Purple non-sulfur bacteria for biotechnological applications

Abstract

In this review, we focus on how purple non-sulfur bacteria can be leveraged for sustainable bioproduction to support the circular economy. We discuss the state of the field with respect to the use of purple bacteria for energy production, their role in wastewater treatment, as a fertilizer, and as a chassis for bioplastic production. We explore their ability to serve as single-cell protein and production platforms for fine chemicals from waste materials. We also introduce more Avant-Garde technologies that leverage the unique metabolisms of purple bacteria, including microbial electrosynthesis and co-culture. These technologies will be pivotal in our efforts to mitigate climate change and circularize the economy in the next two decades.

One-sentence summary: Purple non-sulfur bacteria are utilized for a range of biotechnological applications, including the production of bio-energy, single cell protein, fertilizer, bioplastics, fine chemicals, in wastewater treatment and in novel applications like co-cultures and microbial electrosynthesis.

Keywords: Bioproduction; Biotechnology; Circular economy; Purple bacteria; Sustainability; Waste.

Graphical Abstract

Fig. 1.

The four primary metabolisms of PNSB: (A) Photoheterotrophy utilizing organic sources for electrons and carbon, and light for energy, (B) Photoautotrophy utilizing inorganic electron sources, carbon dioxide for carbon, light for energy, (C) Chemoheterotrophy utilizing organic sources for electrons, carbon and energy, and (D) Chemoautotrophy utilizing inorganic sources for electrons and energy, and carbon dioxide for carbon. In A and B, a simplified photosynthetic electron transport chain (ETC) cycles electrons though the reaction centers (orange) and a cytochrome bc₁ complex (yellow). In C and D, a simplified respiratory ETC directs electrons through NADH dehydrogenase (dark green), a cytochrome bc₁ complex (purple) and a cytochrome cbb₃ oxidase(blue). ATP synthase (light green) uses the proton gradient generated by the ETC to make ATP.

Fig. 2.

Generic chemical structure of poly-hydroxyalkanoate (PHA) and example PHA types with their respective side groups (R). For a comprehensive list of PHA types, refer to Choi et al. (2020).

Fig. 3.

Simplified metabolic pathways for Coenzyme Q10 production in PNSB. The shikimate pathway (left) is used to produce the quinone core, p-hydroxybenzoic acid, to be synthesized with long chain fatty acid decaprenyl diphosphate from the methylerythritol 4-phosphate (MEP) pathway (right) via the ubiquinone pathway to produce Coenzyme Q10 (bottom). Italicized gene names indicate the cluster utilized for that production step.

Fig. 4.

Mechanisms of extracellular electron uptake (EEU) from a cathode in PNSB. (Top) PNSB fix CO₂ to form bioproducts using electrons from (left to right) indirect uptake using H₂ produced from water hydrolysis, indirect uptake using reduced metals or other redox carriers, direct uptake using a conductive material matrix, and direct uptake via cell surface-to-electrode surface contact. (Top, far right) An example of photoautotrophic generation of poly-hydroxybutyrate (PHB) using electrons from the cathode. (Bottom) For R. palustris TIE-1 at potentials below −0.6 V, indirect EEU is favored to occur via hydrolyzed H₂; above −0.6 V direct attachment to the cathode is observed. Adapted with permission from Karthikeyan et al. .