Detailed profiling of carbon fixation of in silico synthetic autotrophy with reductive tricarboxylic acid cycle and Calvin-Benson-Bassham cycle in Esherichia coli using hydrogen as an energy source

Carbon fixation is the main route of inorganic carbon in the form of CO2 into the biosphere.In nature,RuBisCO is the most abundant protein that photosynthetic organisms use to fix CO2 from the atmosphere through the Calvin-Benson-Bassham(CBB)cycle.However,the CBB cycle is limited by its low catalytic rate and low energy efficiency.In this work,we attempt to integrate the reductive tricarboxylic acid and CBB cycles in silico to further improve carbon fixation capacity.Key heterologous enzymes,mostly carboxylating enzymes,are inserted into the Esherichia coli core metabolic network to assimilate CO2 into biomass using hydrogen as energy source.Overall,such a strain shows enhanced growth yield with simultaneous running of dual carbon fixation cycles.Our key results include the following.(i)We identified two main growth states:carbon-limited and hydrogenlimited;(ii)we identified a hierarchy of carbon fixation usage when hydrogen supply is limited;and(iii)we identified the alternative sub-optimal growth mode while performing genetic perturbation.The results and modeling approach can guide bioengineering projects toward optimal production using such a strain as a microbial cell factory.

Use of sewage sludge biochar as a catalyst in production of biodiesel through thermally induced transesterification

Sewage sludge(SS)is a residual/semi-solid material produced from industrial and municipal wastewater treatment processes.SS contains a high content of lipids and earth alkaline metals that can be used as catalysts for various chemical applications;however,its valorization has rarely been the focus of research.This study demonstrates that SS could be a promising raw material for biodiesel production and a biochar catalyst to promote the reaction kinetics of alkylation.Thermally induced transesterification of the SS extract(SSE)was performed in comparison with the conventional homogeneous reaction.SS biochar was fabricated via pyrolysis.The highest yield(33.5 wt.%per SSE)of biodiesel production was achieved in 1 min of reaction at 305℃via thermally induced transesterification in the presence of SS biochar,while the yield of biodiesel from(trans)esterification with 5 wt.%H_(2)SO_(4)was less than 1%even after 24 h.The reaction kinetics(<1 min)of thermally induced transesterification was extraordinarily faster than that of conventional transesterification(3-24 h).The porous structure and high content of alkaline species in the SS biochar expedited the reaction kinetics.Consequently,the integrated/hybridized process for thermally induced transesterification and pyrolysis of the solid residue of SS was experimentally proved for the valorization of SS in this study.Considering that SS is being disposed of as a waste material and generates toxic chemicals in the environment,its valorization into value-added biodiesel and a catalyst could be an environmentally benign and sustainable technique.