The availability of soil phosphorus (P) often limits the productivities of wet tropical lowland forests. Little is known, however, about the metabolomic profile of different chemical P compounds with potentially different uses and about the cycling of P and their variability across space under different tree species in highly diverse tropical rainforests.

We hypothesised that the different strategies of the competing tree species to retranslocate, mineralise, mobilise, and take up P from the soil would promote distinct soil 31P profiles. We tested this hypothesis by performing a metabolomic analysis of the soils in two rainforests in French Guiana using 31P nuclear magnetic resonance (NMR). We analysed 31P NMR chemical shifts in soil solutions of model P compounds.

The results strongly suggest “niches” of soil P profiles associated with physical gradients, mostly topographic position, and with the specific distribution of species along this gradient, which is associated with species-specific strategies of soil P mineralisation, mobilisation, use, and uptake. 

Nuclear magnetic resonance (NMR) spectroscopy remains one of the core analytical platforms for metabolomics, providing complementary chemical information to others, such as mass spectrometry, and offering particular advantages in some areas of research on account of its inherent robustness, reproducibility, and phenomenal dynamic range. While routine experimental protocols for profiling and related statistical analysis pipelines have been established, they often present considerable challenges to the analyst, including spectral overlap, accurate and precise quantification, and chemical shift variation. Consequently, there is still much activity across all areas of NMR spectroscopic analysis in relation to metabolomics. Furthermore, there remain many biological systems and sample types that have not been extensively explored using NMR spectroscopy-based metabolomics.

In this Special Issue, several advances in methodology, and new applications in the field of NMR-based metabolomics, have been presented. In addition, the SI includes authoritative review articles focused on the state-of-the-art of quantitative NMR spectroscopy in biomedical metabolomics applications, and novel applications in the agri-food sector.

Mycorrhizas are known to have a positive impact on plant growth and ability to resist major biotic and abiotic stresses. However, the metabolic alterations underlying mycorrhizal symbiosis are still understudied. By using metabolomics and transcriptomics approaches, cork oak roots colonized by the ectomycorrhizal fungus Pisolithus tinctorius were compared with non-colonized roots. Results from this global metabolomics analysis suggest decreases in root metabolites which are common components of exudates, and in compounds related to root external protective layers which could facilitate plant-fungal contact and enhance symbiosis. Root metabolic pathways involved in defense against stress were induced in ectomycorrhizal roots that could be involved in a plant mechanism to avoid uncontrolled growth of the fungal symbiont in the root apoplast. Several of the identified symbiosis-specific metabolites, such as GABA, may help to understand how ectomycorrhizal fungi such as P. tinctorius benefit their host plants. 

Productivity of tropical lowland moist forests is often limited by availability and functional allocation of phosphorus (P) that drives competition among tree species and becomes a key factor in determining forestall community diversity. We used non-target 31P-NMR metabolic profiling to study the foliar P-metabolism of trees of a French Guiana rainforest. The objective was to test the hypotheses that P-use is species-specific, and that species diversity relates to species P-use and concentrations of P-containing compounds, including inorganic phosphates, orthophosphate monoesters and diesters, phosphonates and organic polyphosphates. Results highlighted the species-specific use of P and the existence of species-specific P-use niches that are driven by the distinct species-specific position in a continuum in the P-allocation from P-storage compounds to P-containing molecules related to energy and anabolic metabolism. 

Oocytes recovered from prepubertal goats are highly heterogeneous in growth and grade of atresia which make them unpredictable for IVEP programs. In our laboratory we have observed that oocytes from 2 month-old goats obtained from > 3 mm follicles develop up to blastocyst stage at a similar percentage than oocytes from adult goats (18% and 21%, respectively), suggesting that the follicle development and the follicular fluid (FF) content are more relevant to oocyte competence than the age of the donor. The aim of this study was to characterize the FF metabolomic profile from different follicular environments. A High-resolution proton nuclear magnetic resonance (1H NMR)-based metabolomic study was carried out. Results showed that metabolomic profiles are different according to the follicle diameter and the female age. Some of these metabolomes could be related to the acquisition of oocyte competence and might be used as biomarkers of oocyte quality.

Plants respond to external environmental conditions such as drought or to the seasonal changes by shifting the foliar C:N:P:K stoichiometry and metabolome. Rates of folivory are usually associated with the water status of plants and/or concentrations of foliar nutrients, especially N, but studies have focused mainly on the concentrations of foliar nutrients or on particular foliar-specific metabolic families. Emerging ecometabolomic techniques allow the study of metabolomes, the total set of metabolites in an organism at a specific moment. This study attempts to integrate stoichiometric and metabolomic techniques, based on NMR spectroscopy and MS spectrometry, to understand the responses of Quercus ilex throughout seasons and under moderate experimental conditions of drought and how these responses affect rates of folivory.  Our data suggest that the increase of sugars and flavonoids in Q. ilex leaves in droughted trees can lead to an increase in herbivorous attack, which implies an indirect relationship between increased drought and rate of folivory produced by metabolomic shifts. The present study represents an advance in understanding the potential cascade effects of drought at different trophic levels and their possible implications in the structure, function and evolution of ecosystems.

Plants defense themselves against herbivory at several levels. One of these levels is the synthesis of inducible chemical defenses. By using NMR metabolomic technique we studied the metabolic changes of plant leaves after a wounding treatment simulating herbivore attack in the Mediterranean sclerophyllous tree Quercus ilex. The study shows the fast metabolic response of Q. ilex to defend its leaves based in a rapid increasing production of quinic acid, quercitol and choline. The results also confirm the suitability of 1H NMR-based metabolomic profiling studies to detect the global metabolome shifts after a biotic stress in tree leaves, and therefore its suitability in ecometabolomic studies.

The ‘exposome’ represents the accumulation of all environmental exposures across a lifetime.  Top-down strategies are required to assess something this comprehensive, and could transform our understanding of how environmental factors affect human health. Metabolic profiling (metabonomics / metabolomics)  defines an individual’s metabolic phenotype, which is influenced by genotype, diet, lifestyle, health and xenobiotic exposure, and could also reveal intermediate biomarkers for disease risk that reflect adaptive response to exposure. We investigated changes in metabolism in volunteers living near a point source of environmental pollution: a closed zinc smelter with associated elevated levels of environmental cadmium. High-resolution 1H NMR spectroscopy (metabonomics) was used to acquire urinary metabolic profiles from 178 human volunteers.

The study showed evidence that an NMR-based metabolic profiling study in an uncontrolled human population is capable of identifying intermediate biomarkers of response to toxicants at true environmental concentrations, paving the way for exposome research.