A Novel Nitrogen-Fixing Bacterium Raoultella electrica Isolated from the Midgut of the Leafhopper Recilia dorsalis

SIMPLE SUMMARY: Nitrogen is a vital element that all living organisms require for growth and development. Although the atmosphere has an abundant supply of gaseous nitrogen, unfortunately, most organisms are unable to utilize it. Some microorganisms are capable of converting nitrogen gas to ammonia, which serves as a nitrogen source available to most organisms. Ammonia is subsequently utilized by organisms to synthesize a range of nitrogen-containing compounds that are necessary for life activities. For example, nitrogen-fixing microorganisms aid leguminous plants in obtaining the required nitrogen. Herbivorous insects face significant growth and development restrictions due to low total nitrogen content in their food. In this study, we isolated a nitrogen-fixing bacterium, Raoultella electrica, from the digestive tract of the leafhopper, Recilia dorsalis. R. electrica has all the nitrogen fixation genes and colonizes the gut lumen of leafhoppers. We further evaluated the growth rate of R. electrica in nitrogen-containing and nitrogen-free media and measured its nitrogenase activity through an acetylene reduction assay. These findings may be useful in studying the biological nitrogen fixation by gut microbes in host insects. ABSTRACT: Nitrogen is a crucial element for the growth and development of insects, but herbivorous insects often suffer from nitrogen nutrition deficiencies in their diets. Some symbiotic microorganisms can provide insect hosts with nitrogen nutrition through nitrogen fixation. Extensive research has clearly demonstrated the process of nitrogen fixation by symbiotic microorganisms in termites, while evidence supporting the occurrence and significance of nitrogen fixation in the diets of the Hemiptera is less conclusive. In this study, we isolated a strain of R. electrica from the digestive tract of a leafhopper, R. dorsalis, and found that it had nitrogen-fixing capabilities. Fluorescence in situ hybridization results showed that it was located in the gut of the leafhopper. Genome sequencing revealed that R. electrica possessed all the genes required for nitrogen fixation. We further evaluated the growth rate of R. electrica in nitrogen-containing and nitrogen-free media and measured its nitrogenase activity through an acetylene reduction assay. The findings of these studies could shed light on how gut microbes contribute to our understanding of nitrogen fixation.