| Abstract: | The possible origin of carbon skeletons for ammonium assimilation in Cyanidium caldarium (Tilden) Geitler was investigated. N-sufficient cells assimilated ammonium at a rate of 182 ± 18 μmol·mL packed cell volume (pcv)-1· h-1. Removal of CO2 or darkening almost immediately prevented ammonium assimilation. N-limited cells in light assimilated ammonium at a rate of 493 ± 45 μmol · mL pcv-1· h-1 in the presence of CO2 and at a lower rate of 168 ± 17 μmol · mL pcv-1· h-1 in the absence of CO2. In darkness they assimilated ammonium at a rate of 293 ± 29 μmol · mL pcv-1 h-1 in the presence of CO2, only 60% of the assimilation rate in light. In the absence of CO2, ammonium was assimilated at a similar rate of 325 ± 14 μmol · mL pcv-1· h-1. Under the latter conditions, however, assimilation was inhibited after 40 min and ceased after 70 min; it resumed upon resupply of CO2. We suggest that N-sufficient cells of C. caldarium obtain carbon skeletons for ammonium assimilation exclusively by photosynthetic reactions. Upon N-limitation they develop the ability, apparently through derepression or activation of regulatory enzyme system(s), to obtain a consistent quantity of additional carbon skeletons and ATP from mobilization of carbon reserves. This enables the N-limited cell to assimilate ammonium not only in light but also in darkness, and at a higher rate than N-sufficient cells. The fact that ammonium assimilation in light occurs at a higher rate than in darkness suggests that ammonium assimilation in light is the sum of both light and dark ammonium assimilation, which implies separate metabolic reactions for the two processes. These results suggest the existence of two distinct and differently controlled pathways in N-limited cells, but not in N-sufficient cells, through which carbon skeletons for ammonium assimilation originate. An important role for dark CO2 fixation in dark or light ammonium assimilation is also indicated. |
