ATP Synthesis Coupled to Nitrate Photoreduction in the AlgaHydrodictyon africanum

Abstract

The presence of exogenous $NO3−$ stimulates O₂ evolution in illuminated H. africanum cells at the CO₂ compensation point, and it is likely that $NO3−$ reduction in the light uses reductant produced in non-cyclic electron flow. When sources of ATP other than fermentation are absent, the presence of $NO3−$ , light, and a functional non-cyclic electron transport pathway stimulates active, ATP-dependent H₂$PO4−$ influx. This is consistent with non-cyclic electron flow associated with $NO3−$ reduction being coupled to ATP synthesis. This $NO3−$ -dependent ATP synthesis may be quantitatively important as a source of ATP during photolithotrophic growth with $NO3−$ as N source in H. africanum and other algal unicells.