The interaction of whole cell metabolism with the distribution of excitation energy between photosystem 2 (PS2) and photosystem 1 (PSI), the light state transition, was investigated in vivo in the green alga Selenastrum minurum. Nitrogen limited cells of S. minutum were presented with a pulse of either N&+ or NO,- in the light. As shown previously, CO, fixation is inhibited and high rates of N assimilation ensue [( 1986) Plant Physiol. 81,273-2791. N%’ assimilation has a much higher requirement ratio for ATP/NADPH than either CO2 or NOa- assimilation and thus drastically increases the demand for ATP relative to reducing power. Room temperature chlorophyll a fluorescence kinetic measurements showed that a reversible non-photochemical quenching of PS2 fluorescence. accompanied the assimilation of Nh+ but not the assimilation of NOa- or COz. 77K fluorescence emission spectra taken from samples removed at regular intervals during N&’ assimilation showed that the non-photochemical quenching of PS2 was accompanied by a complementary increase in the fluorescence yield of PSl, characteristic of a transition to state 2. Our data suggests that S. minutum responds to the increased demand for ATP/NADPH during NH, assimilation by inducing the light state transition to direct more excitation energy to PSI at the expense of PS2 to increase the production of ATP by cyclic electron transport.