Microbial phototrophs, key primary producers on Earth, use H2O, H2, H2S and other reduced inorganic compounds as electron donors. Here we describe a form of metabolism linking anoxygenic photosynthesis to anaerobic respiration that we call ‘syntrophic anaerobic photosynthesis'. We show that photoautotrophy in the green sulfur bacterium Prosthecochloris aestaurii can be driven by either electrons from a solid electrode or acetate oxidation via direct interspecies electron transfer from a heterotrophic partner bacterium, Geobacter sulfurreducens. Photosynthetic growth of P. aestuarii using reductant provided by either an electrode or syntrophy is robust and light-dependent. In contrast, P. aestuarii does not grow in co-culture with a G. sulfurreducens mutant lacking a trans-outer membrane porin-cytochrome protein complex required for direct intercellular electron transfer. Syntrophic anaerobic photosynthesis is therefore a carbon cycling process that could take place in anoxic environments. This process could be exploited for biotechnological applications, such as waste treatment and bioenergy production, using engineered phototrophic microbial communities.
Microbial fuel cells (MFCs) have been used to enrich microbes oxidizing formate with concomitant electricity generation. Medium containing formate was fed continuously to MFCs. MFCs showed approximately 1 mA of current after 4 months of operation. Over 90% of formate supplied was removed in MFCs, while Coulombic efficiency was only 5.3% indicating substantial electron and energy losses rather than electricity generation. Denaturing gradient gel electrophoresis (DGGE) showed that a formate-utilizing acetogenic bacterium (Acetobacterium sp.), an acetate-oxidizing metal reducer (Geobacter sp.), and another formate utilizer (Arcobacter sp.) were mainly detected on the electrode. This result indicates that some formate was consumed by acetogenic bacteria to make acetate, and acetate was used by acetate-utilizing electrochemically active bacteria (EAB) (e.g., Geobacter sp.). Additionally, formate was oxidized by nonelectrochemically active bacteria under microaerobic conditions in the anode compartment of the MFCs.
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