Photosynthetic Nitrogen Metabolism in High and Low CO2-adaptedScenedesmus

Publisher Website
Google Scholar
Abstract
Larsson, M., Larsson, C.-M. and Guerrero, M. G. 1985. Photosynthetic nitrogen metabolism in high and low CO2-adapted Scenedesmus. I. Inorganic carbon-dependent O2 evolution, nitrate utilization and nitrogen recycling.—J. exp Bot. 36: 1373–1386 Scenedesmus obtusiusculus Chod. was grown on an inorganic medium flushed with either air or air supplemented with 3% CO2. In air-grown cells, O2 evolution dependent on low, but not high, HCO−3 concentrations was strongly inhibited by the carbonic anhydrase inhibitor acetazolamide. Cells grown with 3% CO2 exhibited low rates of O2 evolution at low external inorganic C; however, after 30 min in air O2 evolution rates at low inorganic C approached those of air-grown cells. These results are compatible with the view that Scenedesmus develops a ‘CO2 concentrating mechanism’ in air, with carbonic anhydrase as an important constituent When 3% CO2-grown cells were subjected to air-level of CO2, just a transient decline in NO−3 utilization was observed, but in the presence of acetazolamide the rate of the process decreased drastically in response to the decrease in the CO2 level. In CO2-free air NO3− was taken up at high rates but in a deregulated manner, leading to release of NH4+. A portion of the NO3− taken up in the absence of CO2 was apparently assimilated Cellular nitrate reductase (NR) activity initially decreased but subsequently recovered after a transition from 3% CO2 to air. In the presence of acetazolamide, a persistent decrease in NR activity was observed. Cellular glutamine synthetase (GS) activity increased after transition from 3% CO2 to air, the activity increase being unaffected by acetazolamide. NH4+ release to the medium in the presence of L-methionine-D, L-sulphoximine (MSO) transiently increased in 3% CO2-grown cells in response to a transfer to air. MSO-induced NH4+ release was in fact higher in air-grown cells than in 3% CO2-grown cells. Glycollate was initially released after transition from 3% CO2 to air, but there was no difference in glycollate release between MSO-treated and untreated cells. In air-adapted Scenedesmus, N recycling seems to be of minor importance in comparison to primary N assimilation