SEARCH · PubMed Health
Explore indexed PubMed citations for clinical trials, systematic reviews and public health research. Read source abstracts and follow each citation to its original PubMed record.
Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.
Nutrient limitation of phytoplankton growth in aquatic systems is moving towards a higher incidence of P and Si limitation as a result of increased nitrogen loading, a N:P fertilizer use of 26:1 (molar basis), population growth, and relatively stable silicate loading. This result will likely alter phytoplankton community composition, and may compromise diatom-->zooplankton-->fish food webs.
Studies of steady-state metabolic fluxes in Escherichia coli grown in nutrient-limited chemostat cultures suggest remarkable flux alterations in response to changes of growth-limiting nutrient in the medium (Hua et al., J. Bacteriol. 185:7053-7067, 2003). To elucidate the physiological adaptation of cells to the nutrient condition through the flux change and understand the molecular mechanisms underlying the change in the flux, information on gene expression is of great importance. DNA microarray analysis was performed to investigate the global transcriptional responses of steady-state cells grown in chemostat cultures with limited glucose or ammonia while other environmental conditions and the growth rate were kept constant. In slow-growing cells (specific growth rate of 0.10 h(-1)), 9.8% of a total of 4,071 genes investigated, especially those involved in amino acid metabolism, central carbon and energy metabolism, transport system and cell envelope, were observed to be differentially expressed between the two nutrient-limited cultures. One important characteristic of E. coli grown under nutrient limitation was its capacity to scavenge carbon or nitrogen from the medium through elevating the expression of the corresponding transport and assimilation genes. The number of differentially expressed genes in faster-growing cells (specific growth rate of 0.55 h(-1)), however, decreased to below half of that in slow-growing cells, which could be explained by diverse transcriptional responses to the growth rate under different nutrient limitations. Independent of the growth rate, 92 genes were identified as being differentially expressed. Genes tightly related to the culture conditions were highlighted, some of which may be used to characterize nutrient-limited growth.
Nutrient starvation in the yeast Saccharomyces cerevisiae leads to a number of physiological changes that accompany entry into stationary phase. The expression of genes whose products play a role in stress adaptation is regulated in a manner that allows the cell to sense and respond to changing environmental conditions. We have identified a novel yeast gene, YGP1, that displays homology to the sporulation-specific SPS100 gene. The expression of YGP1 is regulated by nutrient availability. The gene is expressed at a basal level during "respiro-fermentative" (logarithmic) growth. When the glucose concentration in the medium falls below 1%, the YGP1 gene is derepressed and the gene product, gp37, is synthesized at levels up to 50-fold above the basal level. The glucose-sensing mechanism is independent of the SNF1 pathway and does not operate when cells are directly shifted to a low glucose concentration. The expression of YGP1 also responds to the depletion of nitrogen and phosphate, indicating a general response to nutrient deprivation. These results suggest that the YGP1 gene product may be involved in cellular adaptations prior to stationary phase and may be a useful marker protein for monitoring early events associated with the stress response.
Liebig's law of the Minimum is reformulated in terms of biomass composition dynamics. The doctrine of the single limiting nutrient is shown to be invalid generally. The nutritional status of a unicellular organism is expressed in terms of state variables; one which represents the subsistence composition and a number of reserve surplus type variables. It is proposed that the property of being limiting should be defined in terms of the reserve surplus variables. On the basis of this definition, it can be decided whether a nutrient, or combination of nutrients, is limiting, both in transient and steady states. The concept of multiple limitation is shown to have two distinct meanings on these definitions. A non-interactive minimum model, based on a 'hard' minimum operator, is introduced. Smooth interactive models may be formulated which have this minimum model as a limiting case. One such model is described. Numerical simulations show how the behaviour of this smooth model can approximate that of the minimum model: apparently hard non-linearities can arise in the smooth model, through time-scale separation.
Initiation of flocculation ability of Saccharomyces cerevisiae MPY1 cells was observed at the moment the cells stop dividing because of nitrogen limitation. A shift in concentration of the limiting nutrient resulted in a corresponding shift in cell division and initiation of flocculence. Other limitations also led to initiation of flocculence, with magnesium limitation as the exception. Magnesium-limited S. cerevisiae cells did not flocculate at any stage of growth. Cell surface hydrophobicity was found to be strongly correlated with the ability of the yeast cells to flocculate. Hydrophobicity sharply increased at the end of the logarithmic growth phase, shortly before initiation of flocculation ability. Treatments of cells which resulted in a decrease in hydrophobicity also yielded a decrease in flocculation ability. Similarly, the presence of polycations increased both hydrophobicity and the ability to flocculate. Magnesium-limited cells were found to be strongly affected in cell surface hydrophobicity. A proteinaceous cell surface factor(s) was identified as a flocculin. This heat-stable component had a strong emulsifying activity, and appears to be involved in both cell surface hydrophobicity and in flocculation ability of the yeast cells.
Using field-collected periphyton and bacterial isolates, we determined adaptation lag periods for microbial transformation of p-cresol. Lag periods were longer for periphyton samples collected from field sites that were low in dissolved inorganic nitrogen and phosphorus. Moreover, lag periods decreased in samples amended with N or P. Our data suggest that adaptation lag periods for microbial transformation of low concentrations of chemicals may correlate with limiting nutrient concentrations, and this correlation may provide a basis for predictive mathematical modeling of lag periods.
Escherichia coli O157:H7 was grown in chemostats as continuous cultures at different controlled growth rates and under different nutrient limitations to determine the effects on lipopolysaccharide (LPS) structure. LPS from whole cells and extracted using the hot aqueous phenol method was examined by sodium dodecyl sulfate--polyacrylamide gel electrophoresis (SDS-PAGE) and by gel filtration after hydrolysis with acetic acid. At low growth rates under glucose limitation (D = 0.1 h-1, doubling time (td), approx. 416 min; or D = 0.4 h-1, td, approx. 104 min), E. coli O157 produced high molecular weight LPS identical to that previously characterized from cells grown in batch culture. At a high growth rate (D = 0.8 h-1, td, approx. 52 min), the ratio of high molecular weight LPS to low molecular weight LPS produced greatly decreased. A small amount of high molecular weight LPS, containing O-polysaccharide which lacked amino sugars, and which thus was chemically different from that previously characterized, was produced by the cells at high growth rates. The predominant form of LPS from these cells was of slightly higher molecular weight than rough LPS, probably S-R LPS, and it consistently formed aggregates on SDS-PAGE. This form of LPS was also predominant when E. coli O157 was grown under Mg2+ limitation at an intermediate growth rate (D = 0.4 h-1, td, approx. 104 min).
In situ experiments of Nutrient Enrichment Bioassays (NEBs) were performed in a morphologically complex reservoir of a temperate region to identify primary limiting nutrient regulating phytoplankton productivity and determine a severeity of the nutrient limitation. Absolute nutrient contents and nutrient ratios of TN:TP and TDN:TDP in the ambient water indicated a potential limitation of phosphorus. This outcome agreed with the results of NEBs; Algal response in the P treatments, expressed as a ratio of CHLf:CHLi, showed significantly (p < 0.05) greater algal response than P + NH4-N or P + NO3-N treatments. The magnitude of the limitation, however, showed large spatial and temporal variations. The response in treatments enriched with phosphorus (P, P + NH4-N, and P + NO3-N) was greatest in the downlake zone and least in the point-source zone, while the response was greater during summer monsoon than any other seasons. Algal growth rate experiments showed that the response in treatments enriched with NO3-N and P + NO3-N never exceeded > 0.50 microg L(-1) per day and was significantly (p < 0.05) less than that in the three treatments with P (1P, 2P, and P + NH4-N), indicating a reduced response in the treatments enriched with NO3-N. The suppression of nitrate on algal growth seemed to be involved with inhibition effect of nitrogenase activity through low-nitrate uptake rate in the high nitrate-rich environment and large additions of potassium with phosphorus in spike. Regression analysis of log10-transformed CHLf:CHLi ratios against ambient nutrient contents showed that in situ algal response in the P treatments was inversely related (r = -0.77; p < 0.001) with ambient TP and positively correlated (r = 0.75; p < 0.001) with TN:TP ratios. This result suggests that in situ response depends largely on external loading of limiting nutrients.
Utilizing a chemostat with a dual nutrient limitation of nitrogen and phosphate, we examined the transient response of the culture following a pulse of one of the limiting nutrients (ammonia). This method provided quantitative evidence that cells can be grown under dual nutrient limitation. Furthermore, the pattern of response was consistent with the hypothesis that phosphate limitation restricts nucleic acid synthesis in the cell and that nitrogen limitation restricts protein synthesis. The net result is that under a phosphate limitation there is a restricted biosynthetic capacity which we feel is closely associated with the RNA content of the cell.
Dual nutrient limited growth, the control of the cell growth rate (kinetic aspect) or the restriction of the amount of biomass (stoichiometric aspect) by two nutrients at the same time, is a relatively unknown ability of the microorganisms and consequently, still not mentioned in textbooks to date. Nevertheless, multiple nutrient limited or controlled growth has been reported for different systems; e.g. ecosystems, batch, fed-batch, and chemostat cultures. Generally, dual nutrient limited growth has been observed when the microorganism of interest: (a) showed a variation of the cellular composition, (b) was able to accumulate a storage compound, (c) changed the cell metabolism, or (d) excreted metabolic intermediates. Consequently, stoichiometric models have been developed to estimate the growth conditions leading to dual nutrient limited growth. A general problem of the kinetic aspect is the accurate measurement of the growth controlling nutrients in the culture broth (microg l(-1) range), as the cells may consume residual nutrients during sampling. Nevertheless, most models of dual limited growth deal with the kinetic aspect although the control experiments are difficult to carry out. The aim of this survey is to introduce this special growth feature with respect to basic models, experimental data, and potential applications in bioprocesses.
Four major proteins with molecular weights of 78 000, 37 000, 34 000 and 20 000 were present in the envelope of Klebsiella aerogenes when cultured at a high specific growth rate. However, at lower growth rates, the protein content and composition of the envelope depended on the imposed nutrient limitation. Under potassium-, carbon-, sulphur- and phosphorus-limited conditions, derepression of synthesis of limitation-specific proteins was observed, their apparent molecular weights being 90 000, 48 000, 41 000 and 36 000, respectively. Nitrogen-limited cells had no additional proteins. For a particular limiting nutrient, expression of the limitation-specific proteins was independent of the chemical or physical form in which the nutrient was supplied. Under potassium or sulphur limitation the specific proteins were present maximally at the lowest imposed growth rate, whereas under carbon limitation a maximum expression of these proteins was found at moderate growth rates. It is concluded that limitation-specific proteins which are associated with the outer membrane function in the uptake of limiting nutrients or, possibly, limitation-releasing compounds.
SNZ1, a member of a highly conserved gene family, was first identified through studies of proteins synthesized in stationary-phase yeast cells. There are three SNZ genes in Saccharomyces cerevisiae, each of which has another highly conserved gene, named SNO (SNZ proximal open reading frame), upstream. The DNA sequences and relative positions of SNZ and SNO genes have been phylogenetically conserved. This report details studies of the expression of the SNZ-SNO gene pairs under various conditions and phenotypic analysis of snz-sno mutants. An analysis of total RNA was used to determine that adjacent SNZ-SNO gene pairs are coregulated. SNZ2/3 and SNO2/3 mRNAs are induced prior to the diauxic shift and decrease in abundance during the postdiauxic phase, when SNZ1 and SNO1 are induced. In snz2 snz3 mutants, SNZ1 mRNA is induced prior to the diauxic shift, when SNZ2/3 mRNAs are normally induced. Under nitrogen-limiting conditions, SNZ1 mRNAs accumulate in tryptophan, adenine, and uracil auxotrophs but not in prototrophic strains, indicating that induction occurs in response to the limitation of specific nutrients. Strains carrying deletions in all SNZ-SNO gene pairs are viable, but snz1 and sno1 mutants are sensitive to 6-azauracil (6-AU), an inhibitor of purine and pyrimidine biosynthetic enzymes, and methylene blue, a producer of singlet oxygen. The conservation of sequence and chromosomal position, the coregulation and pattern of expression of SNZ1 and SNO1 genes, and the sensitivity of snz1 and sno1 mutants to 6-AU support the hypothesis that the associated proteins are part of an ancient response to nutrient limitation.
Polyhydroxyalkanoates (PHAs) are a class of polyesters biosynthesized by microorganisms (esp. Ralstonia eutropha) under an unbalanced growth condition, and which are supposed to partly take the place of traditional plastics made from petroleum in the near future since they are harmless to the environment and biodegradable. Organic acids (mainly butyrate, lactate, propionate and acetate) produced from anaerobic digested food wastes, industrial wastes and sewage may be used as cheap carbon sources since the large amounts of the above wastes disposed by industry and family each year. In order to better understand the process of PHAs formation with acids as carbon sources, so as to increase the yields of PHAs. Biosynthesis of PHAs by R. eutropha during the dual nutrient-limitation-zone was investigated with mixed organic acids (the mass ratio of the four component acids was butyrate: propionate: acetate: lactate = 3: 3: 1: 1, which was simulated as once the result of anaerobic digestion of food wastes) as carbon sources and (NH4)2 SO4 as nitrogen source. Two different manners of maintaining the dual-nutrient-limitation zone were adopted by feeding mixed acids and (NH4 )2SO4 at determined rates to the fermentation culture which were free of carbon sources (manner A) or nitrogen sources (manner B) firstly. The results suggest that, first of all, the meaning of the limitation of mixed acids or (NH4)2 SO4 does not mean to limit the supply of them, but mean to feed as more as possible of carbon and nitrogen sources in order to meet the cell growth and PHAs formation of R. eutropha by the largest extent. However, it's indispensable to make the residual concentration of carbon and nitrogen sources as low as possible since organic acids are inhibitive to the cell growth, and most importantly, only under the presence of nitrogen during the PHAS formation period of the fermentation could R. eutropha produce more PHAs than any other unbalanced growth condition. Secondly, with the increase of the width of the dual-nutrient-limitation zone, the yield of PHAs would also increase, it suggest that most of the PHAs were biosynthesized during the dual-nutrient-limitation zone. Finally, in contrast with the dual-nutrient-limitation manner of limiting the nitrogen source at first (manner B), the dual-nutrient-limitation manner of limiting the carbon source at first (manner A) was more favorable for the production of PHAs, and the maximum production of PHAs of these two manners are 3.72 g/L and 2.55 g/L, respectively. It may be because that PHAs formation required enzymes could not be well developed when R. eutropha grow under the state of nitrogen limitation from the beginning of fermentation. Besides, yield of PHAs produced by the dual-nutrient-limitation fermentation is larger than that of the single-nutrient-limitation batch culture. Therefore, it seems that to increase the output of PHAs production, the strategy of maintaining as wide as possible the width of dual-nutrient (C, N)-limitation zone may be effective.
Expression of the major outer-membrane porins in Escherichia coli is transcriptionally controlled during nutrient limitation. Expression of ompF was more than 40-fold higher under glucose limitation than under nitrogen (ammonia) limitation in chemostat cultures at the same growth rate. In contrast, ompC expression was higher under N limitation. The basis of regulation by nutrient limitation was investigated using mutations affecting expression of porin genes. The influence of cyaA, rpoS, ackA and pta, as well as the two-component envZ-ompR system, was studied under glucose and N limitation in chemostat cultures. A major contributor to low ompF expression under N limitation was negative control by the RpoS sigma factor. RpoS levels were high under N limitation and loss of RpoS resulted in a 19-fold increase in ompF transcription, but little change was observed with ompC. Lack of RpoS under glucose limitation had a lesser stimulatory effect on ompF expression. Porin production was minimally dependent on EnvZ under N limitation due to OmpR phosphorylation by acetyl phosphate. Evidence obtained with pta and ackA mutants suggested that the acetyl phosphate level also regulates porins independently and indirectly via RpoS and other pathways. pta-envZ double mutants had a residual level of porin transcription, implicating alternative means of OmpR phosphorylation under nutrient limitation. Another critical factor in regulation was the level of cAMP, as a cyaA mutant hardly expressed ompF under glucose limitation but boosted ompC. In addition, the role of DNA-binding proteins encoded by hns and himA was tested under glucose limitation: the hns mutation reduced the glucose-limitation peak, but the himA mutation suppressed the hns effect, suggesting a complex web of interrelationships between the DNA-binding proteins. Indeed, multiple inputs and no single regulator were responsible for the high peak of ompF expression under glucose limitation.
Bacteria often encounter environments where nutrient availability is limited, and they must adapt accordingly. To identify Pasteurella multocida genes that are differentially expressed during nutrient limitation, we utilized whole-genome microarrays to compare levels of gene expression during growth in rich and minimal media. Our analysis showed that the levels of expression of a total of 669 genes, representing approximately one-third of the genome, were detectably altered over the course of the experiment. A large number (n = 439) of genes, including those involved in energy metabolism, transport, protein synthesis, and binding, were expressed at higher levels in rich medium, suggesting that, upon exposure to a rich environment, P. multocida immediately begins to turn on many energy-intensive biosynthetic pathways or, conversely, turns these genes off when it is exposed to a nutrient-deficient environment. Genes with increased expression in minimal medium (n = 230) included those encoding amino acid biosynthesis and transport systems, outer membrane proteins, and heat shock proteins. Importantly, our analysis also identified a large number (n = 164) of genes with unknown functions whose expression was altered during nutrient limitation. Overall, the results of our study show that a wide repertoire of genes, many of which have yet to be functionally classified, undergo transcriptional regulation in P. multocida in response to growth in minimal medium and provide a strong foundation to investigate the transcriptional response of this multispecies pathogen to growth in a nutrient-limited environment.
The effects of various nutrient-limiting conditions on expression of the sty operon in Pseudomonas putida CA-3 were investigated. It was observed that limiting concentrations of the carbon source phenylacetic acid, resulted in high levels of phenylacetyl coenzyme A (CoA) ligase activity, this was accompanied also by upper pathway styrene monooxygenase enzyme activity. The introduction of inorganic nutrient limitations, (nitrate, sulfate and phosphate), caused a dramatic reduction in detectable levels of phenylacetyl CoA ligase activity, particularly in the presence of the primary carbon source, succinate. Under these conditions it was no longer possible to detect styrene monooxygenase activity. Reverse transcription PCR analysis of total RNA, isolated under each of the continuous culture conditions examined, revealed that variations in the levels of enzyme activity coincided with altered patterns of corresponding paaK (phenylacetyl CoA ligase) and styA (styrene monooxygenase) gene expression. Transcription of the upper pathway regulatory sensor kinase gene styS was also observed to be growth condition-dependent. These observations suggest that induction/repression of the sty operon in P. putida CA-3, during growth on phenylacetic acid under continuous culture conditions, involves regulatory mechanisms coordinately affecting both the upper and lower pathways and acting at the level of gene transcription.
We used marine phytoplankton from mesocosms seeded with different zooplankton densities to study the impact of mesozooplankton on phytoplankton nutrient limitation. After 7 d of grazing (copepod mesocosms) or 9 d (appendicularian mesocosms) phytoplankton nutrient limitation was studied by enrichment bioassays. After removal of mesozooplankton, bioassay bottles received either no nutrients, phosphorus or nitrogen alone, or a combination of nitrogen and phosphorus and were incubated for 2 d. Phytoplankton reproductive rates in the bottles without nutrient addition were calculated after correction for grazing by ciliates and indicated increasing nitrogen limitation with increasing copepod abundance. No nutrient limitation was found in the appendicularian mesocosms. The increase of nutrient limitation with increasing copepod density seems to be mainly the result of a trophic cascade effect: Copepods released nanoplankton from ciliate grazing pressure, and thereby enhanced nitrogen exhaustion by nanophytoplankton and reduced nitrogen excretion by ciliates. Nitrogen sequestration in copepod biomass, the mechanism predicted by the ecological stoichiometry theory, seems to have been a weaker effect because there was only little copepod growth during the experiment.
Pelagic bacterial production is often positively correlated, or coupled, with primary production through utilization of autotrophically produced dissolved organic carbon. Recent studies indicate that inorganic N or P can directly limit both bacterial and phytoplanktonic growth. Our mesocosm experiments, with whole communities from mesotrophic Calder Lake, test whether this apparent bacterial-algal coupling may be the result of independent responses to limiting inorganic nutrients. In systems without N additions, numbers of bacteria but not phytoplankton increased 2- to 2.5-fold in response to P fertilization (0 to 2.0 mumol of P per liter); this resulted in uncoupled production patterns. In systems supplemented with 10 mumol of NH(4)NO(3) per liter, P addition resulted in up to threefold increases in bacteria and two- to fivefold increases in total phytoplankton biomass (close coupling). P limitation of pelagic bacteria occurred independently of phytoplankton dynamics, and regressions between bacterial abundance and phytoplankton chlorophyll a were nonsignificant in all systems without added N. We describe a useful and simple coupling index which predicts that shifts in phytoplankton and bacterioplankton growth will be unrelated (Delta bacteria/Delta phytoplankton --> either + infinity or - infinity) in systems with inorganic N/P (molar) ratios of < approximately 40. In systems with higher N/P ratios (>40), the coupling index will approach 1.0 and close coupling between bacteria and phytoplankton is predicted to occur.