In Vivo Blue-Light Activation of Chlamydomonas reinhardii Nitrate Reductase

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Abstract
C. reinhardii cells, growing photoautotrophically under air, excreted to the culture medium much higher amounts of NO2- and NH4+ under blue than under red light. Under similar conditions, but with NO2- as the only N source, the cells consumed NO2- and excreted NH4+ at similar rates under blue and red light. In the presence of NO3- and air with 2% CO2 (vol/vol), no excretion of NO2- and NH4+ occurred and, if the bubbling air of the cells that were currently excreting NO2- and NH4+ was enriched with 2% CO2 (vol/vol), the previously excreted reduced N ions were rapidly reassimilated. The levels of total nitrate reductase and active nitrate reductase increased several times in the blue-light-irradiated cells growing on NO3- under air. When tungstate replaced molybdate in the medium (conditions that do not allow the formation of functional nitrate reductase), blue light activated most of the preformed inactive enzyme of the cells. Nitrate reductase extracted from the cells in its inactive form was readily activated in vitro by blue light. Apparently, under high irradiance (90 w m-2) and low CO2 tensions, cells growing on NO3- or NO2- may not have sufficient C skeletons to incorporate all the photogenerated NH4+. Because these cells should have high levels of reducing power, they might use NO3- or, in its absence, NO2- as terminal electron acceptors. The excretion of the products of NO2- and NH4+ to the medium may provide a mechanism to control reductant level in the cells. Blue light is suggested as an important regulatory factor of this photorespiratory consumption of NO3- and possibly of the whole N metabolism in green algae.