After 17 years growing on an electrode, Geobacter evolved faster electron transfer and a smaller genome
A Geobacter sulfurreducens strain was grown continuously on graphite anodes for 17 years. Isolates from the biofilm grew about 1.8 times faster on iron oxide than their ancestor and had smaller genomes, with mutations accumulating in genes not needed on electrodes.
What happened
Geobacter sulfurreducens strain KN400 was grown continuously for 17 years on graphite anodes held at -400 mV. Isolates recovered from the 17-year-old biofilms grew about 1.8 times faster than the parental strain on Fe(III) oxide, and formed more surface-attached biofilm on glass.
During iron oxide reduction the adapted strains strongly upregulated pilA, omcZ and omcB, genes tied to extracellular electron transfer. Their genomes were smaller than KN400's and carried many mutations in genes for c-type cytochromes, signalling proteins and transcriptional regulators. Genes that are dispensable on an electrode, such as those for motility and hydrogen use, had accumulated mutations.
Why it matters
Systems that harvest electricity from waste, soil or sediment are meant to run for long periods, but little is known about how electroactive microbes adapt over that time. This 17-year culture gives a direct look, and it suggests that prolonged life on an electrode selects for more efficient biofilm-based electron transfer.
Caveats
This entry is based on the abstract. The work follows one strain under one set of laboratory conditions, and the abstract does not say how many independent lineages or isolates were compared. The authors describe the link between specific mutations and higher gene expression as likely, not demonstrated.
Evolution for enhanced extracellular electron transfer in Geobacter sulfurreducens over seventeen years of continuous current generation
Read at Frontiers in Microbiology ↗