Enhancement of denitrifying activity in cells of roseobacter denitrificans grown aerobically in the light

The effects of light on denitrifying activity during growth were studied in an aerobic photosynthetic bacterium, Roseobacter denitrificans (formerly Erythrobacter sp. OCh 114). When aerobically grown cells were transferred to anaerobic conditions in the presence of nitrate, this bacterium exhibited denitrifying activity, with either succinate or malate serving as an electron donor in addition to endogenous substrates. The final product of denitrification was identified as nitrous oxide (N₂O), a result that confirms the presence of nitrate and nitrite reductases, but not N₂O reductase, in these cells. Illumination during aerobic growth caused a marked enhancement of the denitrifying activity. The activity increased with increasing intensity of light up to 40 mW cm^-2 and was over 20 times that in dark-grown cells. Enhancement of denitrifying activity in illuminated cells was closely related to increases in levels of components that are involved in the denitrifying pathway, namely, nitrate and nitrite reductases. Development of a denitrifying system under aerobic conditions and the enhancement of denitrifying ability by light in Roseobacter denitrificans are unique characteristics, unlike those of other known denitrifying bacteria.