Prospects and limitations of an isotope tracer technique for understanding sulfur cycling in forested and agro-ecosystems.

Abstract Applications of isotopically distinct sulfur compounds have recently been used for tracing the fate of added sulfur in whole catchments or sub-compartments therein. Basic principles, the analytical methodology, and data evaluation for this isotope tracer technique are briefly described. We recommend that δ(34)S-values of applied and natural sulfur compounds in the investigated ecosystem should differ by more than 20‰ in order to successfully ascertain sulfur fluxes. Where possible, a high ratio of applied sulfur loads versus sulfur pool sizes in the ecosystem should also be realized in order to allow the assessment of sulfur transformations in the study area. Prospects and limitations of this isotope tracer technique are critically discussed by reviewing results from recently or currently conducted lysimeter and field experiments.

Sulphur isotope composition of epiphytic and terricolous lichens and pine bark in Finland.

The sulphur isotope composition of the epiphytic lichens, Hypogymnia physodes (L.) Nyl and Pseudevernia furfuracea (L.) Zopf, of the terricolous lichens, Cladina sp. and Peltigera aphthosa (L.) Willd., and of pine bark, at 31 sites in Finland, appeared to be dependent on lichen and tree species. The negative correlation of sulphur isotope composition with sulphur content was highly significant (P<0.001) in H. physodes and significant (P<0.01) in Pseudevernia furfuracea and Peltigera aphthosa. No correlation between the sulphur isotope composition and sulphur content was found for Cladina sp. and pine bark. The sulphur isotope compositions displayed statistically significant regional variation from south to north. Microenvironmental factors were considered, including phorophyte species and the chemical form of sulphur.

Isotope fractionation by plants and animals: implications for nutrition research.

The isotopic compositions of animal tissues, minerals, and fluids reflect those of ingested food and water and inhaled gases. This relationship is illustrated by a review of data pertaining to five light elements of biological interest (carbon, nitrogen, hydrogen, oxygen, and sulphur). Processes affecting the isotopic composition of inorganic compounds in reservoirs are summarized, and isotope fractionation during transfer of elements from these inorganic reservoirs through progressive trophic levels of food webs is discussed. Variability of delta values within and among individuals, populations, and species of plants and animals is attributed to at least six factors: locality, dietary selectivity, biochemical composition of food, isotope effects in metabolic processes, turnover rates, and stress. Features of a variety of terrestrial and aquatic ecosystems are used to illustrate basic concepts. Future research should seek to clarify specific mechanisms affecting delta values during the transfer of elements through food webs.

Cretaceous cold-seep communities and methane-derived carbonates in the canadian arctic.

Lower Cretaceous cold-seep fossil assemblages have been found in the Canadian Arctic Archipelago. Serpulid worm tubes and bivalves are most abundant in these communities; in contrast, fossils are scarce in the surrounding strata. The fossils are contained in an isotopically light (delta(13)C = -25 to -50 per mil) carbonate rock groundmass that is interpreted to have formed from bacterial oxidation of methane. The rocks were deposited at intermediate depth (

Sulfur isotope fractionation during SO3(2-) reduction by different clostridial species.

Sulfur isotope composition patterns for sulfide evolved from cultures supplemented with 1 mM Na2SO3, suggested that an inducible dissimilatory type SO3(2-) reduction pathway, as previously found in C. pasteurianum, probably exists in many clostridial species. Data are presented for five additional species which include pathogens and nonpathogens.

The Start of Sulfur Oxidation in Continental Environments: About 2.2 x 109 Years Ago.

Whereas dissimilatory sulfate reduction in marine environments has been postulated on the basis of sulfur isotope abundances as commencing before about 2.8 x 10(9) years ago, data from the Huronian Supergroup, Canada, suggest that oxidative-reductive reactions involving sulfur in continental environments were not significant before about 2.2 x 10(9) years ago.

Stable isotope fractionation by Clostridium pasteurianum. 4. Sulfur isotope fractionation during enzymatic S3O6(2-), S2O3(2-), and SO3(2-) reductions.

Cell-free extracts from Clostridium pasteurianum grown on SO3(2-) utilize H2 to reduce S3O6(2-), S2O3(2-), SO3(2-) to H2S at a much faster rate than extracts from SO4(2-)-grown cells. This further supports the concept of an inducible dissimilatory type SO3(2-) reductive pathway in this organism. 35S dilution experiments further support the concept that S3O6(2-) and S2O3(2-) are pathway intermediates. The inducible SO3(2-) reductase is ferredoxin linked and the kinetics of the reduction and the sulfur isotope fractionation of the product can be altered by altering the growth conditions. The attending sulfur isotope fractionations are similar to those observed during the chemical decomposition of these compounds. In the case of S2O3(2-), 35S labelling experiments verified the conclusions derived from the stable isotope fractionation data concerning the relative reduction rates of the sulfane and sulfonate sulfurs. The reduction rates were also affected by enzyme concentration. The integrity of the whole cell is a necessary requirement for the large inverse isotope effects previously reported.

Sulfur isotope fractionation by Proteus vulgaris and Salmonella heidelberg during the reduction of thiosulfate.

Proteus vulgaris metabolized thiosulfate to H2S. The amount evolved and its sulfur isotope composition identified it solely with sulfane sulfur. In contrast, Salmonella heidelberg sequentially reduced the sulfane sulfur of S2O3(-2) with slight enrichment of the evolved sulfide in 32S and then reduced the sulfonate sulfur of S2O3(-2) with large isotopic selectivities and an inverse isotopic fractionation pattern. The inverse isotope fractionation pattern for the H2S derived from the sulfonate sulfur was almost identical to that observed during the reduction of high concentrations of sulfite by S. heidelberg.

Stable isotope fractionation by Clostridium pasteurianum. 3. Effect of SeO32- on the physiology and associated sulfur isotope fractionation during SO32- and SO42- reductions.

Increased SeO32- concentration reduced H2S evolution from SO32- during whole cell and cell-free extract reductions by Clostridium pasteurianum. H2S production from SO42- was completely inhibited by SeO32- in stationary phase cells. Generation times increased with greater SeO32- concentration, the increase with 1 mM SeO32- being a factor of 2.5 for 1 mM SO32-, and over 3 for 1 mM SO42- reductions. In vitro and in vivo experiments with proposed intermediates of the SO32- reduction pathway show that SeO32- inhibited both the S3O62- to S2O32- and S2O32- to S2- reaction sequences with the latter being more pronounced in growth experiments. Both extracts and whole cells reduced SeO32- to Se0 but Se0 granules were not found in the cell's cytoplasm. The formation of S2O32- by an extracellular chemical mechanism appears not to have occurred in these experiments. Increased SeO32- concentration had the effect of compressing the isotopic release pattern for H2S along the H2S production axis and did not significantly alter the maximum and minimum values of delta 34S. Thus, inhibition by SeO32- limited the conversions of sulfur species without altering the isotopic selectivity of rate-controlling steps in the pathway.

Variations in sulphur content and stable sulphur isotope composition of vegetation near a SO2 source at Fox Creek, Alberta, Canada.

The proportion of sulphur found in vegetation increases as an industrial sulphur dioxide emission source is approached, but this trend is modified by topographic extremes. Stable sulphur isotope analysis revealed that increased sulphur concentrations in arborcal and terricolous lichens, andPicea glauca needles were due to incorporation of sulphur emitted as sulphur dioxide by the Kaybob sour gas plant near Fox Creek, Alberta, Canada. It is confirmed thatPicea glauca gets its sulphur from both the soil and the air; arboreal lichens get their sulphur primarily from the atmosphere; and terricolous lichens get theirs from the air and particulate fallout. It is suggested that arboreal lichens selectively excrete isotopically light sulphur when thallus concentrations exceed 1,400 ppm. Accumulation of sulphur in vegetation is dependent, on fumigation episode duration, frequency, and concentration, as well as the plant's ability to avoid injury, but is not closely correlated with average sulphation rates. The implications of terrestrial moss-lichen carpet destruction is discussed.

Sulfur isotope fractionation by Salmonella heidelberg: inverse isotope effects during growth on high concentrations of Na2SO3.

During growth on minimal salts - glucose media supplemented with high concentrations of Na2SO3 (10-3 and 10-2 M), Salmonella heidelberg exhibited cytological and growth responses which indicated increased cellular toxicity with increasing sulfite concentrations. The large quantities of sulfide evolved during growth at both SO32- concentrations were accompanied by large normal and inverse isotope effects. Consistent with earlier findings, this organism was found capable of rapidly metabolizing both the sulfane and sulfonate sulfur of thiosulfate. Therefore, the isotope effects do not appear to be caused by extracellular chemical thiosulfate formation.

The effect of pantothenate on sulfate metabolism and sulfur isotope fractionation by Saccharomyces cerevisiae.

Growth of Saccharomyces cerevisiae in minimal salts-glucose-SO4(2-) medium with varying concentrations of pantothenate (0-1000 microgram/L) produced changes in the cellular lipid content and in the ratio of saturated to unsaturated fatty acids. Substantial differences in SO4(2-) diffusion were also observed with changes in pantothenate concentration. During sulfate reduction, the delta 34S value of the evolved sulfide varied with the pantothenate concentration ranging from -31% in the absence of pantothenate to 0% at 400-1000 microgram/L pantothenate. The isotope selectivity is related to the effect of pantothenate concentration on cellular metabolism.

Stable isotope fractionation by Clostridium pasteurianum. 2. Regulation of sulfite reductases by sulfur amino acids and their influence on sulfur isotope fractionation during SO32- and SO42- reduction.

In addition to an assimilatory sulfite reductase, studies of cultures of Clostridium pasteurianum supplemented with methionine, cysteine, and 35SO42- provides evidence for another reductase which is induced by SO32-. This inducible reductase appears to be dissimaltory because of the copious sulfide production arising when the cells are grown on SO32-. Cysteine can repress the assimilatory sulfite reductase but does not affect the inducible reductase. During late logarithmic growth on 1 mM SO42- + 10mM cysteine, depression of the inducible reductase occurred along with increased sulfide production. The presence of 1 mM cysteine and (or) 1 mM cysteine and (or) 1 mM methionine does not affect the inverse sulfur isotope effect for evolved H2S. However, 5 and 10 mM cysteine reduce the maximum delta34S value for released H2S from +40 to 10%. A small conversion of cysteine to H2S by C. pasteurianum occurs, but only in the stationary phase.

The use of stable sulfur isotope labelling to elucidate sulfur metabolism by Clostridium pasteurianum.

An unique isotope labelling experiment was conducted whereby mixtures of sulfate and sulfite of different isotopic compositions were metabolized by Clostridium pasteurianum. The results showed during reduction of 1 mM SO3= plus 1 mM SO4=, essentially all evolved H2S arose from the sulfite whereas in the case of cellular sulfur, 85% was derived from sulfite and the remainder from sulfate.

Stable isotope fractionation by Clostridium pasteurianum. 1. 34S/32S: inverse isotope effects during SO4-2- and SO3-2- reduction.

During growth on minimal salts--sucrose media supplemented with various concentrations (10-4-10-2 M) of sodium sulfate, Clostridium pasteurianum grew at a normal rate and only evolved sulfide in late stages of growth on 10-2 M SO4-2-. The evolved sulfide was slightly enriched in 34S as compared to the medium sulfur. On the other hand, sulfide was evolved during growth on all concentrations of sulfite tested. Large normal and inverse isotopic effects were observed in the evolved sulfide during SO3-2- reductions. In contrast, the intracellular sulfur showed much smaller fractionations. The complexity of the isotopic patterns suggests that a dissimilatory sulfite reductase system may be induced by high concentrations of sulfite.

Carbon-13 and oxygen-18 in dinosaur, crocodile, and bird eggshells indicate environmental conditions.

We have gathered, from the nests of dinosaurs, and living and fossil birds, some evidence of the environment in which these creatures lived. However, our isotope determinations suggest it will be impossible to resolve the problem as to whether the dinosaurs were warm-or cold-blooded from the oxygen and carbon isotopes content of their shells.

Nitrogen-15: microbiological alteration of abundance.

The abundance of N(15) relative to N(14) is significantly altered during experiments in vitro in which nitrate and nitrite are microbiologically reduced to nitrogen gas. In all studies to date, N(14)O(3)-and N(14)O(2)-species have been preferentially reduced. This selectivity has a complex dependence on conditions in the medium. The results are not only relevant to natural variations in N(15) relative to N(14) but should be seriously considered during N(15) tracer studies in soils.

Sulfur isotope fractionation and kinetic studies of sulfite reduction in growing cells of Salmonella heidelberg.

A pulsed feeding technique was used during studies of sulfite reduction by Salmonella heidelberg in order to realize large percentages of SO(3) (=) conversion while simultaneously maintaining a reasonably stable cell population. As a consequence, much data for conventional kinetic and sulfur isotope fractionation computations were obtained in any one experiment. Under the conditions of supplying 150 mug glucose per ml of medium every 6 hr, anaerobiosis, and varying the SO(3) (=) concentration, the following observations were made: 1. Below 0.01% w/v Na(2)SO(3), the reduction strictly followed first order kinetics with respect to SO(3) (=) concentration. At higher concentrations, the rate of SO(3) (=) reduction fell below that predicted by first order kinetics suggesting that a saturation effect was occurring. 2. At lower concentrations, the ratio of the isotopic rate constants k(1)/k(2) was 1.02 whereas at higher SO(3) (=) levels, k(1)/k(2) values of 1.04 were found. These latter effects are much higher than those obtained in the equivalent chemical reduction. On the basis of these observations, a model is considered which features two isotopically dependent steps and an intermediate reservoir which forms at higher SO(3) (=) concentrations. Results of an experiment under aerobic conditions and an experiment wherein the reduction rate was thermally altered, are also presented.