A Study of Nitrate Reduction in Mould Fungi

Abstract

1. The activity of the nitrate-reducing (nitratase) system in fungal mycelium was measured by incubation of the mycelium (either intact or ground) with nitrate in the presence of sodium fluoride which blocks nitrite reduction and causes nitrite accumulation. 2. The nitrate-reducing system in the fungi examined shows rapid changes in activity in response to some environmental factors. Nitratase activity of mycelium in glucose-nitrate medium falls to a low value within 1 hour after addition of ammonia and remains at this value until all added ammonia has been assimilated. The fall in activity appears to depend on the assimilation of ammonia by the mycelium, since it did not occur in conditions when assimilation was prevented. 3. Nitrate-reducing activity declines rapidly in the absence of assimilable carbohydrate, and recovers equally rapidly when carbohydrate is restored. Recovery is accelerated by nitrate but almost completely prevented by ammonia. 4. Evidence is given that these variations in nitrate reduction are primarily changes in the nitrate reductase link in the enzyme system. Nutritional behaviour of the fungi investigated is closely correlated with the observed changes in activity of their nitrate reductase. 5. Nitrate reductase may be formed by these fungi when grown in complete absence of nitrate.