Growth Characterization and Transcriptomics of Methanotrophic Bacteria as Effected by Carbon and Nitrogen Sources
While industrial activities have shaped our modern world and lifestyles, one of their many important environmental effects is the significant increase in methane emissions and the resultant atmospheric methane concentrations. A common byproduct of many industries, methane is often burned off as waste or simply released to the atmosphere indiscriminately. This is undesirable as methane is a potent greenhouse gas, second only to carbon dioxide in significance to global warming, and current levels
... are more than double the pre-Industrial Revolution levels. As our climate continues to change rapidly, it is more pressing than ever to pursue mitigation and remediation efforts. Methanotrophs are a specialized class of microorganisms that derive both their carbon and energy from methane. Their ecological impact is huge, playing a major role in the regulation of the methane cycle, and serving as the only biological methane sink. Taxonomically, they spread across the tree of life, however one of the largest groups currently known and studied is the aerobic, proteobacterial methanotrophs, also known as the methane oxidizing bacteria (MOB). These bacteria have long been well-known for their incredible potential in the field of biotechnology, bioremediation, and bioconversion. The benefit of MOB-based methane bioconversion is two-fold: mitigation of undesirable methane released from industry as well as production of a vast inventory of green, value-added products to be sold for profit. Despite their hundred-year history in culture, much remains to be understood about MOB, and these gaps in our knowledge hamper attempts to adopt large-scale industrial methanotroph technologies. This includes even fundamental questions about culturing, down to identifying optimal carbon and nitrogen conditions for growth, arguably the two most vital nutrients to cell function. This uncertainty can manifest in undesirable outcomes for bioprocesses, including slow or inhibited growth and diversion of cell resources away from desired bioproducts. The main carbon sources of interest for industry are methane and methanol, the latter also being a common iii industrial byproduct. In terms of nitrogen, both ammonium and nitrate have been investigated for growth, and both are common industrial N-forms, including use as agricultural fertilizers. It is well documented that both C and both N forms can strongly affect methanotroph growth, possessing unique carbon fixation pathways and diverse nitrifying and denitrifying pathways that affect Nmetabolism. There are complicating factors when discussing optimal growth conditions for aerobic proteobacterial MOB. In fact, this group can be further divided into separate types based on taxonomic and physiological differences: the gammaproteobacterial MOB and the alphaproteobacterial MOB. As such, this work begins with a survey of the physiologies of 5 MOB strains, representing two gamma-MOB and three alpha-MOB, in four C-N growth conditions to determine the effects of C source, N source, and combined C-N conditions on growth. In the following chapters studies, two MOB demonstrating unique physiologies were subject to further transcriptomic analysis, allowing insight into the global gene regulation occurring in these strains during growth in each C-N combination. These works take strides in broadening our understanding of how C and N sources affect MOB, and what conditions are best for ensuring efficient, stable growth suitable for bioindustrial implementation. They also highlight how the carbon and nitrogen central assimilation pathways interact in these methanotrophs, and that a more holistic view is required to build optimized bioprocesses around these bacteria. These are necessary steps towards the establishment of best possible growth outcomes, supporting both economical development and environmentally-responsible industrial activities.