Fitness outcomes of competitive interactions of Azotobacter vinelandii depend on nitrogen fixation ability and frequency

Primary author: Chandra Jack
Co-author(s): Maren Friesen

Primary college/unit: Agricultural, Human and Natural Resource Sciences
Campus: Pullman


Cooperation is a core feature of sociality. It is one of the key forces behind the transition to multicellularity, the expansion of ecological niches, and increased genetic biological diversity. Altruism can only survive if both parties gain a benefit through increased fitness. The presence of cheaters, individuals that benefit from the relationship but do not provide any in return, can ultimately lead to the collapse of a population because those individuals have higher fitness compared to cooperative members. Prior to industrialized agriculture, over 90% of the nitrogen used by terrestrial plants was due to biological fixation of nitrogen by soil microbes but that number has dropped dramatically with increased application of external nitrogen. We can apply our understanding of microbial sociality to nitrogen-fixing bacteria to determine the correlation between fixation efficiency and fitness. In this study, we compete lab-generated mutants of Azotobacter vinelandii, that vary in their ability to fix nitrogen, at different frequencies to determine the correlation between fixation efficiency, fitness, and population frequency. We expect our results to show frequency-dependent selection where strains that do not fix as much nitrogen (cheaters relative to their competitive partner) will have the highest fitness when they are rare. Our results will also be used to develop models that can predict the outcomes of competitions based on growth rates which we hope to extrapolate to natural isolates. If we can unlock the complexity of microbial interactions, we can boost biological nitrogen fixation and decrease the economic and ecological toll of chemical fertilization.