Mobilization of beta-1,3-glucan and biosynthesis of amino acids induced byNH4+ addition to N-limited cells of the marine diatom Skeletonema costatum(Bacillariophyceae)

Mobilization of the reserve beta -1,3-glucan (chrysolaminaran) in N-limited cells of the marine diatom Skeletonema costatum (Grev.) Cleve (Bacillariop hyceae) was investigated. The diatom was grown in pH-regulated batch cultur es with a 14:10-h light:dark cycle until N depletion. In a pulse-chase expe riment, the cells were first incubated in high light (200 mu mol photons(.) m(-2.)s(-1)) with C-14-bicarbonate until dissolved inorganic carbon was exh austed. Unlabeled bicarbonate (1 mM) was then added, and the cells were inc ubated in the dark and subsequently in low light (20 mu mol photons-m(-2.)s (-1)) with additions of 40 muM NH4+. In the C-14 pulse phase with high ligh t and N depletion, beta -1,3-glucan accumulated and accounted for 85% of in corporated C-14. In the subsequent C-14 chase phases, added NH4+ was assimi lated at an N-specific rate of 0.11 h(-1) in both the dark and low light, a nd in both cases it caused a significant mobilization of beta -1,3-glucan ( dark, 26%; low light, 19%). Biochemical fractionation of organic C-14 showe d that free amino acids were most rapidly labeled in the early stage of NH4 + assimilation, whereas proteins and polysaccharides were labeled more rapi dly after 1.2 h. Analysis of the cellular free amino acids strongly indicat ed that de novo biosynthesis was occurring, with a Gln:Glu ratio increasing from 0.4 to 10 within 1.2 h. After the NH4+ was exhausted, the cellular po ols of glucan and amino acids became constant or slowly decreased. In anoth er experiment, N-limited cells were first incubated in high light until dis solved inorganic carbon was exhausted and were further incubated in high li ght with 150 muM NH4+ under inorganic carbon limitation. Added NH4+ was ass imilated at an N-specific rate of 0.023 h(-1), and cellular beta -1,3-gluca n decreased by 15% within 6 h. Hence, beta -1,3-glucan was mobilized during NH4+ assimilation, even though inorganic carbon was modifying the metaboli c rates. The results provide new evidence of beta -1,3-glucan supplying ess ential precursors for biosynthesis of amino acids and other components in S . costatum in both the dark and subsaturating light and even saturating lig ht under inorganic carbon limitation.