Homeostasis in nitrogenous uptake assimilation by the green alga Platymonas (Tetraselmis) striata (Prasinophyceae)

When nitrate, ammonium, or urea were the sole nitrogen sources for the growth of Platymonas striata there was a short initial lag period, after which uptake/assimilation occurred at a constant rate. Glycine ‘uptake’ was however non-linear with time. Addition of ammonium or urea markedly inhibited nitrate ‘uptake’ whereas although glycine was initially totally inhibitory, nitrate ‘uptake’ recommenced about 1½ h after addition, but at a greatly reduced rate. Ammonium totally inhibited urea ‘uptake’, but did not inhibit glycine ‘uptake’. When ammonium, urea, or glycine were present at the same time as nitrate the total rate of nitrogen 'uptake/assimilation was within 40% (or closer) of that of nitrate nitrogen ‘uptake’ alone. In the first two cases this reflected a total switching off of nitrate ‘uptake’ whilst the new substrate was assimilated at a rate similar to the initial nitrate rate, whereas in the case of glycine, both substrates were used simultaneously. Once the concentration of any of the inhibitory nitrogen sources had been reduced to a low level the ‘uptake’/assimilation of nitrate recommenced. It is suggested that the ability of P. striata to maintain a more or less constant rate total nitrogen ‘uptake’/assimilation irrespective of the mixture of utilizable nitrogen sources presented to it, is not due to controls at the level of the entry of substrate into the cell, but to intracellular actions at the enzyme level. The relative constancy of, and high rate of ‘uptake’, irrespective of nitrogen source and over a wide range of extracellular concentrations, is tentatively considered to be due to saturation of the assimilatory mechanisms under the conditions employed; all the sources lead, either directly or indirectly, to the production of amino groupings for entry into nitrogenous metabolism. This homeostasis of nitrogen ‘uptake’/assimilation would clearly be of considerable benefit to the alga in nature, enabling it to maximize growth in changing environmental conditions.