Microbial contributions to the nitrogen ecology of a xylophagous beetle

Microbial symbionts contribute to their insect host’s nutritional status, by facilitating attainment of an optimal carbon: nitrogen balance and performing metabolic functions deficient in hosts. The Asian longhorned beetle (Anoplophora glabripennis) (ALB) is an exotic, wood boring insect from China. Larvae spend about a year in their host tree feeding on a diet composed of recalcitrant polysaccharide and lignin polymers with negligible nitrogen and other nutrients. ALB larvae harbor diverse gut microbiota with significant lignocellulolytic capabilities, supplying metabolic carbon. The nitrogen demands of the larvae however are hypothesized to be met through microbial mediated recycling of nitrogenous waste products, like urea.

Microbial ureC genes (a subcomponent of the urease-encoding gene complex) and gene transcripts were successfully detected from the ALB larval guts.  Microbial provisioning of recycled nitrogen as released NH₃ from urea hydrolysis was quantified.  Using larvae fed on a defined diet containing labeled ¹⁵N-urea, incorporation of recycled ¹⁵NH₃ into larval biomass using stable isotope-ratio mass spectrometry was confirmed.  The released ¹⁵NH₃was used to synthesize both essential and non-essential amino acids in ALB larval biomass.

Results indicate that microbes harbored by the ALB larval gut contribute to its nutritional status by recycling nitrogenous waste products. Further studies to determine the origin of essential amino acids in ALB biomass (microbial vs. plant-derived) are ongoing using ¹³C stable isotope fingerprints of amino acids.  Results may provide insight into the nature of this symbiotic association. To our knowledge, this is the first report of microbial urea hydrolysis for a wood boring cerambycid.