CHANGES IN CELL COMPOSITION AND LIPID METABOLISM MEDIATED BY SODIUM AND NITROGEN AVAILABILITY IN THE MARINE DIATOM PHAEODACTYLUM TRICORNUTUM (BACILLARIOPHYCEAE)¹

Abstract

The effects of nitrogen starvation in the presence or absence of sodium in the culture medium were monitored in batch cultures of the marine diatom Phaeodactylum tricornutum Bohlin. During nitrogen starvation in the presence of sodium, cell nitrogen and chlorophyll a decreased, mainly as a consequence of continued cell division. These decreases were accompanied by decreases in the rates of photosynthesis and respiration. There was no change in either cell volume or carbohydrate, but both carbon and lipid increased. During nitrogen starvation in the absence of sodium, cell division ceased. Cell nitrogen and chlorophyll a remained constant, and respiration did not decrease, but the changes in the photosynthetic rate and the lipid content per cell were similar to cultures that were nitrogen‐starved in the presence of sodium. The carbon‐to‐nitrogen ratio increased in both cultures. Nitrogen, in the form of nitrate, and sodium were resupplied to cultures that had been preconditioned in nitrogen‐ and sodium‐deficient medium for 5 d. Control cultures to which neither nitrate or sodium were added remained in a static state with respect to cell number, volume, and carbohydrate but showed slight increases in lipid. Cells in cultures to which 10 mM nitrate alone was added showed a similar response to cultures where no additions were made. Cells in cultures to which 50 mM sodium alone was added divided for 2 d, with concomitant small decreases in all measured constituents. Cell division resumed in cultures to which both sodium and nitrate were added. The lipid content fell dramatically in these cells and was correlated to metabolic oxidation via measured increases in the activity of the glyoxylate cycle enzyme, isocitrate lyase. We conclude that lipids are stored as a function of decreased growth rate and are metabolized to a small extent when cell division resumes. However, much higher rates of metabolism occur if cell division resumes in the presence of a nitrogen source.

Methylcrotonyl-CoA Carboxylase Regulates Triacylglycerol Accumulation in the Model Diatom Phaeodactylum tricornutum

“…Diatoms can also rapidly induce TAGs under Si limitation, which avoids the detrimental effects on photosynthesis associated with other nutrient limitations (Roessler, 1988;Yu et al, 2009). In P. tricornutum, it has been well documented that the increase in TAGs becomes noticeable as a result of nutrient stress (Larson and Rees, 1996;Yu et al, 2009;Burrows et al, 2012;Valenzuela et al, 2012Valenzuela et al, , 2013Mus et al, 2013), especially nitrogen deficiency. In our study, TAG accumulation was initiated after nitrogen was nearly depleted and the lower initial nitrate concentration resulted in an earlier TAG accumulation in cells.…”

“…It is evident that no nitrate was absorbed during TAG accumulation in the mcc2 mutants. Although Valenzuela et al (2013) showed that lipid accumulation in P. tricornutum was arrested upon resupplementation with depleted nutrients, Larson and Rees (1996) indicated that nitrogen-and sodium-deficient cultures resupplemented with nitrate (KNO 3 ) alone continued to accumulate TAGs. It therefore seems that nitrate availability in the medium is not always the determining factor for TAG accumulation and that the lack of nitrate uptake by cells can also result in lipid accumulation.…”

Changes in photosynthetic carbon allocation in algal assemblages of Arctic sea ice with decreasing nutrient concentrations and irradiance

“…Our experiments were initially done under an average of 15 µmol photons m -2 s -1 , with POC:chl a ratios of ca 80 and finally of ca 25, which contrasted with Smith et al conditions. This process led to an enhanced chl a production with a corresponding nitrogen demand for synthesis of the terapyrrol-ring of chl a (Larson & Rees 1996). Therefore, low carbon allocation into proteins after Day 4 could be explained by higher pigment synthesis under lowlight conditions.…”

Identification of a Δ6 fatty acid elongase gene for arachidonic acid biosynthesis localized to the endoplasmic reticulum in the green microalga Myrmecia incisa Reisigl

“…During nitrogen starvation, algal cells have the capacity to utilize the endogenous stored nitrogen for fundamental metabolic activities, e.g., photosynthesis, to survive; however, the photosynthetic rate in these instances has been reported to decline (Ballin et al, 1988;Larson and Rees, 1996). Although nitrogen deficiency can reduce or even stop cell division when the biosynthesis of nucleic acids and proteins is blocked (Ballin et al, 1988;Larson and Rees, 1996), the biomass of Myrmecia incisa H4301 has been shown to increase under a nitrogen starvation stress (Tong et al, 2011).…”