Nitrogenase Activity Associated with Halodule wrightii Roots

Nitrogen fixation (acetylene reduction) associated with roots of the seagrass Halodule wrightii was measured offshore near Beaufort and Moorhead City, N.C. Rates of acetylene reduction were higher in aerobic than in anaerobic assays and were linear for up to 5 days. The temperature range for acetylene reduction was 15 to 35°C with a maximum activity at 35°C. Nitrogenase activity was shown to vary seasonally with highest activities occurring during warmer summer months (23 μg of N₂ fixed per m² per day). At in situ temperature, nitrogenase activities associated with surface-sterilized and non-surface-sterilized roots were similar. One morphological bacterial type was isolated from surface-sterilized roots and identified as Klebsiella pneumoniae type 4B.