Wastewater and green waste biorefinery Degradation of simple sugars using mixed cultures of purple non-sulfur bacteria: an insight into microbial interactions

Purple non-sulfur bacteria (PNSB) are phototrophic organisms that have raised interest for both industrial and domestic wastewater treatment and valorization because of their very high assimilation (~1 g CODX g-1 COD substrate) and valuable compounds production (i.e hydrogen, polyhydroxyalkanoates, pigments). PNSB have been studied mostly in pure- or co-cultures with fermentative bacteria. Mixed cultures have been little described. This work aims to enrich a stable association of fermenters and PNSB on a carbohydrate-rich wastewater in a single-step process with a mixed culture. Batches with glucose and acetate at different concentrations were inoculated with an in-house mixed-culture. A chemostat was continuously supplied with glucose (5 g COD L-1) and run at a dilution rate of 0.05 h-1 after start-up under batch mode and baseline stabilization of the PNSB enrichment with acetate. The reactor was constantly illuminated with a light filtered at wavelengths superior to 700 nm. Biomass, pigments expression and substrate depletion were tracked along the experiment. A model was developed to anticipate the biomass growth and the substrates consumption. In the batches fed with acetate, PNSB were dominant in purple biomass (>80% vs. total sequencing read count). The main PNSB population were Rhodopseudomonas and Rhodobacter. With glucose, the biomass turned white. The fermenter Enterobacter dominated the microbiomes (>95% abundance). In the stirred-tank reactor, batch operation with acetate led to the production of a substantial biomass enriched in PNSB (4.0 g VSS L-1) that assimilated the nutrients in a molar C-N-P ratio of 100:9.5:0.1 (- n/n/n). Under chemostat regime with glucose, PNSB were efficiently associated with fermenters in a biomass of 1.0-1.5 g VSS L-1. Phase-contrast microscopy displayed the typical morphotypes of Rhodopseudomonas and the fermenter Clostridium. The glucose was completely depleted. The main fermentation products were acetate (between 3 and 10 mmol L-1), ethanol (between 6 and 9 mmol L-1) and butyrate (between 2 and 4 mmol L-1). The hydrogen yield was low (0.033±0.012 mol H2 mol-1 glucose). A fermenters-PNSB mixed culture was successfully built by microbial community engineering by harnessing the chemostat regime to establish the ecological niches and interactions of these organisms. It provides an excellent opportunity to clean carbohydrate-rich wastewater in a compact single-stage process. In the case of direct application of PNSB, a concentrated biomass can be produced. The biomass can even find interesting applications for resource recovery in the form of, e.g., a protein-rich fertilizing biomass (on food and agricultural aqueous wastes), biogas, bulk chemicals and biopolymers.