The reductive dechlorination of pentachlorophenol (PCP) by Geobacter sulfurreducens in the presence of different biochars was investigated to understand how biochars affect the bioreduction of environmental contaminants. The results indicated that biochars significantly accelerate electron transfer from cells to PCP, thus enhancing reductive dechlorination. The promotion effects of biochar (as high as 24-fold) in this process depend on its electron exchange capacity (EEC) and electrical conductivity (EC). A kinetic model revealed that the surface redox-active moieties (RAMs) and EC of biochar (900 °C) contributed to 56% and 41% of the biodegradation rate, respectively. This work demonstrates that biochars are efficient electron mediators for the dechlorination of PCP and that both the EC and RAMs of biochars play important roles in the electron transfer process.

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Direct interspecies electron transfer (DIET) may prevail in microbial communities that show methanogenesis and anaerobic methane oxidation and can be an electron source to support anaerobic photosynthesis. Previous mutagenic studies on cocultures of defined Geobacter species indicate that both conductive pili and extracellular cytochromes are essential for DIET. However, the actual functional role of the pili in DIET is uncertain, as the pilus mutation strategy used in these studies affected the extracellular cytochrome profile. Here we repressed the function of pili by deleting the pilus polymerization motor PilB in both Geobacter species. The PilB mutation inhibited the pilus assembly but did not alter the pattern of extracellular cytochromes. We report that the two pilus-free Geobacter species can form aggregates and grow syntrophically with DIET. The results demonstrate that the Gmet_2896 cytochrome of Geobacter metallireducens plays a key role in DIET and that conductive pili are not necessary to facilitate DIET in cocultures of Geobacter species, and they suggest cytochromes by themselves can meditate DIET, deepening the understanding of DIET.

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