Combinatorial use of environmental stresses and genetic engineering to increase ethanol titres in cyanobacteria

Abstract

Current industrial bioethanol production by yeast through fermentation generates carbon dioxide. Carbon neutral bioethanol production by cyanobacteria uses biological fixation (photosynthesis) of carbon dioxide or other waste inorganic carbon sources, whilst being sustainable and renewable. The first ethanologenic cyanobacterial process was developed over two decades ago using Synechococcus elongatus PCC 7942, by incorporating the recombinant pdc and adh genes from Zymomonas mobilis. Further engineering has increased bioethanol titres 24-fold, yet current levels are far below what is required for industrial application. At the heart of the problem is that the rate of carbon fixation cannot be drastically accelerated and carbon partitioning towards bioethanol production impacts on cell fitness. Key progress has been achieved by increasing the precursor pyruvate levels intracellularly, upregulating synthetic genes and knocking out pathways competing for pyruvate. Studies have shown that cyanobacteria accumulate high proportions of carbon reserves that are mobilised under specific environmental stresses or through pathway engineering to increase ethanol production. When used in conjunction with specific genetic knockouts, they supply significantly more carbon for ethanol production. This review will discuss the progress in generating ethanologenic cyanobacteria through chassis engineering, and exploring the impact of environmental stresses on increasing carbon flux towards ethanol production.

Metabolic Engineering of Microorganisms to Produce Pyruvate and Derived Compounds

“…Using photosynthetic organisms provides an alternative production method that eliminates the cost of carbohydrate feedstock. In recent years, cyanobacteria have attracted significant attention because of their ability to use solar energy or CO 2 as the sole energy source to produce biofuels and chemicals [ 102 ]. Researchers successfully used CO 2 as a substrate to produce butanol from cyanobacteria.…”

Cyanobacteria as a Promising Alternative for Sustainable Environment: Synthesis of Biofuel and Biodegradable Plastics

“…Modifying various abiotic parameters like light intensity, carbon source, CO 2 concentration, pH, and medium composition can boost the production of certain molecules (Gao et al, 2012 ). In many situations, optimizing these abiotic and biotic elements together has led to increased yields (Andrews et al, 2021 ). Carrieri et al ( 2010 ) evaluated the influence of salt stress on cyanobacterial fermentation rate.…”

Impact of the carbon flux regulator protein pirC on ethanol production in engineered cyanobacteria

“…The cyanobacteria-based production of the biofuel ethanol is one example, which is commercially promising but is still limited by rather low productivities compared to traditional ethanol production by yeast using plant-derived organic carbon as precursor ( Andrews et al, 2021 ). In fact, the generation of an ethanol-producing Synechococcus elongatus strain by expression of pyruvate decarboxylase (PDC) and ethanol dehydrogenase (ADH) from Zymomonas mobilis was one of the first examples to generate a biofuel-releasing cyanobacterium ( Deng and Coleman, 1999 ).…”

“…PCC 6803 provided better features, particularly because it uses NADPH 2 as the dominating reducing agent in photosynthetic cyanobacterial cells ( Dienst et al, 2014 ). In order to improve ethanol production, several cyanobacterial strains have been engineered, which show different growth rates and resistances towards environmental stresses [reviewed in Andrews et al (2021) ].…”