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EFFECTS OF NH 4 + ASSIMILATION ON DARK CARBON FIXATION AND β‐1,3‐GLUCAN METABOLISM IN THE MARINE DIATOM SKELETONEMA COSTATUM (BACILLARIOPHYCEAE)
Author(s) -
Granum Espen,
Myklestad Sverre M.
Publication year - 1999
Publication title -
journal of phycology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.85
H-Index - 127
eISSN - 1529-8817
pISSN - 0022-3646
DOI - 10.1046/j.1529-8817.1999.3561191.x
Subject(s) - biology , diatom , carbon fixation , assimilation (phonology) , algae , botany , metabolism , glucan , photosynthesis , biochemistry , linguistics , philosophy
The effects of NH 4 + assimilation on dark carbon fixation and β‐1,3‐glucan metabolism in the N‐limited marine diatom Skeletonema costatum (Grev.) Cleve (Bacillariophyceae) were investigated by chemical analysis of cell components and incorporation of 14 C‐bicarbonate. The diatom was grown in pH‐regulated batch cultures with a 14:10 h LD cycle until N depletion. The cells were then incubated in the dark with 14 C‐bicarbonate, but without a source of N for 2 h, then in the dark with 63 μmol·L −1 NH 4 + for 3 h. Without N, the cellular concentration of free amino acids was almost constant (∼4.5 fmol·cell −1 ). Added NH 4 + was assimilated at a rate of 12 fmol·cell −1 ·h −1 , and the cellular amino acid pool increased rapidly (doubled in <1 h, tripled in <3 h). The glutamine level increased steeply (45× within 3 h), and the Gln/ Glu ratio increased from 0.1 to 2.4 within 3 h. The rate of dark C fixation during N depletion was only 1.0 fmol·cell −1 ·h −1 . The addition of NH 4 + strongly stimulated dark C fixation, leading to an assimilation rate of 4.0 fmol·cell −1 ·h −1 , corresponding to a molar C/N uptake ratio of 0.33. Biochemical fractionation of organic 14 C showed no significant 14 C fixation into amino acids during N depletion, but during the first 1–2 h of NH 4 + assimilation, amino acids were rapidly radiolabeled, accounting for virtually all net 14 C fixation. These results indicate that anaplerotic β‐carboxylation is activated during NH 4 + assimilation to provide C 4 intermediates for amino acid biosynthesis. The level of cellular β‐1,3‐d‐glucan was constant (16.5 pg·cell −1 ) during N depletion, but NH 4 + assimilation activated a mobilization of 28% of the reserve glucan within 3 h. The results indicate that β‐1,3‐glucan in diatoms is the ultimate substrate for β‐carboxylation, providing precursors for amino acid biosynthesis in addition to energy from respiration.