Towards sustainable bioplastic production in resource limited environments using the photoferroautotrophic and photoelectroautotrophic bacterium Rhodopseudomonas palustris TIE-1

Abstract

Bioplastics are an attractive alternative to petroleum-derived plastics because of the harmful environmental effects of conventional plastics and the impending fossil fuel crisis. Polyhydroxybutyrate (PHB) is a well-known bioplastic that is produced by several microbes using organic carbon sources. Autotrophic (using carbon dioxide or CO2) PHB production is reported for only a few organisms. Sustainable PHB bioproduction using other autotrophic microbes needs to be explored. Rhodopseudomonas palustris, a metabolically versatile purple non-sulfur bacterium (PNSB) is known to produce PHBs under photoheterotrophic conditions. Rhodopseudomonas palustris strain TIE-1 demonstrates extended metabolic versatility by using electron sources such as ferrous iron and poised electrodes for photoautotrophy. Here we report the ability of TIE-1 to produce PHB under photoferroautotrophic (light - energy source, ferrous iron - electron source and CO2 - carbon source) and photoelectroautotrophic (light - energy source, poised electrodes - electron source and CO2 - carbon source) growth conditions. PHB accumulation was observed both under nitrogen (N2) fixing and non-N2 fixing conditions. For comparison, we determined PHB production under chemoheterotrophic, photoheterotrophic and photoautotrophic conditions using hydrogen as the electron donor. Photoferroautotrophic and photoelectroautotrophic PHB production was on par with that observed from organic carbon substrates such as butyrate. PHB production increased during N2 fixation under photoheterotrophic conditions but not during photoautotrophic growth. Electron microscopy confirmed that TIE-1 cells accumulate PHBs internally under the conditions that showed highest production. However, gene expression analysis suggests that PHB cycle genes are not differentially regulated despite observable changes in biopolymer production.