INSECT-SYMBIONT INTERACTIONS N Discrimination and the Sensitivity of Nitrogen Fixation to Changes in Dietary Nitrogen in Reticulitermes flavipes (Isoptera: Rhinotermitidae)

Xylophagous termites possess symbiotic bacteria that Þx atmospheric nitrogen (N2). Although symbiotic N2 Þxation is central to termite nutrition and ecologically important, it is energetically costly. Using stable isotopes, we tested the hypothesis that symbiotic N2 Þxation would decrease in workers of the eastern subterranean termite, Reticulitermes flavipes Kollar, which were exposed to high nitrogen diets. To calculate the isotope discrimination factor occurring as a result of digestion, dig, and which occurs as the result of N2 Þxation, fix, symbiotic N2 Þxation was inhibited via force feeding termites the antibiotic kanamycin. Antibiotic-treated termites and control (N2Þxing) termites were exposed to different concentrations of dietary N (0, 0.21, and 0.94% N), their N signatures were obtained, and the percent nitrogen derived from the atmosphere within termite samples was calculated. As we hypothesized, symbiotic N2 Þxation rates were negatively correlated with dietary N, suggesting that high concentrations of dietary N suppressed symbiotic N2 Þxation in R. flavipes. A comparison of the N isotope signatures of antibiotic-treated termites with their food sources demonstrated that dig 2.284‰, and a comparison of the N isotope signatures of antibiotic-treated termites with control termites indicated that fix 1.238‰. These are the Þrst estimates of dig for R. flavipes, and the Þrst estimate of fix for any N2-Þxing termite species.