AUTHOR=Doud Devin F. R. , Angenent Largus T. 

TITLE=Single-Genotype Syntrophy by Rhodopseudomonas palustris Is Not a Strategy to Aid Redox Balance during Anaerobic Degradation of Lignin Monomers

JOURNAL=Frontiers in Microbiology

VOLUME=7

YEAR=2016

URL=https://www.frontiersin.org/journals/microbiology/articles/10.3389/fmicb.2016.01082

DOI=10.3389/fmicb.2016.01082

ISSN=1664-302X

ABSTRACT=<p><italic>Rhodopseudomonas palustris</italic> has emerged as a model microbe for the anaerobic metabolism of <italic>p</italic>-coumarate, which is an aromatic compound and a primary component of lignin. However, under anaerobic conditions, <italic>R. palustris</italic> must actively eliminate excess reducing equivalents through a number of known strategies (e.g., CO<sub>2</sub> fixation, H<sub>2</sub> evolution) to avoid lethal redox imbalance. Others had hypothesized that to ease the burden of this redox imbalance, a clonal population of <italic>R. palustris</italic> could functionally differentiate into a pseudo-consortium. Within this pseudo-consortium, one sub-population would perform the aromatic moiety degradation into acetate, while the other sub-population would oxidize acetate, resulting in a single-genotype syntrophy through acetate sharing. Here, the objective was to test this hypothesis by utilizing microbial electrochemistry as a research tool with the extracellular-electron-transferring bacterium <italic>Geobacter sulfurreducens</italic> as a reporter strain replacing the hypothesized acetate-oxidizing sub-population. We used a 2 × 4 experimental design with pure cultures of <italic>R. palustris</italic> in serum bottles and co-cultures of <italic>R. palustris</italic> and <italic>G. sulfurreducens</italic> in bioelectrochemical systems. This experimental design included growth medium with and without bicarbonate to induce non-lethal and lethal redox imbalance conditions, respectively, in <italic>R. palustris</italic>. Finally, the design also included a mutant strain (NifA<sup>*</sup>) of <italic>R. palustris</italic>, which constitutively produces H<sub>2</sub>, to serve both as a positive control for metabolite secretion (H<sub>2</sub>) to <italic>G. sulfurreducens</italic>, and as a non-lethal redox control for without bicarbonate conditions. Our results demonstrate that acetate sharing between different sub-populations of <italic>R. palustris</italic> does not occur while degrading <italic>p</italic>-coumarate under either non-lethal or lethal redox imbalance conditions. This work highlights the strength of microbial electrochemistry as a tool for studying microbial syntrophy.</p>