ABSTRACT
Stimulation of respiratory activity in green unicellular algae is greater in the presence of NO 3 than NH + because NH + is oxidised to a lesser extent than NO 3 (Weger and Turpin, 1989). Moreover, the value of the respiratory quotient (C0 2 /0 2 ) is higher during NO 3 assimilation than NH + assimilation (Bloom et al., 1989; Bloom et al., 1991). These results suggest that the reduction of NO 3 to NH + consumes more reducing power than NH£ assimiliation alone. The TCA cycle provides NADH, which can be used either for reduction of an oxidised molecule, such as nitrate, or can be transformed into ATP via the mitochondrial chain of electron transfer. Thus nitrate assimilation favours TCA cycle activity which produces NADH, while ammonium assimilation favours both the TCA cycle and mitochondrial electron transport to produce NADH and ATP.
