Researchers led by Ming Chen and Chenling Qu tracked two rice varieties—a japonica cultivar called Jinjingyou and an indica cultivar called C-liangyou Simiao—through 360 days of storage under three regimes. One group sat in conventional storage at 25 degrees Celsius and 65 percent relative humidity. A second group received 99 percent nitrogen for 40 days before re-aeration, a protocol designed to mirror the industry standard for rapid pest eradication. A third received 95 percent nitrogen for 180 days, matching the long-term pest suppression guideline from the Chinese standard LS/T 1225–2022. After the nitrogen phase, both treated groups were returned to ordinary air for the remainder of the year-long experiment.

The warning signs emerged at the end of storage. Freshness, measured by a bromothymol blue assay that tracks the acidic substances that accumulate as rice ages, fell significantly below conventional-storage levels in both nitrogen-treated groups by day 360. Germination rate, a proxy for the biological vigor of the embryo, dropped in parallel. The energy chemistry inside the kernels told the same story: ATP content and the cellular energy charge were significantly depleted in the re-aerated rice, while AMP accumulated, a signature of an energy-starved cell scrambling to stay functional.

To understand why, the team zeroed in on central carbon metabolism—the interconnected network of glycolysis, the tricarboxylic acid cycle, the mitochondrial electron transport chain, and the pentose phosphate pathway that converts stored starch into energy and building blocks. Even after harvest, rice kernels remain alive, burning through their own reserves to maintain basic physiology. The researchers measured the activities of seven key enzymes that gate the flow of carbon through this network, from hexokinase at the entry point of glycolysis to ATP synthase at the final step of mitochondrial energy production.

The enzyme data revealed a striking pattern of collapse. By day 360, mitochondrial isocitrate dehydrogenase, a rate-limiting enzyme of the TCA cycle, and ATP synthase, the core engine of oxidative phosphorylation, were both significantly suppressed in the re-aerated rice compared with conventional storage. The pentose phosphate pathway enzymes glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase were also significantly down, cutting the grain’s supply of NADPH, the reducing power that fuels antioxidant defense and nucleotide synthesis. Glycolytic enzymes showed dramatic fluctuations rather than the slow, stable decline seen in untreated rice, suggesting the treated grain was lurching through cycles of metabolic stress and partial recovery.

The most likely culprit, the authors suggest, is a phenomenon well known in plant physiology: the hypoxia–reoxygenation response. When plant tissues are deprived of oxygen and then suddenly re-exposed, reactive oxygen species surge, damaging mitochondrial membranes and inhibiting the very enzymes that generate cellular energy. The transient spike in isocitrate dehydrogenase and ATP synthase activity observed in the high-nitrogen group between days 40 and 180 fits this picture—a stress-repair response firing up when the air returned, followed by progressive exhaustion as the damage accumulated over the remaining months of storage.

Targeted metabolomics using UHPLC-MS/MS added molecular precision to the picture. Of 140 targeted metabolites, 95 passed quality filtering, and principal component and discriminant analyses cleanly separated the re-aerated rice from conventionally stored grain in both varieties. Five metabolites emerged as biomarkers of the accelerated decline. In the high-nitrogen group, fructose-6-phosphate, guanosine monophosphate, and AMP accumulated while cytosine was depleted—evidence of a traffic jam in glycolysis, disrupted nucleotide metabolism, and impaired capacity for DNA repair. In the low-nitrogen group, homogentisic acid, a precursor of the antioxidants tocopherol and plastoquinone, dropped sharply, signaling a weakened antioxidant shield.

The accumulation of fructose-6-phosphate is particularly telling. This glycolytic intermediate piled up in both varieties under the high-nitrogen protocol, likely because phosphofructokinase, the enzyme that should consume it downstream, was significantly inhibited. A stalled pipeline means less carbon flowing toward ATP production and fewer carbon skeletons for biosynthesis, a metabolic bottleneck that echoes findings in cold-stressed peas and nitrogen-starved apple roots, where fructose-6-phosphate likewise accumulates under duress.

The practical implications are immediate. Nitrogen treatment still does its job during the sealed phase—freshness was actually better preserved at day 180 in the long-nitrogen group than in conventional storage—but the benefits evaporate, and then reverse, once the grain meets ordinary air. The authors recommend that paddy rice be processed and consumed promptly after re-aeration rather than returned to long-term storage. They also point to possible remedies: gradual re-aeration to soften the oxidative burst, or exogenous metabolic regulators to restore carbon flux through the compromised pathways.

Caveats remain. The experiment was conducted under tightly controlled laboratory conditions of constant temperature and humidity, and the authors note that verification in real warehouses with full-scale nitrogen systems and longer storage horizons is still needed. Even so, the study reframes a technology often assumed to be purely protective. The nitrogen atmosphere, it turns out, does not simply pause the life of the grain—it reshapes its metabolism in ways that only become costly when the air comes back.

Subject of Research: Effects of nitrogen-modified atmosphere storage and re-aeration on central carbon metabolism and quality deterioration in stored paddy rice

Article Title: Nitrogen-modified atmosphere with re-aeration storage triggers deterioration in paddy rice by disrupting central carbon metabolism

Article References: Chen, M., Xue, F., Li, M., Liu, X., Zhang, Y., & Qu, C. (2026). Nitrogen-modified atmosphere with re-aeration storage triggers deterioration in paddy rice by disrupting central carbon metabolism. Food Chemistry: X, 39, Article 104524. https://doi.org/10.1016/j.fochx.2026.104524

Image Credits: AI Generated

DOI: 10.1016/j.fochx.2026.104524

Keywords: paddy rice, nitrogen-modified atmosphere storage, re-aeration, central carbon metabolism, glycolysis, TCA cycle, pentose phosphate pathway, ATP synthase, targeted metabolomics, grain storage quality, oxidative stress, seed germination

The new study, published in the journal Metabolomics, set out to characterise the global metabolic responses of deuterium-susceptible and deuterium-tolerant E. coli isolates and to pinpoint the pathways and metabolites associated with isotopic stress tolerance. Although earlier work had quantified kinetic isotope effects for individual enzymes in vitro, and a landmark proteomic study had shown down-regulation of translational machinery and up-regulation of redox-active enzymes in fully deuterated medium, the small-molecule layer where isotope effects are first expressed had remained largely unexplored. Untargeted metabolomics is ideally suited to filling that gap, because it captures labile cofactors and pathway intermediates that often change before transcriptional or translational regulation comes into play. The researchers also drew on earlier evidence that D2O induces DNA damage responses, including recA activation, and that central carbon fluxes are rerouted towards anaplerosis in 70 percent heavy water, implicating tricarboxylic acid cycle inhibition.

To build their experimental framework, the team grew five uropathogenic E. coli isolates alongside the reference strain K-12 MG1655 in minimal medium supplemented with glucose and casamino acids, adjusted to contain 0, 20, 40, 60 or 80 percent D2O. Growth was monitored over 24 hours using an automated Bioscreen C system with continuous shaking at 37 degrees Celsius. The results split cleanly into two phenotypes. Isolates 99, 133 and 134 proved susceptible: their increases in optical density at 600 nanometres became progressively delayed and more gradual as deuterium concentrations rose, with effects already visible at 20 percent in some isolates. In contrast, isolates 25 and 148 and the MG1655 reference strain showed comparatively modest changes, growing earlier and maintaining higher final densities even at 80 percent heavy water. Importantly, the authors interpret this as gradual, strain-dependent inhibition rather than a single critical tipping point, consistent with earlier reports of concentration-dependent effects dating back to the 1960s.

For metabolomic analysis, the team selected three susceptible and three tolerant strains and cultured them at 0, 40 and 80 percent D2O. Because heavy water altered growth profiles differently among strains, samples were harvested at strain- and condition-specific time points corresponding approximately to the midpoint of the observed optical density increase, rather than at a fixed chronological time, ensuring that all cultures were captured at broadly comparable positions within their exponential growth. Metabolism was arrested by quenching cultures into pre-chilled 60 percent methanol at minus 48 degrees Celsius, a step validated to prevent metabolite leakage. Cells were lysed through freeze-thaw cycles in liquid nitrogen, extracts were normalised to a common biomass equivalent, and pooled quality-control samples were prepared to monitor analytical drift. Five biological replicates were collected for every strain and condition, providing the statistical power needed for downstream testing.

Analytical chemistry was performed on a Thermo Fisher Vanquish ultra-high-performance liquid chromatography system coupled to a Q Exactive Orbitrap mass spectrometer, using a Hypersil GOLD aQ C18 column and a 15-minute gradient. Data were acquired independently in positive and negative electrospray ionisation modes, with full scans at a resolving power of 70,000 and data-dependent MS/MS fragmentation across four precursor ranges to support compound annotation. Raw data were processed in Compound Discoverer 3.3 using a stable isotope labelling workflow optimised for deuterium-based probing, with retention time alignment, five-part-per-million mass tolerance, blank filtering requiring sample signals to exceed blanks fivefold, and deuterium incorporation detection set to a maximum exchange of 25. Annotations were cross-referenced against mzCloud, ChemSpider, the Human Metabolome Database and an in-house spectral library, with a minimum spectral match score of 70 percent, yielding Level 2 assignments under the metabolomics standards initiative.

Stringent quality filtering followed. Features detected in fewer than 75 percent of quality-control occurrences were excluded, as were those with relative standard deviations above 30 percent or blank signals exceeding 10 percent of QC averages. After gap filling with a k-nearest-neighbours algorithm and correction of signal drift using quality-control-based robust LOESS, the processed data were analysed in MATLAB. Principal component analysis confirmed tight clustering of pooled QC samples, testifying to high reproducibility. Principal component-discriminant function analysis, using the first ten principal components and 18 classes defined by deuterium level, phenotype and strain, revealed a clear gradient along the first discriminant function corresponding to D2O exposure, with control samples at one end and 80 percent cultures displaced furthest at the other. Crucially, the authors note that this separation reflects metabolic changes induced by heavy water rather than isotopic enrichment, since feature areas sum monoisotopic and labelled ions. Susceptible and tolerant isolates also formed distinct clusters along the second discriminant function, most visibly at 40 percent D2O.

Two-way analysis of variance with Benjamini-Hochberg false discovery rate correction identified 1,418 significantly altered features in positive mode, including 38 annotated metabolites, and 1,086 in negative mode, including 17, with 11 metabolites detected in both modes. A strain-level sensitivity analysis using linear mixed-effects models confirmed that phenotype-level pooling was justified for the vast majority of annotated metabolites, with only about 6 percent of tests showing significant within-phenotype strain effects. In total, 44 significantly altered metabolites mapped across nine pathway categories, including glutathione metabolism, arginine and proline metabolism, arginine biosynthesis, nicotinate and nicotinamide metabolism, purine and pyrimidine metabolism, glycolysis, pyruvate metabolism, the TCA cycle, and branched-chain amino acid degradation. A complementary network analysis using the KEGG-derived E. coli metabolic network through MetExplore placed 35 mapped metabolites across 92 of 105 network pathways, with 30 pathways significantly enriched by hypergeometric over-representation testing.

The pathway-level story is one of coordinated stress remodelling. Susceptible isolates showed marked decreases in the dipeptides valylphenylalanine and alanyltyrosine at 40 percent D2O, alongside a striking rise in free valine and elevated succinate, whereas tolerant isolates accumulated those dipeptides. Prolylleucine fell sharply only in susceptible strains. Isoleucine increased in susceptible isolates, consistent with impaired downstream catabolism under reduced cofactor availability. In NAD-related metabolism, tolerant isolates maintained higher baseline abundances of nicotinate, nicotinamide, NAD+ and ADP-ribose, all of which declined with increasing deuterium in both groups, hinting that kinetic isotope effects perturb proton-coupled reactions in NAD+ biosynthesis. Polyamine metabolism provided perhaps the most dramatic signal: susceptible isolates showed a roughly one-hundred-fold surge in cadaverine at 40 percent D2O and strong putrescine upregulation, while tolerant isolates changed far more moderately, patterns consistent with impaired polyamine export under heavy water.

Nucleotide metabolism told a parallel tale of energetic strain. Purine intermediates from IMP through XMP, GMP and AMP declined progressively in both phenotypes, with susceptible isolates declining earlier and more steeply, suggesting mobilisation of purine pools to sustain ATP and GTP synthesis and TCA cycle activity. Adenosine behaved distinctly, rising in susceptible isolates at 40 percent D2O while falling in tolerant ones, hinting at differential handling of AMP breakdown. On the pyrimidine side, orotate, UMP, uracil and cytidine monophosphate shifts pointed to bottlenecks in de novo synthesis, reduced RNA turnover and a late shift toward energetically favourable salvage routes under severe stress. Glutathione-associated metabolites and 5-oxoproline changes further supported differential antioxidant responses, although the authors caution that steady-state abundances cannot directly establish flux, redox state or enzyme activity. Taken together, these findings supply a biochemical foundation for understanding isotopic stress tolerance, with direct implications for optimising stable isotope probing, refining deuterium-based analytical tools, and engineering more robust microbial chassis for biotechnology and synthetic biology.

Subject of Research: Metabolic responses of Escherichia coli to deuterium oxide-induced isotopic stress

Article Title: Metabolic profiling of Escherichia coli under deuterium oxide-induced stress

Article References: Shams, S., Roberts, I., Rehuman, A. N. S., Gotts, N., Xu, Y., Ahmed, S., Jourdan, F., Winder, C., Burke, A., Goodacre, R., & Muhamadali, H. (2026). Metabolic profiling of Escherichia coli under deuterium oxide-induced stress. Metabolomics, 22(5), Article 156. https://doi.org/10.1007/s11306-026-02532-3

Image Credits: AI Generated

DOI: 10.1007/s11306-026-02532-3

Keywords: E. coli, deuterium oxide, metabolomics, stable isotope probing, kinetic isotope effects, LC-MS, central carbon metabolism, polyamine metabolism, nucleotide metabolism, glutathione, NAD metabolism, isotopic stress tolerance