Seasonal variation in nitrogen fixation by Nostoc commune Vaucher at the Vestfold Hills, Antarctica

Studies of in situ nitrogen fixation by the blue-green alga Nostoc, measured by acetylene reduction, were conducted over a 12 month period at the ice-free Vestfold Hills, Antarctica (68°34′S, 77°57′E). Substantial nitrogenase activity occurred during the austral summer, but ceased when vegetation surface temperatures fell below −7°C. Free-living foliose mats of N. commune Vaucher, submerged during times of snowmelt, reduced acetylene during summer at an average rate of 1.49 nmol C2H4 cm−2 h−1. Damp moss-Nostoc associations growing low on west facing, leeward slopes were more active during this period, reducing acetylene at the rate of 2.53 nmol C2H4 cm−2 h−1, whilst the partially desiccated moss-Nostoc cushions at the top of these slopes exhibited the lowest activity (0.28 nmol C2H4 cm−2h−1). The rates of acetylene reduction by Nostoc are determined by temperature and availability of water. Biological nitrogen fixation by N. commune and by moist associations of moss-Nostoc, are estimated to contribute 52 and 119 mg N m−2 yr−1 to the terrestrial ecosystem, respectively.