Purple Non-Sulfur Bacteria for Circular Bioeconomy Applications

Purple non-sulfur bacteria (PNSB) are increasingly being investigated as microbial platforms for circular bioeconomy applications. Their metabolic flexibility enables growth under diverse environmental conditions and supports the use of a wide range of readily bioavailable organic and inorganic substrates, including volatile fatty acids, organic acids, glycerol, and selected carbohydrates, whereas more complex waste-derived feedstocks generally require prior hydrolysis or acidogenic fermentation. These characteristics make PNSB particularly attractive for integrated resource recovery and the sustainable production of value-added compounds. Beyond their traditional role in wastewater treatment, PNSB can be exploited to produce diverse value-added products, including hydrogen, polyhydroxyalkanoates, microbial biomass, pigments, and biofertilizers, typically through process configurations tailored to specific production objectives. Their ability to recover nitrogen and phosphorus from waste streams supports nutrient recycling and can substantially reduce nutrient loads, although additional polishing steps may be required depending on effluent quality targets. This review provides an overview of the biotechnological applications of PNSB, with particular emphasis on waste valorization, hydrogen production, bioplastic synthesis, nutrient recovery, and sustainable biomass generation. Future perspectives involving synthetic biology, artificial intelligence-assisted process control, and integrated biorefinery concepts are also explored.