Reduction of inorganic nitrogen (N0j, N0~) in illuminated Scamdumus obtuu-iuAculuA Chod. is coupled to photosynthetic electron transport. Uptake and reduction of NO^ are in balance with incorporation of reduced N during CO2 fixation; whereas in the absence of CO2, the cells liberate NH^ derived from NOj reduction. During CO2 fixation, the ratio between CO2 fixed and N assimilated is fairly constant, irrespective of which N source (NOj, NO^, NH^) and photon flux that is used.

Cells cultured in high CO2 concentrations and subsequently transferred to air-levels of CO2 develop a ^-concentrating mechanism that enables high rates of CO2 fixation at low ambient CO2 concentrations. The quotient between C and N assimilated is the same in algae grown in high and low CO2 concentrations, respectively. Cells transferred from high to low CO2 concentrations appear to increase their rates of photorespiration. However, C and N liberation in this process are considered to be of minor importance in relation to CO2 fixation and primary N assimilation.

The results presented in this thesis point out the central regulatory role of CO2 fixation in N assimilation, In addition, results are presented suggesting a possible regulatory role for nitrate reductase activity in NOj assimilation.