Nitrogenase activity during chinook snowmelt sequences by Nostoc commune in Stipa–Bouteloa grassland

Abstract

Inactivation of the nitrogenase enzyme in winter has been observed for many terrestrial cyanophytes of north temperate habitats. In southern Alberta frequent winter chinook activity causes repeated snowmelt sequences. The pattern of nitrogenase activity in these periods has been examined (acetylene-reduction technique) over several snowmelt periods. Nostoc collected from snowmelt pockets was immediately assayed for nitrogenase activity under laboratory (20 °C, 200 E∙m−2s−1 photosynthetically active radiation (PAR)) and field (ambient light and temperature) conditions. Air and thallus temperatures for a typical snowmelt sequence rose from −20 to +5 and −5 to +20 °C, respectively, as chinook warming progressed. Activity on initial snowmelt emergence was 420 nmol C2H4∙g−1∙h−1, rising to 1050 nmol after 7 h, monitored in lab conditions. Field activity reached 90 nmol after 4 h, ceasing at 1800 as temperatures fell below 0 °C. Laboratory rates at morning thaw, day 2, were 1040 nmol, while field activity (3 °C, 900 μE) was 800 nmol. By midday laboratory rates reached 1198 nmol, while field rates (10 °C, 1500 μE) were 1077 nmol. Although these rates are well below those at the temperature optimum of 35 °C (6805 nmol), clearly no winter inactivation of the nitrogenase enzyme occurs here. Cumulative activity over winter chinook sequences may well exceed that occurring during infrequent summer hydration.