Inorganic carbon stimulates the metabolic routes related to the polyhdroxybutyrate production in a Synechocystis sp. strain (cyanobacteria) isolated from wastewater

New strategy for bioplastic and exopolysaccharides production: Enrichment of field microbiomes with cyanobacteria

Seven photosynthethic microbiomes were collected from field environmental samples to test their potential in polyhydroxybutyrate (PHB) and exopolysaccharides (EPS) production, both alternatives to chemical-based polymers. Microscope observations together with microbial sequence analysis revealed the microbiome enrichment in cyanobacteria after culture growth under phosphorus limitation. PHB and EPS production were studied under three culture factors (phototrophy, mixotrophy and heterotrophy) by evaluating and optimizing the effect of three parameters (organic and inorganic carbon and days under light:dark cycles) by Box-Behnken design. Results showed that optimal conditions for both biopolymers synthesis were microbiome-dependent; however, the addition of organic carbon boosted PHB production in all the tested microbiomes, producing up to 14 %dcw PHB with the addition of 1.2 g acetate·L-1 and seven days under light:dark photoperiods. The highest EPS production was 59 mg·L-1 with the addition of 1.2 g acetate·L-1 and four days under light:dark photoperiods. The methodology used is suitable for enriching microbiomes in cyanobacteria, and for testing the best conditions for bioproduct synthesis for further scale up.

Phycobiliprotein recovery coupled to the tertiary treatment of wastewater in semi-continuous photobioreactors. Tracking contaminants of emerging concern

This study evaluated a tertiary wastewater treatment technology using cyanobacteria to recover value-added phycobiliproteins. The presence of contaminants of emerging concern (CECs) in wastewater, cyanobacteria biomass and pigments recovered were also analyzed. For this, a wastewater-borne cyanobacterium (Synechocystis sp. R2020) was used to treat secondary effluent from a municipal wastewater treatment plant, with and without nutrients supplementation. Then, the stability of phycobiliprotein production was assessed by operating the photobioreactor in semi-continuous mode. Results showed similar biomass productivity with and without nutrients supplementation (153.5 and 146.7 mg L^-1 d^-1, respectively). Upon semi-continuous operation, the phycobiliprotein content was stable and reached up to 74.7 mg gDW^-1. The phycocyanin purity ratio ranged from 0.5 to 0.8, corresponding to food grade (> 0.7). Out of 22 CECs detected in secondary effluent, only 3 were present in the phycobiliprotein extract. In order to identify applications, prospective research should focus on CECs removal during pigment purification.

Production of phycobiliproteins, bioplastics and lipids by the cyanobacteria Synechocystis sp. treating secondary effluent in a biorefinery approach

Cyanobacteria have been identified as promising organisms to reuse nutrients from waste effluents and produce valuable compounds such as lipids, polyhydroxyalkanoates (PHAs), and pigments. However, almost all studies on cyanobacterial biorefineries have been performed under lab scale and short cultivation periods. The present study evaluates the cultivation of the cyanobacterium Synechocystis sp. in a pilot scale 30 L semi-continuous photobioreactor fed with secondary effluent for a period of 120 days to produce phycobiliproteins, polyhydroxybutyrate (PHB) and lipids. To this end, the harvested biomass from the semi-continuous photobioreactor was transferred into 5 L vertical column batch photobioreactors to perform PHB and lipid accumulation under nutrient starvation. Three hydraulic retention times (HRT) (6, 8 and 10 days) were tested in the semi-continuous photobioreactor to evaluate its influence on biomass growth and microbial community. A maximum biomass concentration of 1.413 g L^-1 and maximum productivity of 173 mg L^-1 d^-1 was reached under HRT of 8 days. Microscopy analysis revealed a shift from Synechocystis sp. to Leptolyngbya sp. and green algae when HRT of 6 days was used. Continuous, stable production of phycobiliproteins in the semi-continuous photobioreactor was obtained, reaching a maximum content of 7.4%_dcw in the biomass. In the batch photobioreactors a PHB content of 4.8%_dcw was reached under 7 days of nitrogen and phosphorus starvation, while a lipids content of 44.7%_dcw was achieved under 30 days of nitrogen starvation. PHB and lipids production was strongly dependent on the amount of nutrients withdrawn from the grow phase. In the case of lipids, their production was stimulated when there was only phosphorus depletion. While Nitrogen and phosphorus limitation was needed to enhance the PHB production. In conclusion, this study demonstrates the feasibility of cultivating cyanobacteria in treated wastewater to produce bio-based valuable compounds within a circular bioeconomy approach.