Stefano M. Bernasconi

Sulfur isotope fractionation during microbial sulfate reduction by toluene-degrading bacteria

Sulfate-reducing bacteria contribute considerably to the mineralization of petroleum hydrocarbons (PHC) in contaminated environments. Stable sulfur isotope fractionation during microbial sulfate reduction was investigated in microcosm experiments with different cultures of sulfate-reducing bacteria for various initial sulfate concentrations using toluene as the sole carbon source. Experiments were conducted with the marine strain Desulfobacula toluolica, the fresh water strain PRTOL1, and an enrichment culture from a PHC-contaminated aquifer. Sulfate reduction rates ranged from 7 ± 1 to 494 ± 9 nmol cm−3 d−1, whereas specific sulfate reduction rates (sSRR) ranged from 8.9 × 10−15 to 3.9 × 10−13 ± 9.2 × 10−14 mol cell−1 d−1. Calculated enrichment factors (ϵ) for the fractionation of stable sulfur isotopes during microbial sulfate reduction ranged from 19.8 ± 0.9 to 46.9 ± 2.1‰. In general, values of ϵ and sSRR obtained in our experiments were similar to those reported previously for sulfate-reducing bacteria incubated with readily available carbon sources under optimal growth conditions. Moreover, we found no obvious correlation between ϵ and sSRR values when data from all our microcosm experiments were combined or when we combined our data with several previously published data sets. In contrast, ϵ values determined in our enrichment culture experiments (average 23.5 ± 4.3‰) agreed well with ϵ values determined in a recent field study performed in situ in a PHC-contaminated aquifer. Thus, results from this laboratory study provide valuable information on stable sulfur isotope fractionation during microbial sulfate reduction under conditions that more closely resemble those in PHC-contaminated environments, i.e., for a variety of sulfate concentrations, including low sulfate concentrations, and for a an important PHC-constituent (toluene) used as sole carbon source.

Oxygen isotopic composition of the Mediterranean Sea since the Last Glacial Maximum: constraints from pore water analyses

Abstract Interstitial waters recovered from Ocean Drilling Program, Leg 161, site 976 in the western Mediterranean Sea are used in conjunction with a numerical model to constrain the δ18O of seawater in the basin since the Last Glacial Maximum, including Sapropel Event 1. To resolve the oxygen isotopic composition of the deep Mediterranean, we use a model that couples fluid diffusion with advective transport, thus producing a profile of seawater δ18O variability that is unaffected by glacial–interglacial variations in marine temperature. Comparing our reconstructed seawater δ18O to recent determinations of 1.0‰ for the mean ocean change in glacial–interglacial δ18O due to the expansion of global ice volume, we calculate an additional 0.2‰ increase in Mediterranean δ18O caused by local evaporative enrichment. This estimate of δ18O change, due to salinity variability, is smaller than previous studies have proposed and demonstrates that Mediterranean records of foraminiferal calcite δ18O from the last glacial period include a strong temperature component. Paleotemperatures determined in combination with a stacked record of foraminiferal calcite depict almost 9°C of regional cooling for the Last Glacial Maximum. Model results suggest a decrease of ∼1.1‰ in seawater δ18O relative to the modern value caused by increased freshwater input and reduced salinity accompanying the formation of the most recent sapropel. The results additionally indicate the existence of isotopically light water circulating down to bottom water depths, at least in the western Mediterranean, supporting the existence of an ‘anti-estuarine’ thermohaline circulation pattern during Sapropel Event 1.

DIET OF THE COMMON WARTHOG (PHACOCHOERUS AFRICANUS) ON FORMER CATTLE GROUNDS IN A TANZANIAN SAVANNA

Abstract In otherwise nutrient-poor savannas, fertile vegetation patches are particularly attractive to ungulates because of the higher-quality food they provide. We investigated forage plants and diet of the common warthog (Phacochoerus africanus) on an abandoned cattle ranch in coastal Tanzania. The forage grasses of highest nutritional quality occurred in former paddock enclosures (bomas) where cattle had been herded at night. In the dry season, grass samples from bomas contained approximately 4 times as much nitrogen and phosphorus as those of the surrounding vegetation. δ15N values of soil and plants also were highest in bomas and decreased significantly with distance, and high δ15N values in feces suggest that warthogs preferentially fed in the vicinity of the former bomas. δ13C values of warthog feces indicate that warthogs ingested on average 83% (77–98%) C4 grasses, with this proportion varying regionally but not seasonally. We conclude that, for medium-sized selective grazers such as warthogs, bomas represent attractive feeding grounds. We also hypothesize that by promoting nutrient turnover in these patchily distributed areas, grazing animals help to maintain them as sources of high-quality forage.

Nitrate-consuming processes in a petroleum-contaminated aquifer quantified using push-pull tests combined with 15N isotope and acetylene-inhibition methods

Nitrate consumption in aquifers may result from several biogenic and abiotic processes such as denitrification, assimilatory NO3- reduction, dissimilatory NO3- reduction to ammonium (DNRA), or abiotic NO3- (or NO2-) reduction. The objectives of this study were to investigate the fate of NO3- in a petroleum-contaminated aquifer, and to assess the feasibility of using single-well push-pull tests (PPTs) in combination with 15N isotope and C2H2 inhibition methods for the quantification of processes contributing to NO3- consumption. Three consecutive PPTs were performed in a monitoring well of a heating oil-contaminated aquifer in Erlen, Switzerland. For each test, we injected 500 l of test solution containing 0.5 mM Br- as conservative tracer and either 0.5 mM unlabeled NO3- or approximately 0.3 mM 15N-labeled NO3- as reactant. Test solutions were sparged during preparation and injection with either N2, Ar or 10% C2H2 in Ar. After an initial incubation period of 1.5-3.2 h, we extracted the test solution/groundwater mixtures from the same location and measured concentrations of relevant species including Br-, NO3-, NO2-, N2O, N2, and NH4+. In addition, we determined the 15N contents of N2, N2O, NH4+, and suspended biomass from 15N/14N isotope-ratio measurements. Average total test duration was 50.5 h. First-order rate coefficients (k) were computed from measured NO3- consumption, N2-15N production and N2O-15N production. From measured NO3- consumption we obtained nearly identical estimates of k for all PPTs with small 95% confidence intervals, indicating good reproducibility and accuracy for the tests. Estimates of k from N2-15N production and N2O-15N production indicated that denitrification accounted for only 46-49% of observed NO3- consumption. Production of N2-15N in the presence of C2H2 was observed during one of the tests, which may be an indicator for abiotic NO3- reduction. Moreover, 15N isotope analyses confirmed occurrence of assimilatory NO3- reduction (0.58 at.% 15N in suspended biomass) and to a smaller extent DNRA (up to 4 at.% 15N in NH4+). Our results indicated that the combination of PPTs, 15N-isotope and C2H2 inhibition methods provided improved information on denitrification as well as alternative fates of NO3- in this aquifer.

Characterizing Intrinsic Bioremediation in a Petroleum Hydrocarbon-Contaminated Aquifer by Combined Chemical, Isotopic, and Biological Analyses

Chemical, isotopic, and biological parameters were evaluated over a 1-year period to characterize microbial processes associated with intrinsic bioremediation in a petroleum hydrocarbon-contaminated aquifer located in Studen, Switzerland. Chemical parameters measured included oxidants such as O2, NO3 −, and SO4 2−, reduced species such as Fe2+ and CH4, and dissolved inorganic carbon (DIC). Stable carbon isotope analyses of DIC were used to differentiate between different processes that contribute to DIC production. Microbial populations were identified by sequence analysis of archaeal 16S rDNA and in situ hybridization using a general DNA binding dye (DAPI) and specific probes targeting the domain Archaea (Arch915) and Bacteria (Eub338), as well as the species Methanosaeta concilii (Rotcl1) and Methanospirillum sp. (Rotcl2). Groundwater exhibited reduced conditions and elevated concentrations of DIC within the contaminated zone. Spatially distinct values of δ13C ranging from −16.5l%c to −4.44%o were found, indicating the presence of different ongoing microbial processes. Detected microbial populations (% of DAPI-stained cells) within the contaminated zone belonged to Archaea (9±2% to 31±13%) and Bacteria (13±3% to 32±13%). In wells with methanogenic activity, Methanosaeta concilii accounted for up to 26% of all DAPI-detected microorganisms. These results demonstrate that this novel combination of chemical, isotopic, and biological analysis provides valuable insights that can be used for the characterization of microbial processes in contaminated aquifers.