Engineering the diazotroph Azotobacter vinelandii to enhance its utility as a biofertilizer

Abstract

The growing world population will necessitate an increased demand for food production which will require extensive application of nitrogen-based fertilizers. Synthetic chemical fertilizers can deliver nitrogen to plants, but the production of these fertilizers is energy intensive due to reliance on methods like the Haber-Bosch Process that contribute to CO2 emissions and a substantial portion of nitrogen that is delivered is lost to the environment where it can wreak havoc on ecosystems. Thus, there is an urgent need to develop fertilization practices that increase nitrogen use efficiency in plants as well as limit the impact on climate change. Biofertilizers present an eco-friendly alternative to synthetic chemical fertilizers as they are formulations that contain microorganisms that can deliver nutrients that stimulate plant growth, benefit crop productivity, and maintain soil fertility without compromising environmental health. The genus Azotobacter has demonstrated utility as a biofertilizer due in large part to its ability to aerobically fix atmospheric nitrogen gas into ammonia via the enzyme nitrogenase in a process called biological nitrogen fixation (BNF). The research presented here aims to enhance the properties of A. vinelandii as a biofertilizer. The first chapter discusses the ability to engineer A. vinelandii to extracellularly produce the biopolymer γ-polyglutamic acid (γ-PGA), which has utility as a slow-release nitrogen fertilizer, through heterologous expression of γ-PGA producing genes from Bacillus subtilis. The second chapter discusses the ability of A. vinelandii strains that have been deregulated to promote BNF to grow and excrete high levels of ammonium on central metabolites and TCA cycle intermediates, many of which are common plant root exudates. The results show progress in enhancing the capabilities of A. vinelandii as a biofertilizer, but challenges remain before this work could be applied at an industrial scale.