TRANSCRIPTOMIC AND GENETIC ANALYSIS OF DIRECT INTERSPECIES ELECTRON TRANSFER
The possibility that metatranscriptomic analysis could distinguish between direct interspecies electron transfer (DIET) and H2 interspecies transfer (HIT) in anaerobic communities was investigated by comparing gene transcript abundance in co-cultures in which Geobacter sulfurreducens was the electron-accepting partner for either Geobacter metallireducens, which performs DIET, or Pelobacter carbinolicus, which relies on HIT. Transcript abundance for G. sulfurreducens uptake hydrogenase genes were 7-fold lower in co-cultures with G. metallireducens than with P. carbinolicus, consistent with DIET and HIT, respectively, in the two co-cultures. Transcript abundance for the pilus-associated cytochrome OmcS, which is essential for DIET, but not HIT, was 240-fold higher in the co-cultures with G. metallireducens than with P. carbinolicus. The pilin gene pilA was moderately expressed despite a mutation that might be expected to repress pilA expression. Lower transcript abundance for G. sulfurreducens genes associated with acetate metabolism in the co-cultures with P. carbinolicus was consistent with repression of these genes by the H2 during HIT. Genes for biogenesis of pili and flagella and several c-type cytochrome genes were among the most highly expressed in G. metallireducens. Mutant strains that lacked the ability to produce pili or flagella or outer-surface c-type cytochrome Gmet_2896 were not able to form co-cultures with G. sulfurreducens. These results demonstrate that there are unique gene expression patterns that distinguish DIET from HIT and suggest that metatranscriptomics may be a promising route to investigate interspecies electron transfer pathways in more complex environments.