Harnessing Fermentation May Enhance the Performance of Biological Sulfate-Reducing Bioreactors

Biological sulfate
reduction (BSR) represents a promising strategy
for bioremediation of sulfate-rich waste streams, yet the impact of
metabolic interactions on performance is largely unexplored. Here,
genome-resolved metagenomics was used to characterize 17 microbial
communities in reactors treating synthetic sulfate-contaminated solutions.
Reactors were supplemented with lactate or acetate and a small amount
of fermentable substrate. Of the 163 genomes representing all the
abundant bacteria, 130 encode 321 NiFe and FeFe hydrogenases and all
genomes of the 22 sulfate-reducing microorganisms (SRM) encode genes
for H2 uptake. We observed lactate oxidation solely in
the first packed bed reactor zone, with propionate and acetate oxidation
in the middle and predominantly acetate oxidation in the effluent
zone. The energetics of these reactions are very different, yet sulfate
reduction kinetics were unaffected by the type of electron donor available.
We hypothesize that the comparable rates, despite the typically slow
growth of SRM on acetate, are a result of the consumption of H2 generated by fermentation. This is supported by the sustained
performance of a predominantly acetate-supplemented stirred tank reactor
dominated by diverse fermentative bacteria encoding FeFe hydrogenase
genes and SRM capable of acetate and hydrogen consumption and CO2 assimilation. Thus, addition of fermentable substrates to
stimulate syntrophic relationships may improve the performance of
BSR reactors supplemented with inexpensive acetate.