Oxygen-18 Composition of Oceanic Sulfate
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Abstract:
Comparison of experimental data with analyses of oceanic sulfate indicates that oceanic sulfate is not in oxygen isotope equilibrium with ocean water. Preliminary experiments suggest that the turnover of sulfate in the sulfur cycle is too rapid to allow equilibrium to be established. If this is so, the sulfur cycle must exert a significant influence on the oxygen balance of the oceanatmosphere system.Keywords:
Sulfur Cycle
Bacteria that disproportionate elemental sulfur fractionate sulfur isotopes such that sulfate is enriched in sulfur-34 by 12.6 to 15.3 per mil and sulfide is depleted in sulfur-34 by 7.3 to 8.6 per mil. Through a repeated cycle of sulfide oxidation to S0 and subsequent disproportionation, these bacteria can deplete sedimentary sulfides in sulfur-34. A prediction, borne out by observation, is that more extensive sulfide oxidation will lead to sulfides that are more depleted in sulfur-34. Thus, the oxidative part of the sulfur cycle creates circumstances by which sulfides become more depleted in sulfur-34 than would be possible with sulfate-reducing bacteria alone.
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Isotopes of sulfur
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The sulfur cycle is an important and complex biogeochemical cycle involving both inorganic and organic species in both oxic and anoxic environments. However, due to the lack of research regarding the sulfur cycle in freshwater systems, the contributions of organic sulfur compounds to the sulfur cycle are underappreciated. Recent studies have suggested organic sulfur compounds likely fuels sulfate reduction, especially in low-sulfate oligotrophic freshwater systems, through a possible cryptic sulfur cycle. To determine the contributions that organic sulfur compounds may have in this environment, we used Lake Superior sediment to analyze for the presence of and expression of sulfur cycling genes. In these metagenomes, we found genes for sulfur reduction, oxidation and organic sulfur compound degradation. Metabolic pathway analysis showed presence of not only organic sulfur compounds contributing to the sulfur cycle, but tetrathionate, thiosulfate, and polysulfides playing a role as well. Using Lake Superior sediments, we also conducted sediment incubations to measure the biotransformation capability of sulfur-containing amino acids, sulfonates, and an analog for a common sulfolipid. Taurine and sodium dodecyl sulfate produced higher sulfate values in incubations, suggesting that microbes prefer sulfonates over sulfur-containing amino acids, in addition to a possible partiality towards oxidized organic sulfur compounds over reduced forms regarding sulfate production. The preference of sulfonates is supported by the commonality of taurine genes present as well as the low, but present transcription values of sulfoacetaldehyde degradation. While sulfur-containing amino acids do not produce sulfate values near that of sulfonates or sulfolipids, there are still present and transcriptionally active genes that can contribute to sulfate reduction in the system. Regarding methyl-sulfurs, metatranscriptomic data shows that methyl-mercaptan (intermediate within dimethyl sulfide and methionine degradation) degradation is transcriptionally active across genomes. By combining biotransformation incubation data, metagenomics, and metatranscriptomics, we analyzed how methylated sulfurs, sulfur-containing amino acids and sulfonates can fuel a sulfur cycle in a low-sulfate environment, informing us on how pathways may have operated in our Earth’s geologic past.
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Biogeochemical Cycle
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Four model constructed wetlands (CWs) were designed to investigate the effects of sulfate load on sulfate and sulfide removal. The results showed that as the sulfate load increased from 1.42 to 7.01 g S m–3 d–1, the sulfate removal rate increased from 1.42 to 3.16 g S m–3 d–1, and the sulfide discharge rate increased from 0.08 to 1.46 g S m–3 d–1. The total sulfur removal rate ranged between 1.29 and 1.74 g S m–3 d–1. The sulfide in the effluent only accounted for 5.55%–46.9% of the removed sulfate. This indicated that CWs can effectively reduce sulfide discharge while removing sulfate. The conversion of dissolved sulfide into deposited sulfur by CW matrix was a main way for sulfide removal. Elemental sulfur, acid volatile sulfide (AVS), and pyrite-sulfur were the main forms of sulfur deposition in this study. The accumulations of these three sulfur compounds were 16.6–36.2, 22.3–36.0, and 49.7–63.6 mg S kg–1 gravel, respectively. Sulfur balance analysis showed that 42.9%–71.1% of the removed sulfate was deposited in the matrix, and only 0.84%–2.34% was absorbed by the plant.
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Onion (Allium cepa L.) was exposed to low levels of H2S in order to investigate to what extent H2S could be used as a sulfur source for growth under sulfate-deprived conditions. Sulfate deprivation for a two-week period resulted in a decreased biomass production of the shoot, a subsequently decreased shoot to root ratio and an increased dry matter content in shoot and roots. Furthermore, it resulted in decreased contents of total sulfur, sulfate and organic sulfur and in a decreased sulfate to total sulfur ratio. Symptoms of sulfur deficiency disappeared upon simultaneous exposure to relatively low levels of H2S (0.05, 0.1 and 0.15 mu l l(-1)), which showed that H2S could be used as a sulfur source for growth. H2S exposure even resulted in a slightly increased biomass production in sulfate-sufficient plants. The observed accumulation of sulfate and organic sulfur upon H2S exposure in both sulfate-sufficient and sulfate-deprived plants is discussed.
Sulfur Metabolism
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Sulfur Cycle
Sulfate-Reducing Bacteria
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The Georgia Tech/Goddard Global Ozone Chemistry Aerosol Radiation and Transport (GOCART) model is used to simulate the atmospheric sulfur cycle. The model uses the assimilated meteorological data from the Goddard Earth Observing System Data Assimilation System (GEOS DAS). Global sulfur budgets from a 6‐year simulation for SO 2 , sulfate, dimethylsulfide (DMS), and methanesulfonic acid (MSA) are presented in this paper. In a normal year without major volcanic perturbations, about 20% of the sulfate precursor emission is from natural sources (biogenic and volcanic), and 80% is anthropogenic; the same sources contribute 33% and 67%, respectively, to the total sulfate burden. A sulfate production efficiency of 0.41–0.42 is estimated in the model, an efficiency which is defined as a ratio of the amount of sulfate produced to the total amount of SO 2 emitted and produced in the atmosphere. This value indicates that less than half of the SO 2 entering the atmosphere contributes to the sulfate production, the rest being removed by dry and wet depositions. In a simulation for 1990 we estimate a total sulfate production of 39 Tg S yr −1 , with 36% and 64% from in‐air and in‐cloud oxidation, respectively, of SO 2 . We also demonstrate that major volcanic eruptions, such as the Mount Pinatubo eruption in 1991, can significantly change the sulfate formation pathways, distributions, abundance, and lifetime. Comparison with other models shows that the parameterizations for wet removal or wet production of sulfate are the most critical factors in determining the burdens of SO 2 and sulfate. Therefore a priority for future research should be to reduce the large uncertainties associated with the wet physical and chemical processes.
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Methanesulfonic acid
Sulfate aerosol
Atmospheric chemistry
Acid rain
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Sulfur Cycle
Cycling
Salt marsh
δ34S
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Sulfur Cycle
Sulfate-Reducing Bacteria
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Sulfur Cycle
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Sulfur Cycle
Atmospheric chemistry
Dimethyl sulfide
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