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ABSTRACT Rhodopseudomonas palustrisis renowned for its metabolic versatility, supported by a genome that contains pathways for numerous processes. In a recent study, Oda et al. (Appl Environ Microbiol 91:e02056-24, 2025,https://doi.org/10.1128/aem.02056-24) examineR. palustrisDSM127, a strain with a dramatically reduced gene inventory, to study how environmental pressures have influenced gene loss. Missing more than a quarter of its genome reduces its versatility. However, it may improve its efficiency under the conditions in which it still thrives, enhancing its aptitude as a chassis for biotechnology. 

ABSTRACT Polyhydroxyalkanoates are a diverse class of microbially synthesized polymers that are used to make bioplastics with a wide range of applications. As interest in polyhydroxyalkanoates (PHAs) grows, researchers are faced with a challenge: how best to use the resources at their disposal to reliably quantify PHA produced by their microbe(s) of choice. Investigators must weigh the pros and cons of each method against logistical constraints (e.g., time, money, and equipment) and technical concerns (e.g., accuracy and sensitivity). At the same time, the broader community of scientists researching PHAs should aspire to land on a set of best practices. To this end, we must continually audit our methods. Here, we offer readers a snapshot of popular and emerging approaches for quantifying PHA in the lab. For each method, we provide an overview,list the primary equipment, briefly describe the methods, including improvements or iterations, and discuss the pros and cons of the approach. Along the way, we highlight gaps in research and make recommendations about best practices and future directions. 

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 summaryPurple 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. 

Purple phototrophic bacteria and microbial electrochemical technologies: A new biorefinery concept for wastewater treatmentThe shift towards sustainable wastewater treatment focuses on nutrient recovery through biorefineries, highlighting the importance of microalgae, cyanobacteria, and, more recently, purple phototrophic bacteria for their metabolic flexibility and adaptability. Activated sludge has been the primary strategy for wastewater treatment worldwide for the last century. The efficiency of this process has improved the quality of life and reduced the impact of wastewater on the ecosystem by preventing eutrophication processes. However, given the energetic demand for wastewater treatment, the strategy is now shifting towards nutrient recovery from wastewater instead of pollutant removal (Verstraete et al., 2009). 

Role of extracellular electron transfer in the nitrogen cycleExtracellular electron transfer impacts the nitrogen cycle by enhancing microbial processes and connecting to other biogeochemical cycles. Understanding EET mechanisms provides insights into ecosystem functioning and potential advancements; Arpita Bose and Zhecheng (Robert) Zhang explain. Nitrogen is a fundamental element required by all living species. It can be found in amino acids, proteins, and nucleic acids. The nitrogen cycle promotes nitrogen transformation and transit across the environment, making it available for biological activity. Key steps in the cycle include nitrogen fixation (conversion of atmospheric nitrogen to ammonia), nitrification (oxidation of ammonia to nitrate), denitrification (reduction of nitrate to nitrogen gas), and anaerobic ammonium oxidation (anammox), which then converts ammonium and nitrite directly to nitrogen gas. Extracellular electron transfer (EET) is the mechanism by which microorganisms transmit electrons from their cells to accept electrons from external donors. This ability allows microorganisms to interact with insoluble substrates, which, in turn, influences a variety of biogeochemical cycles, including the nitrogen cycle. Understanding EET’s function sheds light on microbial ecology and environmental processes. 

ABSTRACT With the rising demand for sustainable renewable resources, microorganisms capable of producing bioproducts such as bioplastics are attractive. While many bioproduction systems are well-studied in model organisms, investigating non-model organisms is essential to expand the field and utilize metabolically versatile strains. This investigation centers onRhodopseudomonas palustrisTIE-1, a purple non-sulfur bacterium capable of producing bioplastics. To increase bioplastic production, genes encoding the putative regulatory protein PhaR and the depolymerase PhaZ of the polyhydroxyalkanoate (PHA) biosynthesis pathway were deleted. Genes associated with pathways that might compete with PHA production, specifically those linked to glycogen production and nitrogen fixation, were deleted. Additionally, RuBisCO form I and II genes were integrated into TIE-1’s genome by a phage integration system, developed in this study. Our results show that deletion ofphaRincreases PHA production when TIE-1 is grown photoheterotrophically with butyrate and ammonium chloride (NH₄Cl). Mutants unable to produce glycogen or fix nitrogen show increased PHA production under photoautotrophic growth with hydrogen and NH₄Cl. The most significant increase in PHA production was observed when RuBisCO form I and form I & II genes were overexpressed, five times under photoheterotrophy with butyrate, two times with hydrogen and NH₄Cl, and two times under photoelectrotrophic growth with N₂. In summary, inserting copies of RuBisCO genes into the TIE-1 genome is a more effective strategy than deleting competing pathways to increase PHA production in TIE-1. The successful use of the phage integration system opens numerous opportunities for synthetic biology in TIE-1.IMPORTANCEOur planet has been burdened by pollution resulting from the extensive use of petroleum-derived plastics for the last few decades. Since the discovery of biodegradable plastic alternatives, concerted efforts have been made to enhance their bioproduction. The versatile microorganismRhodopseudomonas palustrisTIE-1 (TIE-1) stands out as a promising candidate for bioplastic synthesis, owing to its ability to use multiple electron sources, fix the greenhouse gas CO₂, and use light as an energy source. Two categories of strains were meticulously designed from the TIE-1 wild-type to augment the production of polyhydroxyalkanoate (PHA), one such bioplastic produced. The first group includes mutants carrying a deletion of thephaRorphaZgenes in the PHA pathway, and those lacking potential competitive carbon and energy sinks to the PHA pathway (namely, glycogen biosynthesis and nitrogen fixation). The second group comprises TIE-1 strains that overexpress RuBisCO form I or form I & II genes inserted via a phage integration system. By studying numerous metabolic mutants and overexpression strains, we conclude that genetic modifications in the environmental microbe TIE-1 can improve PHA production. When combined with other approaches (such as reactor design, use of microbial consortia, and different feedstocks), genetic and metabolic manipulations of purple nonsulfur bacteria like TIE-1 are essential for replacing petroleum-derived plastics with biodegradable plastics like PHA. 

Extracellular electron transfer explainedArpita Bose, PhD from Washington University in St. Louis, guides us through host-associated impacts and biotechnological applications of extracellular electron transfer in electrochemically active bacteria. Electron flow and oxidative and reductive reactions, referred to as “redox reactions,” collectively impact the outcomes of biochemical pathways essential for cell growth, energy conservation, and stress response throughout various organisms. An example of these organisms is electrochemically active bacteria (EAB), which can link internal redox reactions with external electron acceptors or donors via a process known as extracellular electron transfer (EET). 

Purple bacteria and their less known applicationsJungwoo Lee, High-School Student, and Arpita Bose, Associate Professor at Washington University in St. Louis, guide us through purple bacteria and their less-known applications, including wastewater treatment and biofertilization. Purple bacteria, also known as purple photosynthetic bacteria, which belong to the phylum Proteobacteria, can be classified into purple sulfur bacteria (PSB) and purple non-sulfur bacteria (PNSB). In contrast to PSB, PNSB demonstrate the ability to utilize various electron donors and acceptors, which further expands their applications. Their adaptable metabolism, coupled with well-defined genetic manipulation techniques, positions PNSB as ideal models for elucidating the intricacies of metabolic pathways, which hold significant implications for diverse biotechnological applications, including wastewater treatment, and as biofertilizers. 

Abstract Petroleum‐based plastics levy significant environmental and economic costs that can be alleviated with sustainably sourced, biodegradable, and bio‐based polymers such as polyhydroxyalkanoates (PHAs). However, industrial‐scale production of PHAs faces barriers stemming from insufficient product yields and high costs. To address these challenges, we must look beyond the current suite of microbes for PHA production and investigate non‐model organisms with versatile metabolisms. In that vein, we assessed PHA production by the photosynthetic purple non‐sulfur bacteria (PNSB)Rhodomicrobium vannieliiandRhodomicrobium udaipurense.We show that both species accumulate PHA across photo‐heterotrophic, photo‐hydrogenotrophic, photo‐ferrotrophic, and photo‐electrotrophic growth conditions, with either ammonium chloride (NH₄Cl) or dinitrogen gas (N₂) as nitrogen sources. Our data indicate that nitrogen source plays a significant role in dictating PHA synthesis, with N₂fixation promoting PHA production during photoheterotrophy and photoelectrotrophy but inhibiting production during photohydrogenotrophy and photoferrotrophy. We observed the highest PHA titres (up to 44.08 mg/L, or 43.61% cell dry weight) when cells were grown photoheterotrophically on sodium butyrate with N₂, while production was at its lowest during photoelectrotrophy (as low as 0.04 mg/L, or 0.16% cell dry weight). We also find that photohydrogenotrophically grown cells supplemented with NH₄Cl exhibit the highest electron yields – up to 58.89% – while photoheterotrophy demonstrated the lowest (0.27%–1.39%). Finally, we highlight superior electron conversion and PHA production compared to a related PNSB,Rhodopseudomonas palustrisTIE‐1. This study illustrates the value of studying non‐model organisms likeRhodomicrobiumfor sustainable PHA production and indicates future directions for exploring PNSB metabolisms. 

Purple non-sulfur bacteria and the circular economyArpita Bose, Associate Professor at Washington University in St. Louis, discusses the potential of microbial solutions in supporting sustainable and environmentally responsible alternatives to the traditional linear economy. Earth’s climate is undergoing unprecedented changes due to human activities, primarily the emission of greenhouse gases. Widespread petroleum-based production of fuels and plastics releases large amounts of pollution, contributing to rising global temperatures, extreme weather events, and ecosystem disruptions. Finding feasible solutions to the climate crisis is crucial to preserve essential resources and protect human and environmental health. Harnessing and strengthening the natural capabilities of microorganisms and microbial communities with synthetic biology will be the key to reducing and upcycling waste for a greener global economy.