar light semiconducting minerals, such as metal oxides and metal sulfides, generates photoelectrons which could be used by non-phototrophic microorganisms to support their metabolisms. The growth of microorganism was closely related to photon quantity and energy, and the microorganism growth and mineral light absorption spectra were fitted well under different light wavelengths. The overall energy efficiency from photon to biomass was 0.13‰ to 1.9‰. Further studies revealed that in natural soil systems, semiconducting mineral photocatalysis could influence the microbial population. Solar energy utilization pathway by nonphototrophic microorganisms mediated by semiconducting mineral photocatalysis provides a new concept to evaluate the origin and evolution of life. Semiconducting minerals are ubiquitous on Earth’s surface and widely participate in redox reactions following photoelectron-photohole pairs excited by solar light. As photoholes can be easily scavenged by environmental reductive substances and microorganisms possess multiple strategies to utilize extracellular electrons, the highly reductive photoelectrons serve as potential energy source for microbial life. The discovery of this pathway extends our knowledge on the use of solar energy by nonphototrophic microorganisms, and provides important clues to evaluate life on the early Earth. Microorganisms, minerals and solar light constitute a complex but important ternary system through Earth history. The discovery of the novel energy conversion pathway in this system demonstrates how nonphototrophic microorganisms directly or indirectly utilized photoelectrons as the solar energy source. The fully comprehending of nonphototrophic bacteria solar energy utilization conducted by semiconducting minerals in present environment will greatly help us to better understand the energy transform mechanism among interfaces of lithosphere, pedosphere, hydrosphere and biosphere.