Enhancement of phycocyanobilin biosynthesis in Escherichia coli by strengthening the supply of precursor and artificially self-assembly complex

Phycocyanobilin(PCB)is widely used in healthcare,food processing,and cosmetics.Escherichia coli is the common engineered bacterium used to produce PCB.However,it still suffers from low production level,pre-cursor deficiency,and low catalytic efficiency.In this study,a highly efficient PCB-producing strain was created.First,chassis strains and enzyme sources were screened,and copy numbers were optimized,affording a PCB titer of 9.1 mg/L.Most importantly,the rate-limiting steps of the PCB biosynthetic pathway were determined,and the supply of precursors necessary for PCB synthesis was increased from endogenous sources,affording a titer of 21.4 mg/L.Then,the key enzymes for PCB synthesis,HO1 and PcyA,were assembled into a multi-enzyme complex using the short peptide tag RIAD-RIDD,and 23.5 mg/L of PCB was obtained.Finally,the basic con-ditions for PCB fermentation were initially determined in 250 mL shake flasks and a 5-L bioreactor to obtain higher titers of PCB.The final titer of PCB reached 147.0 mg/L,which is the highest reported titer of PCB so far.This research provided the foundation for the industrial production of PCB and its derivatives.

Excessive greenhouse gas emissions from wastewater treatment plants by using the chemical oxygen demand standard

Chemical oxygen demand(COD)is widely used as an organic pollution indicator in wastewater treatment plants.Large amounts of organic matter are removed during treatment processes to meet environmental standards,and consequently,substantial greenhouse gases(GHGs)such as methane(CH_(4))are released.However,the COD indicator covers a great amount of refractory organic matter that is not a pollutant and could be a potential carbon sink.Here,we collected and analysed COD data from 86 worldwide municipal wastewater treatment plants(WWTPs)and applied a model published by the Intergovernmental Panel on Climate Change to estimate the emission of CH_(4) due to recalcitrant organic compound processing in China’s municipal wastewater treatment systems.Our results showed that the average contribution of refractory COD to total COD removal was55%in 86 WWTPs.The amount of CH_(4) released from the treatment of recalcitrant organic matter in 2018 could have been as high as 38.22 million tons of carbon dioxide equivalent,which amounts to the annual carbon sequestered by China’s wetlands.This suggests that the use of COD as an indicator for organic pollution is undue and needs to be revised to reduce the emission of GHG.In fact,leaving nontoxic recalcitrant organic matter in the wastewater may create a significant carbon sink and will save energy during the treatment process,aiming at carbon neutrality in the wastewater treatment industry.