Relationships between sulfate-reducing and methane-producing bacteria

Summary Sulfate ions in the muddy sediments of Lake Vechten are consumed by sulfate-reducing bacteria of which the abundance is limited by the concentration of these ions. Methane producers are found deeper in the mud at lower concentrations of hydrogen sulphide. The turnover rate constant (k) of L-lactate, calculated from the decline in specific activity of labeled acid, was 2.37 h⁻¹. The average L-lactate pool size was 12.2 μg per gram of wet mud, giving a turnover rate of 28.9 μg of lactate/gram of mud per h. The turnover rate constant of acetate was 0.35 h⁻¹ and the average pool size 5.7 μg per gram of wet mud, giving a rate of disappearance of 2.0 μg of acetate/gram of mud per h. The formation of C¹⁴H₄ from [U-C¹⁴]-L-lactate, suggests a substrate relationship between sulfate-reducing and methane-producing bacteria. Results of chemostat experiments gave further supporting evidence of such a relationship. The influence of an acetate-producing organism,Desulfovibrio desulfuricans, on the fermentation of limiting amounts of acetate by a methane-producing organism,Methanobacterium sp., was studied in mixed continuous cultures. The results of these experiments indicated the existence of a commensalism. Paper read at the Symposium on the Sulphur Cycle, Wageningen, May 1974.     

Interrelations between sulfate-reducing and methane-producing bacteria in bottom deposits of a fresh-water lake. III. Experiments with 14C-labeled substrates

An ecological substrate relationship between sulfate-reducing and methane-producing bacteria in mud of Lake Vechten has been studied in experiments using ¹⁴C-labeled acetate and lactate as substrates. Fluoroacetate strongly inhibited the formation of ¹⁴CO₂ from [U-¹⁴C]-acetate and β-fluorolactate gave an inhibition of similar magnitude of the breakdown of [U-¹⁴C]-l-lactate to ¹⁴CO₂ thus confirming earlier results on the specific action of these inhibitors. The turnover-rate constant of l-lactate was 2.37 hr^-1 and the average l-lactate pool size was 12.2 μg per gram of wet mud, giving a turnover rate of 28.9 μg of lactate/gram of mud per hr. The turnover-rate constant of acetate was 0.35 hr^-1 and the average pool size was 5.7 μg per gram of wet mud, giving a rate of disappearance of 1.99 μg of acetate/gram of mud per hr. Estimations of the acetate turnover rate based upon the formation of ¹⁴CO₂ from [U-¹⁴C]-acetate or [1-¹⁴C]-acetate yielded figures of the same magnitude (range 0.45 to 1.74). These and other results suggest that only a portion of the lactate dissimilated is turned over through the acetate pool. The ratio of ¹⁴CO₂/¹⁴CH₄ produced from [U-¹⁴C]-acetate by mud was 1.32; indicating that 0.862 moles of CH₄ and 1.138 moles of CO₂ are formed per mole of acetate. From the rate of disappearance of acetate (0.027 μmoles/gram wet mud per hr) and the rate of methane production (0.034 μmoles/gram wet mud per hr), it may be concluded that acetate is an important precursor of methanogenesis in mud (approximately 70%). A substrate relationship between the two groups of bacteria is likely since ¹⁴CH₄ was formed from [U-¹⁴C]-l-lactate.     

Comparison of Acetate Turnover in Methanogenic and Sulfate-Reducing Sediments by Radiolabeling and Stable Isotope Labeling and by Use of Specific Inhibitors: Evidence for Isotopic Exchange

Acetate turnover in the methanogenic freshwater anoxic sediments of Lake Vechten, The Netherlands, and in anoxic sediments from the Tamar Estuary, United Kingdom, and the Grosser Jasmunder Bodden, Germany, the latter two dominated by sulfate reduction, was determined. Stable isotopes and radioisotopes, inhibitors (chloroform and fluoroacetate), and methane flux were used to provide independent estimates of acetate turnover. Pore water acetate pool sizes were determined by gas chromatography with a flame ionization detector, and stable isotope-labeled acetate was determined by gas chromatography-mass spectrometry. The appearance of acetates with a different isotope labeling pattern from that initially added demonstrated that isotopic exchange occurred during methanogenic acetate metabolism. The predominant exchange processes were (i) D-H exchange in the methyl group and (ii) (sup13)C-(sup12)C exchange at the carboxyl carbon. These exchanges are most probably caused by the activity of the enzyme complex carbon monoxide dehydrogenase and subsequent methyl group dehydrogenation by tetrahydromethanopterine or a related enzyme. The methyl carbon was not subject to exchange during transformation to methane, and hence acetate with the methyl carbon labeled will provide the most reliable estimate of acetate turnover to methane. Acetate turnover rate estimates with these labels were consistent with independent estimates of acetate turnover (acetate accumulation after inhibition and methane flux). Turnover rates from either radioisotope- or stable isotope-labeled methyl carbon isotopes are, however, dependent on accurate determination of the acetate pool size. The additions of large amounts of stable isotope-labeled acetate elevate the acetate pool size, stimulating acetate consumption and causing deviation from steady-state kinetics. This can, however, be overcome by the application of a non-steady-state model. Isotopic exchange in sediments dominated by sulfate reduction was minimal.     

Quantitative High-Pressure Liquid Chromatography-Fluorescence Determination of Some Important Lower Fatty Acids in Lake Sediments

For the quantitative determination of traces of fatty acids in pore water, several gas and liquid chromatographic methods were tested and discussed. Direct determination by gas-liquid chromatography with the use of formic acid-saturated carrier gas was found to be the least laborious method, but it is only recommended for the determination of volatile acids such as acetate and higher homologs. For the determination of lactate and formate, a derivatization procedure is necessary. The determination of these acids as phenacyl or benzyl esters was complicated by contaminants in the reagents. For this reason, a high-pressure liquid chromatography procedure with 4-bromomethyl-7-methoxycoumarin as a fluorescent labeling reagent is preferred. With this method, lactic, acetic, and formic acids could be demonstrated simultaneously at the nanogram level in 5-ml samples. Profiles of these acids in the sediment of Lake Vechten were measured, and they showed correlations with sulfate-reducing and methanogenic bacterial activities.     

Aspects of aerobic mineralization during spring in Lake Vechten with special reference to the 14C-labelling technique

Aerobic mineralization, i.e. seston respiration, microbial breakdown of detritus and microbial assimilation-dissimilation of photosynthetically derived D(issolved) O(rganic) C(arbon) was measured in concentrated samples from the pelagic zone of Lake Vechten. The samples were described by cell numbers of dominant algae prior to concentrating by centrifuge with continuous rotor. The concentrated samples were incubated in the laboratory at in situ temperature and a light intensity of 30 W · m^–2 for measuring primary production and photosynthetically derived DOC. After filtration the particulate fraction was incubated in unlabelled lake water far measuring respiratory production of DI¹⁴C. Portions of the same particulate fraction were sterilized and incubated in unlabelled sample concentrate for measuring microbial breakdown of detritus. The seston respiration amounted to 19–30% loss of the particulate fraction over 15–45 h. Microbial breakdown of detritus amounted to 28–40% loss of the particulate fraction over 24–168 h. In both cases P(articulate) O(rganic) ¹⁴C(arbon) was transformed to mainly DI¹⁴C. Microbial assimilation-dissimilatioh of photosynthetically derived DOC could not be measured reliably.     

Sulfate reduction and S-oxidation in a moorland pool sediment

In an oligotrophic moorland pool in The Netherlands, S cycling near the sediment/water boundary was investigated by measuring (1) SO₄ ^2– reduction rates in the sediment, (2) depletion of SO₄ ^2– in the overlying water column and (3) release of³⁵S from the sediment into the water column. Two locations differing in sediment type (highly organic and sandy) were compared, with respect to reduction rates and depletion of SO₄ ^2– in the overlying water.Sulfate reduction rates in sediments of an oligotrophic moorland pool were estimated by diagenetic modelling and whole core³⁵SO₄ ^2– injection. Rates of SO₄ ^2– consumption in the overlying water were estimated by changes in SO₄ ^2– concentration over time in in situ enclosures. Reduction rates ranged from 0.27–11.2 mmol m^–2 d^–1. Rates of SO₄ ^2– uptake from the enclosed water column varied from –0.5, –0.3 mmol m^–2 d^–1 (November) to 0.43–1.81 mmol m^–2 d^–1 (July, August and April). Maximum rates of oxidation to SO₄ ^2– in July 1990 estimated by combination of SO₄ ^2– reduction rates and rates of in situ SO₄ ^2– uptake in the enclosed water column were 10.3 and 10.5 mmol m^–2 d^–1 at an organic rich and at a sandy site respectively.Experiments with³⁵S^2– and³⁵SO₄ ^2– tracer suggested (1) a rapid formation of organically bound S from dissimilatory reduced SO₄ ^2– and (2) the presence of mainly non SO₄ ^2–-S derived from reduced S transported from the sediment into the overlying water. A³⁵S^2– tracer experiment showed that about 7% of³⁵S^2– injected at 1 cm depth in a sediment core was recovered in the overlying water column.Sulfate reduction rates in sediments with higher volumetric mass fraction of organic matter did not significantly differ from those in sediments with a lower mass fraction of organic matter.Corresponding author     

Comparing global models of terrestrial net primary productivity (NPP): the importance of water availability

Given that neither absolute measures nor direct model validations of global terrestrial net primary productivity (NPP) are feasible, intercomparison of global NPP models provides an effective tool to check model consistency. For this study, we tested the assumption that water availability is the primary limiting factor of NPP in global terrestrial biospheric models. We compared a water balance coefficient (WBC), calculated as the difference of mean annual precipitation and potential evapotranspiration to NPP for each grid cell (0.5° × 0.5° longitude/latitude) in each of 14 models. We also evaluated different approaches used for introducing water budget limitations on NPP: (1) direct physiological control on evapotranspiration through canopy conductance; (2) climatological computation of constraints from supply/demand for ecosystem productivity; and (3) water limitation inferred from satellite data alone. Plots of NPP vs. WBC showed comparable patterns for the models using the same method for water balance limitation on NPP. While correlation plots revealed similar patterns for most global models, other environmental controls on NPP introduced substantial variability.     

Interrelations between sulfate-reducing and methane-producing bacteria in bottom deposits of a fresh-water lake. I. Field observations

Observations on the seasonal periodicity in bottom deposits of Lake Vechten indicated an ecological relationship between sulfate-reducing and methane-producing bacteria. Sulfate reducers are most abundant at depths of 0 to 2 cm in the mud at pS^2- values of about 11 and redox potential values of-100 to-150 mV. Maximum number of methane producers are situated at depths of 3 to 6 cm in the mud at pS^2- values of about 14, redox potential values of-250 to-300 mV and maximum values of the methane concentration. During summer stratification the numbers of bacteria increased considerably. However the number of methane producers rose much more than that of the sulfate reducers. Sulfate in the interstitial water of the sediments is reduced by the sulfate reducers and the sulfate concentration limited the latter's abundance. Methane producers are found deeper in the mud at lower concentrations of hydrogen sulphide. Therefore the different localities of the two bacterial groups may be due to sensitivity of methane producers to hydrogen sulphide. Differential counting of the mixed population of methane-producing bacteria showed that acetate-and methyl-alcohol-fermenting types are most abundant at a depth of 5, and formate-and CO₂/H₂-fermenting types at a depth of 3 cm in the mud.     

Sources of organic carbon in the littoral of Lake Gooimeer as indicated by stable carbon isotope and carbohydrate compositions

The relative importance of potential carbon sources in the littoral of Lake Gooimeer, a lake in the centre of the Netherlands, was studied using a combination of ¹³C/¹²C-ratio analysis and carbohydrate composition analysis. The littoral is covered on the land side by a 80 m wide Phragmites australis bed. Potential carbon sources were macrophyte litter, seston and benthic algae. Samples of potential carbon sources, sediments and benthic macrofauna from inside and outside the bed were analyzed for their¹³C/¹²C-ratio and some for their carbohydrate composition. Results indicate that inside the bed, macrophyte litter was the main source of carbon for both the sediment organic matter and the benthic macrofauna, and that algal material was of minor importance. Outside the bed, production by benthic algae was the main carbon source, with seston as a second source. No macrophyte derived material could be detected outside the reed bed.Abbreviations DOC (Dissolved Organic Carbon) - SOM (Sediment Organic Matter)