The Asian longhorned beetle's (Anoplophora glabripennis) association with nitrogen fixing bacteria

Many insects rely on symbiotic relationships with microbes to provide digestive capabilities or nutrients the insect cannot synthesize or obtain from feeding on suboptimal diets. The Asian longhorned beetle (Anoplophora glabripennis, ALB), a wood-boring insect introduced into the U.S. from China, feeds on wood of living trees that have not been contaminated with decay fungi. This begged the question of how ALB obtains sufficient nitrogen for growth and development living on a substrate that is strikingly limited in nitrogen. My hypothesis is that ALB, similar to termites, harbors nitrogen-fixing bacteria to meet this nutritional requirement. To test this hypothesis, I used two approaches. First, indirect evidence of nitrogen fixation by ALB larvae was obtained using the acetylene reduction assay. Second, direct evidence of nitrogen-fixing bacteria was obtained by PCR amplification and sequencing of nifH genes from microbial DNA extracted from larval ALB guts. NifH genes encode sub components of the nitrogenase enzyme complex, which converts atmospheric nitrogen into ammonia. In addition, using 16S sequencing of gut bacteria grown anaerobically on nitrogen-free media, I identified the bacterial taxa responsible for nitrogen fixation in the ALB gut. Subsequently, I will quantify incorporation of radio-labeled atmospheric nitrogen that is converted to larval biomass. Diverse forms of nitrogen-fixing microbes are likely more widespread in arthropod guts than previously thought. Nitrogen fixation may represent a significant contribution both to the growth of terrestrial arthropods and to their ecosystem functions of processing carbon and nitrogen.