Bacteria isotopic composition

Sigman et al. [134] have described a bacterial method for measuring the isotopic composition of seawater nitrate at the natural-abundance level. The method is based on the analysis of nitrous oxide gas (N2O) produced quantitatively from nitrate by denitrifying bacteria. The classical denitrification pathway consists of the stepwise reduction of nitrate (NOp to nitrite (N02), nitric oxide (NO), nitrous oxide (N2O), and dinitrogen (N2) ... 

Although the sediments in these systems accumulate Se over time, the small isotopic contrast suggests that dissimilatory reduction is not the dominant accumulation mechanism. If dissimilatory reduction of Se(VI) and/or Se(IV) to Se(0) by bacteria were the dominant mechanism, one would expect the accumulated Se(0) to be enriched in the lighter isotope. In the San Francisco Estuary case, this assumes that the isotopic fractionations measured by Ellis et al. (2003) can be extrapolated to much lower concentrations. Incorporation of Se into algae and macrophyte tissues, followed by decay of some material and conversion of its Se to Se(0), is more consistent with the observed Se isotope data. Notably, the mean Se isotope composition of the Se(0) in the sediments of the Herbel et al. (2002) study was identical to that of the macrophytes. 

Figure 3. Effects on Fe isotope compositions of partial dissolution of hornblende in the presence of various organic ligands, as well as Streptomyces and Arthrobacter bacteria (identification of Arthrobacter sp. has been revised to Bacillus sp. S. Brantley, pers. commun. 2004). For abiotic experiments, Fe isotope data are plotted relative to the association constant for the indicated ligands. Fe isotope composition of...

An end-member case would be precipitation of ferric oxyhydroxide by photosynthetic Fe(II)-oxidizing bacteria through simple Rayleigh fractionation, with no external Fe(II) flux or return of Fe(II) to the pool from Fe(III)-reducing bacterial activity, which will produce extreme Fe isotope compositions, but only in the latest fluids and precipitates (Fig. 17). In... 

The relatively smaller 15N enrichments of Cyclops compared to phytoplankton and the other zooplankton in the leucine experiment are consistent with the idea that Cyclops feeds on microheterotrophs that are less labeled with 15N than the phytoplankton. As with the 15N, Cyclops was less enriched in 13C than phytoplankton. But the 13C proved less useful as a tracer because phytoplankton took up the label and because the actual isotopic compositions of bacteria and microheterotrophs were unknown. 

The H2S concentration and isotopic composition profiles in the euxinic sediment are constant with depth indicating that no significant reduction is occurring in the sediment. The high H2S concentration (110 mmol/L) may have an inhibitory effect on the sulfate-reducing bacteria. 

Relative stable carbon isotope compositions of individual amino acids in bacteria cultured on alanine and glutamic acid (from Macko et al 1987). Amino acids provide a good example of the wide range in 5 C values that can exist within a compound class, largely owing to the many different biochemical pathways by which various amino acids are formed. 

The isotopic compositions closely follow those of heterotrophic carbon sources, and this can be exploited to provide new information about the roles of bacteria in aquatic food webs (Coffin et al. 1994, 1997). 

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