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Photosynthetic energy supply for NO 3 − assimilation in Scenedesmus
Author(s) -
Larsson Marie,
Ingemarsson Björn,
Larsson CarlMagnus
Publication year - 1982
Publication title -
physiologia plantarum
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.351
H-Index - 146
eISSN - 1399-3054
pISSN - 0031-9317
DOI - 10.1111/j.1399-3054.1982.tb00296.x
Subject(s) - scenedesmus , antimycin a , incubation , photosynthesis , electron transport chain , chemistry , metabolism , biochemistry , biophysics , medicinal chemistry , nuclear chemistry , biology , botany , algae
NO 3 − ‐dependent O 2 in synchronous Scenedesmus obtusiusculus Chod. in the absence of CO 2 is stoichiometric with NH 4 + excretion, indicating a close coupling of NO 3 − reduction to non‐cyclic electron flow. Also in the presence of CO 2 , NO 3 − stimulates O 2 evolution as manifested by an increase in the O 2 /CO 2 ratio from 0.96 to 1.11. This quotient was increased to 1.36 by addition of NO 2 − , without competitive interaction with CO 2 fixation, indicating that the capacity for non‐cyclic electron transport at saturating light is non‐limiting for simultaneous reduction of NO 3 − and CO 2 at high rates. During incubation with NO 3 − + CO 2 , no NH 4 + is released to the outer medium, whereas during incubation with NO 2 − + CO 2 , excess NH 4 + is formed and excreted. NO 3 − uptake is stimulated by CO 2 , and this stimulation is also significant when the cellular energy metabolism is restricted by moderate concentrations of carbonyl cyanide‐ p ‐trifluoromethoxyphenylhydrazone, whereas NO 3 − uptake in the absence of CO 2 is severely inhibited by the uncoupler. Also under energy‐restricted conditions NO 3 − uptake is not competitive with CO 2 fixation. Antimycin A is inhibitory for NO 3 − uptake in the absence of CO 2 , and there is no enhancement of NO 3 − uptake by CO 2 in the presence of antimycin A . It is assumed that the energy demand for NO 3 − uptake is met by energy fixed as triosephosphates in the Calvin cycle. Antimycin A possibly affects the transfer of reduced triose phosphates from the chloroplast to the cytoplasm. Active carbon metabolism also seems to exert a control effect on NO 3 − assimilation, inducing complete incorporation of all NO 3 − taken up into amino acids. This control effect is not functional when NO 2 − is the nitrogen source. Active carbon metabolism thus seems to be essential both for provision of energy for NO 3 − uptake and for regulation of the process.