The Assimilation of Nitrogen from Ammonium Salts and Nitrate by Fungi

Abstract

SUMMARY 1. The assimilation of inorganic nitrogen by Scopulariopsis brevicaulis and some physiologically similar species has been studied. Their failure to assimilate completely from ammonium sulphate has been shown to be due to the fall in pH of the medium induced by the initial uptake of ammonia. 2. Complete assimilation of ammonia takes place in the presence of the neutral salts of each of thirteen organic acids investigated. The organic acids act prima rily through their buffering effect which prevents or slows down the fall in pH. They are not specifically required for ammonia assimilation by these fungi and can be effectively replaced by certain inorganic buffers. 3. The influence of several external factors on the rate of assimilation of ammo nia, nitrate, and nitrite has been studied in S. brevicaulis. In corresponding condi tions the mycelium assimilates ammonia more rapidly than .nitrate over a wide range of conditions. 4. Ammonia, even in very low concentration, completely suppresses nitrate assimilation when both sources of nitrogen are present together. Nitrite, how ever, is assimilated simultaneously with ammonia. It is therefore concluded that ammonia blocks the reduction of nitrate to nitrite by the fungus. 5. The suppression of nitrate assimilation in the presence of ammonia is com mon to many mould fungi besides S. brevicaulis, and is believed to have adaptive significance in natural habitats. 6. The nitrate-reducing and assimilating system is formed, even when S. brevi caulis is grown in complete absence of nitrate (ammonia medium with organic acid). It comes into action rapidly when the inhibiting effect of ammonia is removed. Similarly, nitrate-grown mycelium is capable of assimilating ammonia at maximal rate without any adaptive lag.