Carbon cost of nitrogenase activity in Frankia–Alnus incana root nodules

Abstract

The carbon cost of nitrogenase activity was investigated to determine symbiotic efficiency of the actinorhizal root nodule symbiosis between the woody perennial Alnus incana and the soil bacterium Frankia. Respiration (CO2 production) and nitrogenase activity (H2 production) by intact nodulated root systems were continuously recorded in short-term assays in an open-flow gas exchange system. The assays were conducted in N2:O2, thus under N2-fixing conditions, in all experiments except for one. This avoided the declines in nitrogenase activity and respiration due to N2 deprivation that occur in acetylene reduction assays and during extended Ar:O2 exposures in H2 assays. Two approaches were used: (i) direct estimation of root and nodule respiration by removing nodules, and (ii) decreasing the partial pressure of O2 from 21 to 15% to use the strong relationship between respiration and nitrogenase activity to calculate CO2/H2. The electron allocation of nitrogenase was determined to be 0.6 and used to convert the results into moles of CO2 produced per 2e− transferred by nitrogenase to reduction of N2. The results ranged from 2.6 to 3.4 mol CO2 produced per 2e−. Carbon cost expressed as g C produced per g N reduced ranged from 4.5 to 5.8. The result for this actinorhizal tree symbiosis is in the low range of estimates for N2-fixing actinorhizal symbioses and crop legumes. Methodology and comparisons of root nodule physiology among actinorhizal and legume plants are discussed.