The effects of dietary nitrogen on symbiotic nitrogen fixation in the eastern subterranean termite, Reticulitermes flavipes (Isoptera: Rhinotermitidae)

Xylophagous termites possess symbiotic bacteria that fix atmospheric nitrogen. Although nitrogen fixation is both central to termite nutrition and ecologically important, it is energetically costly; therefore we hypothesized that a high nitrogen diet would reduce termite dependence on symbiotic nitrogen fixation. We reared workers of the eastern subterranean termite (Reticulitermes flavipes) on three food substrates (pine wood, pine needles, and aspen leaves) that differed significantly in nitrogen content (0.07, 0.36, and 0.93% nitrogen, respectively). Termites and food samples were analyzed using an isotope ratio mass spectrometer. Initial results indicated that nitrogen fixation rates were negatively correlated with dietary nitrogen.

We conducted a follow-up study to assess the effect of nitrogen source on symbiotic nitrogen fixation, as well as to calculate two terms: the isotope discrimination factor occurring as a result of digestion, Δdig, and the isotope discrimination factor occurring as a result of nitrogen fixation, Δfix. In order to eliminate N-fixing symbionts, a subset of R. flavipes workers were force fed the antibiotic kanamicin. Antibiotic-treated termites and control (N-fixing) termites were exposed to different concentrations (0.1 and 1.0% N) of either urea or ammonium nitrate. A comparison of the δ15N signatures of antibiotic-treated termites with their food sources demonstrated that Δdig=2.264, and a comparison with control termites indicated that Δfix=-1.125. Symbiotic nitrogen fixation was negatively correlated with dietary nitrogen for termites exposed to both N sources. These studies are significant as they are the first to calculate Δdig and Δfix for any nitrogen-fixing termite species.