Onset Of Nitrogen Starvation Research Articles

Effect of initial biomass-specific photon supply rate on fatty acid accumulation in nitrogen depleted Nannochloropsis gaditana under simulated outdoor light conditions

Triacylglycerol (TAG) accumulation in the microalgae Nannochloropsis gaditana is induced by nitrogen starvation and dependent on the light supplied. We studied under simulated outdoor light conditions the effect of supplied light on the TAG yield by varying the biomass-specific photon supply rate present at the onset of nitrogen starvation. High, intermediate and low average biomass-specific photon supply rates (26, 11 and 6 μmol g−1 s−1) were achieved by applying equal incident light intensity to different biomass concentrations (1.2, 2.9 and 5.4 g L−1). The intermediate biomass-specific photon supply rate resulted in the highest time-averaged TAG yield on light; 0.09 gTAG molph−1. Sub-optimal yields were attributed to photosaturation, photoinhibition, light falling through reactor without being absorbed and high maintenance requirements. The biomass-specific photon supply rate is important to optimize TAG production by microalgae.

Influence of abscisic acid on growth, biomass and lipid yield of Scenedesmus quadricauda under nitrogen starved condition

Scenedesmus quadricauda, accumulated more lipid but with a drastic reduction in biomass yield during nitrogen starvation. Abscisic acid (ABA) being a stress responsible hormone, its effect on growth and biomass with sustainable lipid yield during nitrogen depletion was studied. The result revealed that the ABA level shoots up at 24h (27.21pmol/L) during the onset of nitrogen starvation followed by a sharp decline. The external supplemented ABA showed a positive effect on growth pattern (38×106cells/ml) at a lower concentration. The dry biomass yield is also increasing up to 2.1 fold compared to nitrogen deficient S. quadricauda. The lipid content sustains in 1 and 2μM concentration of ABA under nitrogen-deficient condition. The fatty acid composition of ABA treated S. quadricauda cultures with respect to nitrogen-starved cells showed 11.17% increment in saturated fatty acid content, the desired lipid composition for biofuel application.

Profiling of the Early Nitrogen Stress Response in the Diatom Phaeodactylum tricornutum Reveals a Novel Family of RING-Domain Transcription Factors.

Diatoms often inhabit highly variable habitats where they are confronted with a wide variety of stresses, frequently including starvation of nutrients such as nitrogen. In this study, the transcriptome of the model diatom Phaeodactylum tricornutum was profiled during the onset of nitrogen starvation by RNA sequencing, and overrepresented motifs were determined in promoters of genes that were early and strongly up-regulated during the nitrogen stress response. One of these motifs could be bound by a nitrogen starvation-inducible RING-domain protein termed RING-GAF-Gln-containing protein (RGQ1), which was shown to act as a transcription factor and belongs to a previously uncharacterized family that is conserved in heterokont algae.

Photosynthetic efficiency and carbon partitioning in nitrogen-starved Scenedesmus obliquus

This work investigates how the photoacclimated state at the onset of nitrogen starvation and the light intensity during nitrogen starvation influence carbon partitioning and photosynthetic efficiency in nitrogen-starved Scenedesmus obliquus. Nitrogen-depleted batch cultivations were performed at an incident light intensity of 200 and 1000μmolm−2s−1. These nitrogen-depleted batch cultivations were started with biomass that was photoacclimated to an incident light intensity of either 200 or 1000μmolm−2s−1 under nitrogen replete conditions. During initial nitrogen starvation, fatty acids and starch were produced in a 1:4 ratio. This ratio progressively increased towards only fatty acid synthesis. Hereafter, the initially accumulated starch was degraded and likely used as a substrate for fatty acid synthesis. The ratio between starch and fatty acid synthesis correlated strongly to the biomass nitrogen content. Slightly more carbon was partitioned to starch synthesis at high light intensities. Carbon partitioning was not affected by the photoacclimated state at the onset of nitrogen starvation. The photosynthetic efficiency decreased 2-fold at high versus low light intensities but was not affected by photoacclimation. Likely, the observed carbon partitioning is caused by competition between fatty acid and starch synthesis for a common carbon pre-cursor.

Both the autophagy and proteasomal pathways facilitate the Ubp3p-dependent depletion of a subset of translation and RNA turnover factors during nitrogen starvation in Saccharomyces cerevisiae.

Protein turnover is an important regulatory mechanism that facilitates cellular adaptation to changing environmental conditions. Previous studies have shown that ribosome abundance is reduced during nitrogen starvation by a selective autophagy mechanism termed ribophagy, which is dependent upon the deubiquitinase Ubp3p. In this study, we asked whether the abundance of various translation and RNA turnover factors are reduced following the onset of nitrogen starvation in Saccharomyces cerevisiae. We found distinct differences in the abundance of the proteins tested following nitrogen starvation: (1) The level of some did not change; (2) others were reduced with kinetics similar to ribophagy, and (3) a few proteins were rapidly depleted. Furthermore, different pathways differentially degraded the various proteins upon nitrogen starvation. The translation factors eRF3 and eIF4GI, and the decapping enhancer Pat1p, required an intact autophagy pathway for their depletion. In contrast, the deadenylase subunit Pop2p and the decapping enzyme Dcp2p were rapidly depleted by a proteasome-dependent mechanism. The proteasome-dependent depletion of Dcp2p and Pop2p was also induced by rapamycin, suggesting that the TOR1 pathway influences this pathway. Like ribophagy, depletion of eIF4GI, eRF3, Dcp2p, and Pop2p was dependent upon Ubp3p to varying extents. Together, our results suggest that the autophagy and proteasomal pathways degrade distinct translation and RNA turnover factors in a Ubp3p-dependent manner during nitrogen starvation. While ribophagy is thought to mediate the reutilization of scarce resources during nutrient limitation, our results suggest that the selective degradation of specific proteins could also facilitate a broader reprogramming of the post-transcriptional control of gene expression.

Metabolomic analysis of fission yeast at the onset of nitrogen starvation.

Microorganisms naturally respond to changes in nutritional conditions by adjusting their morphology and physiology. The cellular response of the fission yeast S. pombe to nitrogen starvation has been extensively studied. Here, we report time course metabolomic analysis during one hour immediately after nitrogen starvation, prior to any visible changes in cell morphology except for a tiny increase of cell length per division cycle. We semi-quantitatively measured 75 distinct metabolites, 60% of which changed their level over 2-fold. The most significant changes occurred during the first 15 min, when trehalose, 2-oxoglutarate, and succinate increased, while purine biosynthesis intermediates rapidly diminished. At 30–60 min, free amino acids decreased, although several modified amino acids—including hercynylcysteine sulfoxide, a precursor to ergothioneine—accumulated. Most high-energy metabolites such as ATP, S-adenosyl-methionine or NAD+ remained stable during the whole time course. Very rapid metabolic changes such as the shut-off of purine biosynthesis and the rise of 2-oxoglutarate and succinate can be explained by the depletion of NH4Cl. The changes in the levels of key metabolites, particularly 2-oxoglutarate, might represent an important mechanistic step to trigger subsequent cellular regulations.

The response of diatom central carbon metabolism to nitrogen starvation is different from that of green algae and higher plants.

The availability of nitrogen varies greatly in the ocean and limits primary productivity over large areas. Diatoms, a group of phytoplankton that are responsible for about 20% of global carbon fixation, respond rapidly to influxes of nitrate and are highly successful in upwelling regions. Although recent diatom genome projects have highlighted clues to the success of this group, very little is known about their adaptive response to changing environmental conditions. Here, we compare the proteome of the marine diatom Thalassiosira pseudonana (CCMP 1335) at the onset of nitrogen starvation with that of nitrogen-replete cells using two-dimensional gel electrophoresis. In total, 3,310 protein spots were distinguishable, and we identified 42 proteins increasing and 23 decreasing in abundance (greater than 1.5-fold change; P < 0.005). Proteins involved in the metabolism of nitrogen, amino acids, proteins, and carbohydrates, photosynthesis, and chlorophyll biosynthesis were represented. Comparison of our proteomics data with the transcriptome response of this species under similar growth conditions showed good correlation and provided insight into different levels of response. The T. pseudonana response to nitrogen starvation was also compared with that of the higher plant Arabidopsis (Arabidopsis thaliana), the green alga Chlamydomonas reinhardtii, and the cyanobacterium Prochlorococcus marinus. We have found that the response of diatom carbon metabolism to nitrogen starvation is different from that of other photosynthetic eukaryotes and bears closer resemblance to the response of cyanobacteria.

Time-dependent regulation of yeast glycolysis upon nitrogen starvation depends on cell history

In this study, the authors investigated how the glycolytic flux was regulated in time upon nitrogen starvation of cells with different growth histories. We have compared cells grown in glucose-limited chemostat cultures under respiratory conditions (low dilution rate of 0.1/h) to cells grown under respirofermentative conditions (high dilution rate of 0.35/h). The fermentative capacity was lower in cells grown under respiratory conditions than in cells grown under respirofermentative conditions, yet more resilient to prolonged nitrogen starvation. The time profiles revealed that the fermentative capacity even increased in cells grown under respiratory conditions during the first hours of nitrogen starvation. In cells grown under respirofermentative conditions the fermentative capacity decreased from the onset of nitrogen starvation. We have applied time-dependent Regulation Analysis to follow the fermentative capacity during nitrogen starvation. In both experiments, diverse categories of regulation were found. However, in the cells grown under respiratory conditions regulation was predominantly metabolic, whereas in the cells grown under respirofermentative conditions hierarchical regulation was dominant. To study the metabolic regulation, concentrations of intracellular metabolites, including allosteric regulators, were measured. The obtained results can explain some aspects of the metabolic regulation, but not all.

The phosphoproteome of Fusarium graminearum at the onset of nitrogen starvation

Fusarium graminearum grown under stress, such as nutrient deprivation, activates, among others, the trichothecene pathway that produces the mycotoxin deoxynivalenol and its derivatives. The kinase inhibitor staurosporine reduced the production of trichothecenes by 39% compared with control in vitro. On the other hand, phosphatase inhibitor okadaic acid increased the amount by 72% compared with the control in vitro. This suggests that phosphorylation events are involved in the signalling pathway, leading to the activation of the trichothecene pathway. Three approaches were used to study the phosphoproteome of F. graminearum under nitrogen-limiting conditions: 2-DE (2-DE: IEFxSDS-PAGE) in combination with MS protein identification; SDS-PAGE in combination with off-line IMAC and TiO(2) enrichment and gel electrophoresis LC-MS analysis; and a gel-free approach using strong anion exchange chromatography, IMAC and LC-MS. A total of 348 phosphorylation sites localized in 301 peptides from 241 proteins were identified. By 2-DE, 20 phosphoproteins were identified, nine of which underwent changes during the time course examined. Using gel electrophoresis LC-MS 231 phosphopeptides were identified from three samples (ten gel slices each) at time points of nitrogen starvation t=0, 6, and 12 h. The gel-free analysis added 70 peptides from 65 proteins to the total. Proteins of unknown function and enzymes of known function comprised the largest groups overall. Ten protein kinases and seven transcription factors were identified. This is the first reported phosphoproteome of F. graminearum.

Protein Turnover in Chlorella fusca var. vacuolata: Detection of a Rapidly Degraded Class of Protein in Growing and Non-growing Cells

Chlorella fusca var. vacuolata protein was labelled by incorporation of 14C-labelled leucine or tyrosine. On subsequent incubation with unlabelled amino acid in otherwise nitrogen-free medium, radioactive amino acid accumulated in the medium as a consequence of intracellular protein degradation (turnover). The kinetics of loss of label from protein was first order with two major components. By varying the period of incorporation of 14C-labelled amino acid into protein of growing cells, it was deduced that about 8% of protein was degraded at about 20% h−1 in growing cells. The relative size of the ‘unstable’ fraction decreased at the onset of nitrogen starvation. The remaining cell protein was degraded much more slowly during nitrogen starvation and may have been essentially stable in growing cells.

The Induction of Urea Carboxylase and Allophanate Hydrolase in Saccharomyces cerevisiae

Abstract Saccharomyces cerevisiae can utilize urea as a sole nitrogen source; this compound is degraded to CO2 and NH3 by a multienzyme complex (Whitney, P. A., and Cooper, T. G. (1972) Biochem. Biophys. Res. Commun. 49, 45) composed of urea carboxylase and allophanate hydrolase. Since these activities are present in cultures grown in media containing urea, arginine, or a purine base, the nature of the inducer was investigated. The data presented here indicate that it is the intermediate compound, allophanate, which is the inducer of both urea carboxylase and allophanate hydrolase. This conclusion is based on the observation that: (a) urea induces both activities in a wild type strain, (b) urea has no effect on the basal levels of allophanate hydrolase produced by mutants lacking urea carboxylase, and (c) urea has no effect on the high constitutive level of urea carboxylase produced by a mutant lacking allophanate hydrolase. Consistent with this view is the observation that a urea analogue, formamide, cannot induce allophanate hydrolase in a strain lacking urea carboxylase activity, whereas an allophanate analogue, hydantoic acid, is able to serve as an inducer in such a strain. Both of these compounds are able to effect the induction of both activities in a wild type strain. Mutants blocked in either arginase or urea carboxylase were also used to show that the derepression of allophanate hydrolase following the onset of nitrogen starvation is, in fact, internal induction, and is contingent upon the ability of the cells to produce the inducer, allophanate, from internal arginine and urea pools.