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Geothermal waters contain numerous potential electron donors capable of supporting chemolithotrophybased primary production. Thermodynamic predictions of energy yields for specific electron donor and acceptor pairs in such systems are available, although direct assessments of these predictions are rare. This study assessed the relative importance of dissolved H 2 and H 2 S as energy sources for the support of chemolithotrophic metabolism in an acidic geothermal spring in Yellowstone National Park. H 2 S and H 2 concentration gradients were observed in the outflow channel, and vertical H 2 S and O 2 gradients were evident within the microbial mat. H 2 S levels and microbial consumption rates were approximately three orders of magnitude greater than those of H 2 . Hydrogenobaculum-like organisms dominated the bacterial component of the microbial community, and isolates representing three distinct 16S rRNA gene phylotypes (phylotype ‫؍‬ 100% identity) were isolated and characterized. Within a phylotype, O 2 requirements varied, as did energy source utilization: some isolates could grow only with H 2 S, some only with H 2 , while others could utilize either as an energy source. These metabolic phenotypes were consistent with in situ geochemical conditions measured using aqueous chemical analysis and in-field measurements made by using gas chromatography and microelectrodes. Pure-culture experiments with an isolate that could utilize H 2 S and H 2 and that represented the dominant phylotype (70% of the PCR clones) showed that H 2 S and H 2 were used simultaneously, without evidence of induction or catabolite repression, and at relative rate differences comparable to those measured in ex situ field assays. Under in situ-relevant concentrations, growth of this isolate with H 2 S was better than that with H 2 . The major conclusions drawn from this study are that phylogeny may not necessarily be reliable for predicting physiology and that H 2 S can dominate over H 2 as an energy source in terms of availability, apparent in situ consumption rates, and growth-supporting energy.Thermophiles dominate the deepest and shortest branches of the Bacteria and Archaea domains in the tree of life, suggesting that they are likely ancestors of Earth's contemporary microbial populations (8,35). Consequently, these organisms have attracted considerable attention due to interest in the origin of enzymes and metabolic pathways that are thought to have evolved from such organisms. Chemolithotrophic metabolism is foundational to primary productivity in geothermal environments where temperatures exceed the limit of photosynthesis. The bioenergetics of such systems have been examined from the perspective of theoretical energy yield as a way of discussing the relative importance of the various electron donors and acceptors that could support primary productivity (3)(4)(5)22). Other studies have sought to link the inferred physiology of microbial populations with the predicted energy yields obtainable from the inorganic constituents present (4...

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Oceanic dimethylsulfoniopropionate (DMSP) is the precursor to dimethylsulfide (DMS), which plays a role in climate regulation through transformation to methanesulfonic acid (MSA) and nonseasalt sulfate (NSS-SO 4 2− ) aerosols. Here, we report measurements of the abundance and sulfur isotope compositions of DMSP from one phytoplankton species (Prorocentrum minimum) and five intertidal macroalgal species (Ulva lactuca, Ulva linza, Ulvaria obscura, Ulva prolifera, and Polysiphonia hendryi) in marine waters. We show that the sulfur isotope compositions (δ 34 S) of DMSP are depleted in 34 S relative to the source seawater sulfate by ∼1-3‰ and are correlated with the observed intracellular content of methionine, suggesting a link to metabolic pathways of methionine production. We suggest that this variability of δ 34 S is transferred to atmospheric geochemical products of DMSP degradation (DMS, MSA, and NSS-SO 4 2− ), carrying implications for the interpretation of variability in δ 34 S of MSA and NSS-SO 4 2− that links them to changes in growth conditions and populations of DMSP producers rather than to the contributions of DMS and non-DMS sources. COO − ] is a secondary metabolite that is produced and stored in large amounts by marine macroalgae (1) and microalgae (2). This β-sulfonium compound is widespread among marine taxa but is particularly abundant within specific groups of phytoplankton, zooplankton, macroalgae, halophytic plants, macroinvertebrates, and fishes (3-5). DMSP plays important ecophysiological functions in marine algae by acting as an antioxidant (6), a cryoprotectant, an osmolyte, and a precursor to an activated defense system (3). It is also an important carbon and sulfur source for marine bacterioplankton (7).The synthesis of DMSP by algae has been reviewed previously (3,8). It starts with the assimilation of seawater sulfate into the cytoplasm. The sulfate is subsequently transported into the chloroplasts, where it is reduced to sulfide in the presence of glutathionine and then transformed into cysteine. Cysteine is used to synthesize methionine, which is then transformed into DMSP via one of three pathways that differ among taxonomic groups of plants and algae (9-12). Thus, the biosynthesis of DMSP ultimately depends on the activity of the sulfate assimilation pathway; however, little is known about how DMSP synthesis differs among algae from diverse origins, except that the whole molecule is derived from sulfur amino acids.DMSP and its cleavage product dimethylsulfide [DMS; (CH 3 ) 2 S] have attracted much research interest because of their possible role in climate regulation (13,14). Since the introduction of the Charlson, Lovelock, Andreae, Warren (CLAW) hypothesis, which argues for feedback between biological DMS production, Earth's solar radiation, and the regulation of global climate (15), there has been an increasing emphasis by environmental scientists on determining the strength of the sea-to-air biogeochemical sources of DMS. This sea-to-air exchange of DMS is mediated through turbu...

Thermochemical sulfate reduction experiments with simple amino acid and dilute concentrations of sulfate reveal significant degrees of mass-independent sulfur isotope fractionation. Enrichments of up to 13‰ for 33 S are attributed to a magnetic isotope effect (MIE) associated with the formation of thiol-disulfide, ion-radical pairs. Observed 36 S depletions in products are explained here by classical (mass-dependent) isotope effects and mixing processes. The experimental data contrasts strongly with multiple sulfur isotope trends in Archean samples, which exhibit significant 36 S anomalies. These results support an origin other than thermochemical sulfate reduction for the mass-independent signals observed for early Earth samples.anomalous | sulfur radical | thermolysis | spin-selective | hyperfine coupling S ince the report by Farquhar et al., (1) that significant deviations from the terrestrial fractionation line are observed in samples older than approximately 2.32-2.45 Ga (2, 3), considerable effort has been dedicated to identifying the origin and significance of this signal (4-10). The mass-independent signal in these ancient samples is expressed as variations in both Δ 33 S and Δ 36 S (1).* Given the observations that gas-phase reactions can produce mass-independent signals for both Δ 33 S and Δ 36 S, the first studies on this subject attributed this ancient signal to photolytic reactions in the early atmosphere. Subsequent studies also pointed out that the mass-independent reactions may also be produced by variations in the spectrum of light that drives atmospheric photolytic reactions (10-12), and other studies speculated that liquid phase reactions involving weakly bound transition states may account for these variations (7,13).In a recent report, Watanabe et al. (7) demonstrated that high temperature reduction of sulfate using alanine and glycine as organic substrates caused moderate mass-independent sulfur isotope fractionations. These authors did not identify the origin of the effect, but suggested that it was either a magnetic isotope effect (MIE) (14) or another type of isotope effect accompanying heterogeneous reactions such as adsorption of S-bearing compounds on surfaces of solids (13). Magnetic isotope effects are expressed in rare cases for isotopes with nuclear magnetic moments, like 13 C, 17 O, 29 Si, 199 Hg, 201 Hg, 73 Ge, 235 U, and 33 S (14, 15, 16). The effect is expressed when the lifetime of a radical pair is sufficient for hyperfine coupling between magnetic nuclei and unpaired electrons to influence interconversions between singlet and triplet states. This coupling in turn changes the proportion of reactive intermediates that can participate in spin-selective reactions. The 33 S nucleus has a spin of 3∕2 and a magnetic moment of 0.643 nuclear magnetons and has been implicated in at least one well-characterized example of a 33 S MIE (14,(17)(18)(19). The alternative suggestion relates to a proposal that anomalous isotope effects may be associated with heterogeneous reactions as a resu...

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We have investigated the quadruple sulfur isotopic composition of inorganic sulfur-bearing phases from 13 carbonaceous chondrites of CM type. Our samples include 4 falls and 9 Antarctic finds. We extracted sulfur from sulfides, sulfates, and elemental sulfur (S 0) from all samples. On average, we recover a bulk sulfur (S) content of 2.11±0.39 wt.% S (1). The recovered sulfate, S 0 and sulfide contents represent 25±12%, 10±7% and 65±15 % of the bulk S, respectively (all 1). There is no evidence for differences in the bulk S content between falls and finds, and there is no correlation between the S speciation and the extent of aqueous alteration. We report ranges of  33 S and  36 S values in CMs that are significantly larger than previously observed. The largest variations are exhibited by S 0 , with  33 S values ranging between-0.104±0.012‰ and +0.256±0.018‰ (2). The  36 S 33 S ratios of S 0 are on average-3.1±1.0 (2). Two CMs show distinct  36 S 33 S ratios, of +1.3±0.1 and +0.9±0.1. We suggest that these mass independent S isotopic compositions record H2S photodissociation in the nebula. The varying  36 S 33 S ratios are interpreted to reflect photodissociation that occurred at different UV wavelengths. The preservation of these isotopic features requires that the S-bearing phases were heterogeneously accreted to the CM parent body. Non-zero 33 S values are also preserved in sulfide and sulfate, and are positively correlated with S 0 values. This indicates a genetic relationship between the S-bearing phases: We argue that sulfates were produced by the direct oxidation of S 0 (not sulfide) in the parent body. We describe two types of models that, although imperfect, can explain the major features of the CM S isotope compositions, and can be tested in future studies. Sulfide and S 0 could both be condensates from the nebula, as the residue and product, respectively, of incomplete H2S photodissociation by UV light (wavelength < 150 nm). This idea requires that FeS formation and the S 0 condensation co-occur. As an alternative, ice accretion to the CM parent body could allow the delivery of S-MIF in CMs. In that case, sulfides would have been the only S-bearing condensate in CM precursors, and S 0 would have been derived from the oxidation of H2S trapped in ices, after its photodissociation at low temperature (< 500 K) in the nebula. In our models, the observations of H2S UV photodissociation is required to occur at the disk surface, and allowed in nebular environments with canonical C/O ratios. Vertical motions in the disk would redistribute phases that condensed at high altitude to the midplane, where they accreted in the phases that make up the chondritic matrix.