15N Discrimination and the Sensitivity of Nitrogen Fixation to Changes in Dietary Nitrogen in Reticulitermes flavipes (Isoptera: Rhinotermitidae)

Xylophagous termites possess symbiotic bacteria that fix atmospheric nitrogen (N-2). Although symbiotic N-2 fixation is central to termite nutrition and ecologically important, it is energetically costly. Using stable isotopes, we tested the hypothesis that symbiotic N-2 fixation 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, Delta(dig), and which occurs as the result of N-2 fixation, Delta(fix), symbiotic N-2 fixation was inhibited via force feeding termites the antibiotic kanamycin. Antibiotic-treated termites and control (N-2-fixing) termites were exposed to different concentrations of dietary N (0, 0.21, and 0.94% N), their N-15 signatures were obtained, and the percent nitrogen derived from the atmosphere within termite samples was calculated. As we hypothesized, symbiotic N2 fixation rates were negatively correlated with dietary N, suggesting that high concentrations of dietary N suppressed symbiotic N-2 fixation in R. flavipes. A comparison of the N-15 isotope signatures of antibiotic-treated termites with their food sources demonstrated that Delta(dig) = 2.284 parts per thousand, and a comparison of the N-15 isotope signatures of antibiotic-treated termites with control termites indicated that Delta(fix) = 1.238 parts per thousand. These are the first estimates of Delta(dig) for R. flavipes, and the first estimate of Delta(fix) for any N-2-fixing termite species.