Testing the Potential of Regulatory Sigma Factor Mutants for Wastewater Purification or Bioreactor Run in High Light

Abstract

It is shown that a freshly inoculated culture of the model cyanobacterium Synechocystis sp. PCC 6803 consumed almost all phosphate and 50% of nitrate within 6 days from the nutrient-rich BG-11 growth medium, indicating potential of cyanobacteria to purify wastewaters. Synechocystis sp. PCC 6803 control strain also collected nutrients efficiently from a landfill leachate wastewater KA2 (5.9-6.9 mM ammonium and 0.073-0.077 mM phosphate). Wastewaters might induce oxidative stress to microalgae, which prompted us to test growth of sigma factor inactivation strains, as ΔsigBCE and ΔsigCDE strains show superior growth in chemically induced oxidative stress. All cyanobacterial strains, including a stress-sensitive strain ΔsigBCDE, grew well in KA2 for four days, indicating that KA2 did not cause immediate oxidative stress. Completely arrested growth and bleaching of ΔsigBCDE cells after one week in KA2 wastewater point to the importance of group 2 sigma factor-mediated changes in gene expression during wastewater treatment. The growth of ΔsigBCD was arrested early in un-buffered and Hepes buffered (pH 7.5) KA2. In ΔsigBCD, all phosphate transporter genes are upregulated in standard conditions, and ΔsigBCD cells showed growth defects in low-phosphate BG-11 medium. ΔsigBCD cells removed phosphate slower from KA2 than the control strain, but phosphate supplementation of KA2 did not improve growth of ΔsigBCD. The ΔsigBCE strain showed superior growth in a laboratory-scale bioreactor in bright light and removed phosphate even slightly more efficiently than the control strain if KA2 was Hepes buffered although ΔsigBCE grew slowly in un-buffered KA2 and in low-phosphate BG-11 medium. The results indicate that engineering expression of regulatory group 2 sigma factor(s) might be useful for practical applications.

Roles of Close Homologues SigB and SigD in Heat and High Light Acclimation of the Cyanobacterium Synechocystis sp. PCC 6803

“…The sigB gene is induced in high light (Figure 1), although to a lesser extent than the sigD gene, and similar induction of the RNAP-SigB holoezyme is not seen in high light as is seen for RNAP-SigD [3,16]. Our previous results have shown that high amounts of carotenoids, as well as Flv2, Flv4, and Sll0218 proteins, in ∆sigCDE reduce the production of singlet oxygen and confer resistance of PSII against light-induce damage; simultaneously, reduced contents of glutathione and Flv1/3 proteins led to more pronounced oxidative damage in ∆sigCDE than in the control strain.…”

“…The ∆sigBCE strain remains greener in high light than the other strains because the phycobilins and chlorophyll a are not degraded as much as in the other strains and the carotenoid content remains low. Previous studies have shown that the ∆sigBCE strain is more resistant against chemicallyinduced singlet oxygen and H 2 O 2 stresses than the control strain [3,16]. We have previously shown that the amount of the SigD protein is two times as high in the ∆sigBCE strain as the CS strain [16], suggesting that overproduction of SigD might offer protection against some reactive oxygen species.…”

“…Municipal, piggery, and dairy wastewaters, as well as anaerobic digestion reject waters, are typically nutrient rich, containing plenty of ammonium, nitrate, and phosphate [1,2]. Cyanobacteria are efficient in collecting nutrients [3] and many valuable products, including ethanol, isopropanol, butanol, biohydrogen and alkanes, have already been produced in cyanobacteria [4]. In the most tempting scenario, cyanobacteria are engineered to produce high value compounds, not only biomass, and simultaneously remediate wastewaters.…”