Biodegradability screening of soil amendments through coupling of wavelength-scanned cavity ring-down spectroscopy to multiple dynamic chambers

A system was developed for the automatic measurements of 13CO? efflux to determine biodegradation of extra carbon amendments to soils. The system combines wavelength-scanned cavity ring down laser spectroscopy (WS-CRDS) with the open-dynamic chamber (ODC) method. The WS-CRDS instrument and a batch of 24 ODC are coupled via microprocessor-controlled valves. Determination of the biodegradation requires a known δ13C value and the applied mass of the carbon compounds, and the biodegradation is calculated based on the 13CO? mixing ratio (ppm) sampled from the headspace of the chambers. The WS-CRDS system provided accurate detection based on parallel samples of three standard gases (13CO? of 2, 11 and 22 ppm) that were measured simultaneously by isotope ratio mass spectrometry (linear regression R2 = 0.99). Repeated checking with the same standards showed that the WS-CRDS system showed no drift over seven months.The applicability of the ODC was checked against the closed static chamber (CSC) method using the rapid biodegradation of cane sugar-δ13C-labeled through C4 photosynthesis. There was no significant difference between the results from 7-min ODC and 120-min CSC measurements. Further, a test using samples of either cane sugar (C4) or beetroot sugar (C3) mixed into standard soil proved the target functionality of the system, which is to identify the biodegradation of carbon sources with significantly different isotopic signatures.     

Incorporation of carbon originating from CO2 into different compounds of soil microbial biomass and soil organic matter

In general, soils without the vegetation growing on them are regarded as sources of CO2. However, there are indications that CO2 is also fixed by soil microorganisms. Although this process is not significant from a quantitative point of view, it may change the isotopic composition of soil organic matter. Therefore, we conducted an incubation study with soil and 13C-labeled CO2 to investigate this process. We found that the label was transferred from CO2 into organic compounds in soil. At the end of a 61-day incubation period, 1.3 micromol C g(-1) soil, corresponding to approximately 0.08% of the soil organic carbon, had been fixed. CO2 may, therefore, be an additional source of soil organic carbon. Compound-specific analysis of amino sugars, amino acids, and fatty acids indicated that the label is incorporated into microbial, mainly bacterial, biomass. All groups of microorganisms were involved in the assimilation of CO2, but the relatively high enrichment of mono-unsaturated and mid-chain branched fatty acids indicates that gram negative bacteria and actinomycetes may be slightly more important in this process than other groups of microorganisms.     

Isolation of bicarbonate from equine urine for isotope ratio mass spectrometry

Sodium bicarbonate administration to horses prior to competition in order to enhance the buffer capacity of the organism is considered as a doping offence. The analysis of the isotopic composition of urinary bicarbonate/CO(2) (TCO(2)) may help to identify an exogenous bicarbonate source, as technical sodium bicarbonate exhibits elevated delta(13)C values compared with urinary total carbon. The isolation of TCO(2) from 60 equine urine samples as BaCO(3) followed by an isotopic analysis shows a significant variability of delta(13)C for TCO(2) of more than 10 per thousand. The delta(13)C of total carbon and TCO(2) seem to reflect different proportions of C3 and C4 plant material in the diet. The isotopic analysis of different mixtures of technical NaHCO(3) and equine urine shows that TCO(2) can be easily isolated without major isotopic fractionation; however, attention has to be paid to the storage time of urine samples, as a shift of delta(13)C of TCO(2) to lower values may occur.     

Influences of heterotrophic and autotrophic resource use on carbon and hydrogen isotopic compositions of tropical tree leaves.

The delta13C and SD values of newly emerging to senescing tree leaves produced during a rainy season were obtained in dry seasonal and moist forest in Panamá. Newly emerging leaves had less negative delta13C values than older leaves yet instantaneous pi/pa was never lowest in the youngest leaves. Furthermore, isotopic enrichment during early growth may have a detectable influence on the delta13C values of mature leaves. The deltaD values of cellulose nitrate were only related to deltaD values of leaf water if leaf age was also considered so that, for a given deltaD of leaf water, deltaD values were highest in the youngest leaves (R2 = 98%). There was no correlation between leaf age and deltaD values of leaf water. Investment of translocated organic carbon is a factor likely to be associated with both 13C and deuterium enrichment effects in new leaves. A coarse, mass balance approach can estimate the proportional heterotrophic investment in leaf growth and improve estimates of integrated pi/pa by approximating delta13C for the most autotrophic phase of leaf growth. Delta13C values of the predominantly sucrose mobile organic fraction in new leaves were less negative than in older leaves, thereby suggesting that the enrichment did not occur at the original site of production of the substrate for new leaf growth. Although the delta values of early leaf growth must be influenced by inputs of translocated organic carbon, enrichment effects, per se, are apparently caused by other mechanisms such as, for carbon, de novo sucrose synthesis and anaplerotic replenishment. Better recognition of metabolic causes of isotopic enrichment in leaves promises to increase the power and accuracy of inferences about carbon and water use of tropical trees from delta analyses.     

Double-quantum filtered rotational-echo double resonance

The homonuclear scalar coupling of a directly bonded 13C-13C pair has been used to create a double-quantum filter (DQF) to remove the natural-abundance 13C background in 13C{15N} rotational-echo double-resonance (REDOR) experiments. The DQF scalar and REDOR dipolar evolution periods are coincident which is important for sensitivity in the event of weak 13C-15N dipolar coupling. Calculated and observed 13C{15N} DQF-REDOR dephasings were in agreement for a test sample of mixed recrystallized labeled alanines. Glycine metabolism in a single uniform-15N soybean leaf labeled for 6 min by 13CO2 was measured quantitatively by 13C{15N} DQF-REDOR with no background interferences.     

中红外波段大气碳同位素激光吸收光谱研究

大气碳同位素在环境污染源汇示踪和地球化学发展等方面的应用越来越广泛,在其探测技术方面,激光吸收光谱技术具有体积小、可在线、灵敏度高等优点,在气体同位素探测中越来越受到重视。工作中研究了2.7 μm波段的分布式反馈激光器(distributed feedback laser, DFB)可调谐半导体激光器的性能,在遵循12CO₂和13CO₂同位素分子吸收谱线特征和同位素分子谱线选择原则的基础上,确定了合适的激光器输出波长。结合光程390.3 m的新型多次反射池,实现了大气中CO₂分子的δ13C同位素丰度探测。     

Microcombustion of ng Amounts of Carbon in Non-Volatile Materials for isotope Ratio Evaluation

Abstract A novel microcombustion technique for carbon isotopic analysis of nanogram amounts of carbon in non-volatile materials based on isotope ratio monitoring (irm) mass spectrometry is described. Liquid or solid samples placed in a quartz sleeve are combusted at 1000°C in a continuous stream of helium and oxygen. CO(2) removed from the carrier gas stream by cryogenic trapping is transferred onto a GC column. Following GC separation, the CO(2) is transferred via an open split to the ion source of a gas isotope ratio mass spectrometer. Reproducibility for samples >25 nmol carbon is <1‰. Problems associated with blanks from various sources and with reproducible deposition of small sample amounts led to variable accuracy, which was dependent on the compound class being analysed. Minimum sample size was in the range from 5 to 10 nmol carbon. Measurements of dissolved organic carbon (DOC) of groundwater from Germany yielded consistent values of δ(13)C = -28.8‰.     

What are the instrumentation requirements for measuring the isotopic composition of net ecosystem exchange of CO2 using eddy covariance methods?

Better quantification of isotope ratios of atmosphere-ecosystem exchange of CO2 could substantially improve our ability to probe underlying physiological and ecological mechanisms controlling ecosystem carbon exchange, but the ability to make long-term continuous measurements of isotope ratios of exchange fluxes has been limited by measurement difficulties. In particular, direct eddy covariance methods have not yet been used for measuring the isotopic composition of ecosystem fluxes. In this article, we explore the feasibility of such measurements by (a) proposing a general criterion for judging whether a sensor's performance is sufficient for making such measurements (the criterion is met when the contribution of sensor error to the flux measurement error is comparable to or less than the contribution of meteorological noise inherently associated with turbulence flux measurements); (b) using data-based numerical simulations to quantify the level of sensor precision and stability required to meet this criterion for making direct eddy covariance measurements of the 13C/12C ratio of CO2 fluxes above a specific ecosystem (a mid-latitude temperate forest in central Massachusetts, USA); (c) testing whether the performance of a new sensor-a prototype pulsed quantum cascade laser (QCL) based isotope-ratio absorption spectrometer (and plausible improvements thereon)-is sufficient for meeting the criterion in this ecosystem. We found that the error contribution from a prototype sensor (approximately 0.2 per thousand, 1 SD of 10 s integrations) to total isoflux measurement error was comparable to (1.5 to 2x) the irreducible 'meteorological' noise inherently associated with turbulent flux measurements above this ecosystem (daytime measurement error SD of approximately 60% of flux versus meteorological noise of 30-40% for instantaneous half-hour fluxes). Our analysis also shows that plausible instrument improvements (increase of sensor precision to approximately 0.1 per thousand, 1 SD of 10 s integrations, and increased sensor stability during the half-hour needed to integrate eddy covariance measurements) should decrease the contribution of sensor error to the point where it is less than the contribution from meteorological noise. This suggests that new sensors using QCL-based isotope ratio absorption spectroscopy should make continuous long-term observations of the isotopic composition of CO2 fluxes via eddy covariance methods feasible.     

Isotope effect of the phonons mean free path in graphene by micro-Raman measurement

The isotope labeled graphene was synthesized in the concentration of ¹³C carbon atom in 1%, 25%, 50%, 75% and 99%. The isotope effect on the phonon behavior in graphene was investigated based on the micro-Raman analysis of ¹³C isotope labeled graphene samples. We found that the phonon scattering is affected by the isotopic carbon atom as a point defect. Based on the experiment results, the Klemens-Callaway model and uncertainty principle were used to obtain the mean free path of the G and D phonons. The results agree with the thermal conductivity measurement by non-contact optical method and with other theoretical calculations.     

Continuous field measurements of delta(13)C-CO(2) and trace gases by FTIR spectroscopy

Continuous analysis of the (13)C/(12)C ratio of atmospheric CO(2) (delta(13)C-CO(2)) is a powerful tool to quantify CO(2) flux strengths of the two major ecosystem processes assimilation and respiration. Traditional laboratory techniques such as isotope ratio mass spectrometry (IRMS) in combination with flask sampling are subject to technical limitations that do not allow to fully characterising variations of atmospheric delta(13)C-CO(2) at all relevant timescales. In our study, we demonstrate the strength of Fourier transform infrared (FTIR) spectroscopy in combination with a PLS-based calibration strategy for online analysis of delta(13)C-CO(2) in ambient air. The ability of the instrument to measure delta(13)C-CO(2) was tested on a grassland field-site and compared with standard laboratory-based IRMS measurements made on field-collected flask samples. Both methods were in excellent agreement, with an average difference of 0.4 per thousand (n=81). Simultaneously, other important trace gases such as CO, N(2)O and CH(4) were analysed by FTIR spectroscopy.     

The effect of physical back-diffusion of 13CO2 tracer on the coupling between photosynthesis and soil CO2 efflux in grassland

Pulse labelling experiments provide a common tool to study short-term processes in the plant–soil system and investigate below-ground carbon allocation as well as the coupling of soil CO₂ efflux to photosynthesis. During the first hours after pulse labelling, the measured isotopic signal of soil CO₂ efflux is a combination of both physical tracer diffusion into and out of the soil as well as biological tracer release via root and microbial respiration. Neglecting physical back-diffusion can lead to misinterpretation regarding time lags between photosynthesis and soil CO₂ efflux in grassland or any ecosystem type where the above-ground plant parts cannot be labelled in gas-tight chambers separated from the soil. We studied the effects of physical ¹³CO₂ tracer back-diffusion in pulse labelling experiments in grassland, focusing on the isotopic signature of soil CO₂ efflux. Having accounted for back-diffusion, the estimated time lag for first tracer appearance in soil CO₂ efflux changed from 0 to 1.81±0.56 h (mean±SD) and the time lag for maximum tracer appearance from 2.67±0.39 to 9.63±3.32 h (mean±SD). Thus, time lags were considerably longer when physical tracer diffusion was considered. Using these time lags after accounting for physical back-diffusion, high nocturnal soil CO₂ efflux rates could be related to daytime rates of gross primary productivity (R²=0.84). Moreover, pronounced diurnal patterns in the δ¹³C of soil CO₂ efflux were found during the decline of the tracer over 3 weeks. Possible mechanisms include diurnal changes in the relative contributions of autotrophic and heterotrophic soil respiration as well as their respective δ¹³C values. Thus, after accounting for physical back-diffusion, we were able to quantify biological time lags in the coupling of photosynthesis and soil CO₂ efflux in grassland at the diurnal time scale.     

Isotopic ((13)C) fractionation during plant residue decomposition and its implications for soil organic matter studies

Carbon isotopic fractionations in plant materials and those occurring during decomposition have direct implications in studies of short-and longer-term soil organic matter dynamics. Thus the products of decomposition, the evolved CO(2) and the newly formed soil organic matter, may vary in their (13)C signature from that of the original plant material. To evaluate the importance of such fractionation processes, the variations in (13)C signatures between and within plant parts of a tropical grass (Brachiaria humidicola) and tropical legume (Desmodium ovalifolium) were measured and the changes in (13)C content (signatures) during decomposition were monitored over a period of four months. As expected the grass materials were less depleted in (13)C (-11.4 to -11.9 per thousand) than those of the legume (-27.3 to -25.8 per thousand). Root materials of the legume were less (1.5 per thousand) depleted in (13)C compared with the leaves. Plant lignin-C was strongly depleted in (13)C compared with the bulk material by up to 2.5 per thousand in the legume and up to 4.7 per thousand in the grass. Plant materials were subsequently incubated in a sand/nutrient-solution/microbial inoculum mixture. The respiration product CO(2) was trapped in NaOH and precipitated as CaCO(3), suitable for analysis using an automated C/N analyser coupled to an isotope ratio mass spectrometer. Significant depletion in (13)C of the evolved CO(2) was observed during the initial stages of decomposition probably as a result of microbial fractionation as it was not associated with the (13)C signatures of the measured more decomposable fractions (non-acid detergent fibre and cellulose). While the cumulative CO(2)-(13)C signatures of legume materials became slightly enriched with ongoing decomposition, the CO(2)-C of the grass materials remained depleted in (13)C. Associated isotopic fractionation correction factors for source identification of CO(2-)C varied with time and suggested errors of 2-19% in the estimation of the plant-derived C at 119 days of incubation in a soil of an intermediate (-20.0 per thousand) (13)C signature. Analysis of the residual material after 119 days of incubation showed little or no change in the (13)C signature partly due to the incomplete decomposition at the time of harvesting. Copyright 1999 John Wiley & Sons, Ltd.     

Determination of multiple ***φ***-torsion angles in proteins by selective and extensive (13)C labeling and two-dimensional solid-state NMR.

We describe an approach to efficiently determine the backbone conformation of solid proteins that utilizes selective and extensive (13)C labeling in conjunction with two-dimensional magic-angle-spinning NMR. The selective (13)C labeling approach aims to reduce line broadening and other multispin complications encountered in solid-state NMR of uniformly labeled proteins while still enhancing the sensitivity of NMR spectra. It is achieved by using specifically labeled glucose or glycerol as the sole carbon source in the protein expression medium. For amino acids synthesized in the linear part of the biosynthetic pathways, [1-(13)C]glucose preferentially labels the ends of the side chains, while [2-(13)C]glycerol labels the C(alpha) of these residues. Amino acids produced from the citric-acid cycle are labeled in a more complex manner. Information on the secondary structure of such a labeled protein was obtained by measuring multiple backbone torsion angles phi; simultaneously, using an isotropic-anisotropic 2D correlation technique, the HNCH experiment. Initial experiments for resonance assignment of a selectively (13)C labeled protein were performed using (15)N-(13)C 2D correlation spectroscopy. From the time dependence of the (15)N-(13)C dipolar coherence transfer, both intraresidue and interresidue connectivities can be observed, thus yielding partial sequential assignment. We demonstrate the selective (13)C labeling and these 2D NMR experiments on a 8.5-kDa model protein, ubiquitin. This isotope-edited NMR approach is expected to facilitate the structure determination of proteins in the solid state.     

Using a laser-based CO2 carbon isotope analyser to investigate gas transfer in geological media

CO₂ stable carbon isotopes are very attractive in environmental research to investigate both natural and anthropogenic carbon sources. Laser-based CO₂ carbon isotope analysis provides continuous measurement at high temporal resolution and is a promising alternative to isotope ratio mass spectrometry (IRMS). We performed a thorough assessment of a commercially available CO₂ Carbon Isotope Analyser (CCIA DLT-100, Los Gatos Research) that allows in situ measurement of δ ¹³C in CO₂. Using a set of reference gases of known CO₂ concentration and carbon isotopic composition, we evaluated the precision, long-term stability, temperature sensitivity and concentration dependence of the analyser. Despite good precision calculated from Allan variance (5.0 ppm for CO₂ concentration, and 0.05 ‰ for δ ¹³C at 60 s averaging), real performances are altered by two main sources of error: temperature sensitivity and dependence of δ ¹³C on CO₂ concentration. Data processing is required to correct for these errors. Following application of these corrections, we achieve an accuracy of 8.7 ppm for CO₂ concentration and 1.3 ‰ for δ ¹³C, which is worse compared to mass spectrometry performance, but still allowing field applications. With this portable analyser we measured CO₂ flux degassed from rock in an underground tunnel. The obtained carbon isotopic composition agrees with IRMS measurement, and can be used to identify the carbon source.     

Stable carbon isotopic composition of tree rings from a pine tree from Augustów Wilderness, Poland, as a temperature and local environment conditions indicator

Tree rings can be used as archives of climatic and environmental data with annual resolution. Tree rings widths, maximum late wood density and other parameters as stable composition in tree rings can be used for the reconstruction of past climatic and environmental changes. Stable carbon isotope ratios in tree rings may provide valuable information on past climatic conditions. 13C/12C ratios of plant organic matter can reflect corresponding 13C/12C ratio of atmospheric CO2 during formation of the rings. Investigations of isotopic carbon composition in tree rings from in the ecologically clean the Augustów Wilderness region in the north-eastern part of Poland (22 degrees 58'E, 53 degrees 51'N) (nowadays a sanctuary) were undertaken. Series of delta13C in alpha-cellulose and in wholewood were acquired. Those measurements constituted a part of more complex investigations of carbon isotope composition in tree rings including the measurements of radiocarbon concentration and tree ring widths. This article presents preliminary results. It is argued that contrary to the tree ring widths and delta13C in wholewood that do not reveal significant correlation with temperature, the variation of delta13C in the latewood alpha-cellulose is correlated with combined July and August temperatures.     

Use of the optogalvanic effect (OGE) for isotope ratio spectrometry of 13CO2 and 14CO2

The use of isotopic carbon dioxide lasers for determination of carbon (and oxygen) isotope ratios was first demonstrated in 1994. Since then a commercial device called LARA, has been manufactured and used for Helicobacter pylori breath tests using (13)C-labelled urea. The major advantages of the optogalvanic effect compared with other infrared absorption isotope ratio measurement techniques are its lack of optical background and its high sensitivity resulting from a signal gain proportional to laser power. Continuous normalisation using two cells, a standard and sample, lead to high accuracy as well as precision. Recent advances in continuous flow measurement of (13)C/(12)C ratios of CO(2) in air and extensions of the technique to (14)C, which can be analysed as a stable isotope, are described.     

Triple resonance experiments for aligned sample solid-state NMR of (13)C and (15)N labeled proteins

Initial steps in the development of a suite of triple-resonance (1)H/(13)C/(15)N solid-state NMR experiments applicable to aligned samples of (13)C and (15)N labeled proteins are described. The experiments take advantage of the opportunities for (13)C detection without the need for homonuclear (13)C/(13)C decoupling presented by samples with two different patterns of isotopic labeling. In one type of sample, the proteins are approximately 20% randomly labeled with (13)C in all backbone and side chain carbon sites and approximately 100% uniformly (15)N labeled in all nitrogen sites; in the second type of sample, the peptides and proteins are (13)C labeled at only the alpha-carbon and (15)N labeled at the amide nitrogen of a few residues. The requirement for homonuclear (13)C/(13)C decoupling while detecting (13)C signals is avoided in the first case because of the low probability of any two (13)C nuclei being bonded to each other; in the second case, the labeled (13)C(alpha) sites are separated by at least three bonds in the polypeptide chain. The experiments enable the measurement of the (13)C chemical shift and (1)H-(13)C and (15)N-(13)C heteronuclear dipolar coupling frequencies associated with the (13)C(alpha) and (13)C' backbone sites, which provide orientation constraints complementary to those derived from the (15)N labeled amide backbone sites. (13)C/(13)C spin-exchange experiments identify proximate carbon sites. The ability to measure (13)C-(15)N dipolar coupling frequencies and correlate (13)C and (15)N resonances provides a mechanism for making backbone resonance assignments. Three-dimensional combinations of these experiments ensure that the resolution, assignment, and measurement of orientationally dependent frequencies can be extended to larger proteins. Moreover, measurements of the (13)C chemical shift and (1)H-(13)C heteronuclear dipolar coupling frequencies for nearly all side chain sites enable the complete three-dimensional structures of proteins to be determined with this approach.     

Development of Methods to Measure Carbon Isotope Ratios of Bacterial Biomarkers in the Environment

Abstract

The δ¹³C value of bacterial carbon is an important parameter in microbial ecology for studying the carbon flow within a microbial community and for the identification of ecological important strains involved in the mineralization of certain carbon pools in the environment. In our study, biomarkers were isolated from bacteria from a microbial consortium derived from two chemostats and δ¹³C values were measured. Similar isotope ratios between biomarkers such as fatty acids and outer membrane protein, biomass and substrate were observed. The δ¹³C analyses of outer membrane protein F of Pseudomonas and biomarker fatty acids were combined to follow bacterial assimilation of ¹³C labelled 4-chlorocatechol. This new approach was also used in the environment where soil samples were cultivated with different ¹³C traced substrates.

The isotopic analyses of bacterial biomarkers indicated that carbons of histidine were widely incorporated into bacterial biomarkers, in contrast to 4-chlorocatechol which was less often used as a substrate. Results indicate that by isolating bacterial biomarkers and measuring their δ¹³C values, activities of microbial communities in a complex environmental sample can be determined. This new method has the potential to elucidate individual carbon sources for individual bacterial taxa in microbial ecology.     

Potential for assessing long-term dynamics in soil nitrogen availability from variations in delta15N of tree rings

Numerous researchers have used the isotopic signatures of C, H, and O in tree rings to provide a long-term record of changes in the physiological status, climate, or water-source use of trees. The frequently limiting element N is also found in tree rings, and variation in its isotopic signature may provide insight into long-term changes in soil N availability of a site. However, research has suggested that N is readily translocated among tree ring of different years; such infidelity between the isotopic compositions of the N taken up from the soil and the N contained in the ring of that growth year would obscure the long-term N isotopic record. We used a 15-year 15N-tracer study to assess the degree of N translocation among tree rings in ponderosa pine (Pinus ponderosa) trees growing in a young, mixed-conifer plantation. We also measured delta13C and delta15N values in unlabeled trees to assess the degree of their covariance in wood tissue, and to explore the potential for a biological linkage between them. We found that the maximum delta15N values in rings from the labeled trees occurred in the ring formed one-year after the 15N was applied to the roots. The delta15N value of rings from labeled trees declined exponentially and bidirectionally from this maximum peak, toward younger and older rings. The unlabeled trees showed considerable interannual variation in the delta15N values of their rings (up to 3 and 5 per thousand), but these values correlated poorly between trees over time and differed by as much as 6 per thousand. Removal of extractives from the wood reduced their delta15N value, but the change was fairly small and consistent among unlabeled trees. The delta13C and delta15N values of tree rings were correlated over time in only one of the unlabeled trees. Across all trees, both delta13C values of tree rings and annual stem wood production were well correlated with annual precipitation, suggesting that soil water balance is an important environmental factor controlling both net C gain and transpirational water loss at this site. Our results suggest that interannual translocation of N among tree rings is substantial, but may be predictable enough to remove this source of variation from the tree-ring record, potentially allowing the assessment of long-term changes in soil N availability of a site.     

Diurnal variations in the photosynthesis-respiration activity of a cyanobacterial bloom in a freshwater dam reservoir: an isotopic study

The stable isotopic analyses of molecular oxygen dissolved in water (delta18O(DO)) and dissolved inorganic carbon (delta13C(DIC)), supplemented with basic chemical measurements, have been carried out on a diurnal basis to better understand the dynamics of photosynthesis and respiration in freshwater systems. Our observations have been carried out in a lowland dam reservoir, the Sulejow Lake (central Poland), during the summer cyanobacterial bloom. All data obtained, isotopic, hydrochemical, and biological, show a high mutual consistency. Namely, the lowest delta18O(DO) values, obtained at 10:00 and 14:00 (16.0 and 15.5 per thousand, respectively), correspond to the highest amount of cyanobacterial cells observed (66 and 63 mg dm(-3), respectively), whereas the minimum delta13C(DIC) (-10.6 per thousand) obtained at 22:00 corresponds to the maximum content of organic matter (110 mg dm(-3)). This evidence suggests that isotopic assays of delta18O(DO) and delta13C(DIC) are a reliable tool for the quantitative study of biochemical processes in freshwater systems.