Mechanistic Whole Cell Models of Nitrogen Fixation and Amino Acid Metabolism in Bacteria.

Feeding the growing human population is a pressing issue; one of the BBSRC's strategic research goals is increasing food security. Crop plants are limited in their ability to grow largely by a lack of bio-available nitrogen in the soil. To compensate, large quantities of nitrogen rich fertiliser is added: a practice that is costly both financially and environmentally. Leguminous plants enter a symbiotic relationship with nitrogen fixing bacteria, Diazotrophs, whereby they provide the bacteria with a carbon source in exchange for bio-available nitrogen, allowing these plants to grow
in nitrogen poor soil.

A mechanistic whole cell model of a Diazotroph has been built to simulate its total nitrogen balance in the context of nitrogen fixation and symbiosis with a plant. The aims of this model are to shed light on the mathematics governing symbiosis and determine strategies to maximise the nitrogen output of the system within biological constraints. It has been found that there is a trade-off between the yield of fixed nitrogen and the growth of the Diazotroph population.

Separately, an equation modelling the production and use of amino acids in the cell has been constructed. The amino acid model now represents all major nitrogen pools in the cell. Parameter searches are in progress to improve biological accuracy of the model. The existing nitrogenase equations will be added to the finished amino acid model as the inclusion of the significant pool of nitrogen in the form of amino acids is hypothesised to change the results discovered previously.

Finally, game theory will be used to examine the ecologically stable strategy of the two party symbiosis of bacteria and plant. The conditions necessary for a stable plant/Diazotroph relationship on an ecological level will be explored, then extended to include a third party: an invading bacteria that does not fix nitrogen but is otherwise indistinguishable. The aim for this work is to determine what situations allow an invading bacteria to successfully supplant the established Diazotroph.