Metabolism
of Multiple Aromatic Compounds in Corn
Stover Hydrolysate by <i>Rhodopseudomonas palustris</i>
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Abstract
Lignocellulosic
biomass hydrolysates hold great potential as a
feedstock for microbial biofuel production, due to their high concentration
of fermentable sugars. Present at lower concentrations are a suite
of aromatic compounds that can inhibit fermentation by biofuel-producing
microbes. We have developed a microbial-mediated strategy for removing
these aromatic compounds, using the purple nonsulfur bacterium Rhodopseudomonas palustris. When grown photoheterotrophically
in an anaerobic environment, R. palustris removes most of the aromatics from ammonia fiber expansion (AFEX)
treated corn stover hydrolysate (ACSH), while leaving the sugars mostly
intact. We show that R. palustris can
metabolize a host of aromatic substrates in ACSH that have either
been previously described as unable to support growth, such as methoxylated
aromatics, and those that have not yet been tested, such as aromatic
amides. Removing the aromatics from ACSH with R. palustris, allowed growth of a second microbe that could not grow in the untreated
ACSH. By using defined mutants, we show that most of these aromatic
compounds are metabolized by the benzoyl-CoA pathway. We also show
that loss of enzymes in the benzoyl-CoA pathway prevents total degradation
of the aromatics in the hydrolysate, and instead allows for biological
transformation of this suite of aromatics into selected aromatic compounds
potentially recoverable as an additional bioproduct