J. Gwynfryn Jones

Bacterial Reduction of Ferric Iron in a Stratified Eutrophic Lake

SUMMARY: The rate of release of Fe(II) from anoxic lake sediments was lower in the presence than in the absence of nitrate. The reduction of Fe(III) by the sediments had a temperature optimum of 30 °C and was inhibited by HgCl2, suggesting that the process was largely biological in nature. Of the iron sources tested with cultures of anaerobic iron-reducing bacteria, FeCl3 was the most readily reduced and goethite the least. Reduction was faster in the presence of a chelating agent and was suppressed by the addition of NO- 3, ClO- 3, MnO2, Mn2O3 and O2. An iron-reducing chemoorganotroph, tentatively identified as a member of the genus Vibrio, was isolated. Physical contact between the bacterium and iron particles was essential to ensure maximum rates of Fe(III) reduction but > 30% of the activity appeared to be associated with extracellular components. Although Fe(III) reduction by whole cells and cell-free extracts was decreased in the presence of electron transport inhibitors, the molar growth yield of the organism was unaffected by the presence of Fe(III). It is assumed that the organism used the Fe(III) as a hydrogen sink. A second organism, an anaerobic facultative chemolithotroph, appeared to conserve energy by the reduction of Fe(III). Biomass yield (measured as ATP) was greater in the presence of Fe(III), and the organism was able to use H2 as a source of reducing power.

Comparison of fatty acid content and DNA homology of the filamentous gliding bacteriaVitreoscilla, Flexibacter, Filibacter

DNA hybridization experiments showed that there was a high degree of homology amongVitreoscilla strains but not with DNA fromFilibacter limicola. Flexibacter spp were much more heterogeneous indicating a low genetic similarity. These results were also reflected in the membrane fatty acids of the bacteria. TheVitreoscilla strains were very similar with the 16:1ω7c fatty acid being dominant. The membrane fatty acids ofF. limicola were dominated by a15:0 and a17:0 components which provided additional support for its relatedness to the genusBacillus. There was much greater diversity in the fatty acid patterns of theFlexibacter spp.F. aurantiacus, F. ruber andF. elegans shared the common dominant fatty acids 16:1ω7c with theVitreoscilla strains, but this was replaced by the 16:1ω6c acid inF. flexilis. F. ruber was distinguished by the absence of branched odd-chain monounsaturated fatty acids andF. elegans by the dominance of the β-OH i15:0 acid. Precise determination of fatty acid double bond positions and geometry are essential for correct interpretation of increasingly complex ecological and taxonomic data sets.

Differences in Microbial Decomposition Processes in Profundal and Littoral Lake Sediments, with Particular Reference to the Nitrogen Cycle

An investigation of sediments from the littoral (shallow water) and profundal (deep water) zones of Blelham Tarn, a shallow eutrophic lake, showed marked differences in the microbial decomposition processes. These differences were due largely to differences in the degree of oxygenation, supply of electron acceptors, and mean summer temperature at the two sites. The changes in the hypolimnion (the deep water zone formed on thermal stratification, which may be treated essentially as a closed system) could be used to calculate profundal rates of aerobic respiration, NO- 3 and SO2- 4 reduction, and methanogenesis, relative to the accumulation of CO2. Laboratory measurements demonstrated that NH+ 4 accumulation, SO2- 4 reduction and methanogenesis were more intense in the profundal than in the littoral zone. Anaerobic processes that occurred in the littoral sediments did so at greater depths than in the profundal sediments. The release of CH4 and N2 bubbles also provided estimates of the importance of these processes at the two sites. At both sites aerobic respiration was the most important component (about 50%) of carbon mineralization; SO2- 4 reduction was the least important, accounting for only a small percentage of carbon turnover. Pathways of NO- 3 reduction and methanogenesis accounted for approximately equal proportions (varying between 15 and 25%) of the carbon mineralized. When the results were adjusted to account for the relative areas of the profundal and littoral zones, the former was the more important site of methanogenesis and SO2- 4 reduction, whereas aerobic respiration and NO- 3 reduction were greater in the littoral zone. The major end-product of NO- 3 reduction was NH+ 4 in the profundal and N2 in the littoral zone. The higher and continued levels of nitrification, which recycled the NH+ 4 in the littoral sediments, were thought to contribute to this.

Distribution and regulation of nitrate and nitrite reduction by Desulfovibrio and Desulfotomaculum species

Rates of nitrate and nitrite reduction by cultures and washed suspensions of both natural isolates and culture collection strains of sulphate-reducing bacteria have been determined. Neither activity was detected in the Desulfotomaculum strains, but all Desulfovibrio strains reduced nitrite. Only the Desulfovibrio natural isolate FBA 20a was also able to reduce nitrate. Nitrate reduction by washed suspensions of strain FBA 20a was far more rapid than previously reported rates for sulphate-reducing bacteria [6.6 μmol NO3-reduced h-1 (mg dry weight)-1] and was regulated by nitrate induction and sulphate repression: it was insensitive to the product of nitrate reduction, ammonium ions. Cell yields from sulphate-limited cultures were proportional to the concentration of nitrate added with a yield coefficient of 28.0 g bacterial dry weight per mol of nitrate reduced. These results indicate that although the ability of strain FBA 20a to reduce nitrate is a physiologically significant process, it is a specialized property of only a few strains of Desulfovibrio isolates.

Factors Affecting Methanogenesis and Associated Anaerobic Processes in the Sediments of a Stratified Eutrophic Lake

SUMMARY: Factors affecting methanogenesis in the sediments of a eutrophic lake were studied during late summer, a period during which CH4 gas production slowed down dramatically or stopped completely. The most active methanogenesis occurred in the surface sediments and the temperature optimum for the process in these deeper sediments was 30 °C. Addition of H2 or formic acid to sediment slurries stimulated CH4 production to a greater extent than did acetic or pyruvic acid. Analysis of the kinetics of the conversion of H2 to CH4 suggested that the sediments were severely limited in H2, the concentration being considerably less than 2·5 μmol 1-1, the Km for the process. Methanogenesis was not stimulated by the addition of trace quantities of Ni2+, Co2+, MoO4 2- or Fe2+ ions but was inhibited by 0·5 mmol SO4 2- 1-1. Under natural conditions the sediments were also limited in SO4 2- and sulphate reducers acted as net H2 donors to the methanogens; addition of SO4 2- allowed the sulphate reducers to compete effectively for H2. The addition of 20 mmol Na2MoO4 1-1 to sediments inhibited methanogenesis but this was not due entirely to its effect in the H2 transfer from sulphate reducers; it also inhibited CO2 uptake by sediments and the production of CH4 from CH3COOH and CO2 by cultures of methanogens. It is therefore inadvisable to use MoO4 2- at this concentration as a specific inhibitor of sulphate reducers in such freshwater sediments. Experiments with other inhibitors of methanogens suggested that they may interact with sulphate reducers, acetogens or anaerobic bacteria involved in fatty acid decomposition. Small, sealed sediment cores, which were used to reproduce natural conditions, particularly of available H2 concentration, were injected with trace quantities of H14CO3 - and 14CH3COOH. The results suggested that more than 75% of the CH4 was derived from CO2 and the remainder from CH3COOH. The overall rates of methanogenesis in the small cores agreed well with results from the field.

Microbial Nitrate Reduction in Freshwater Sediments

Summary: Nitrate reductase activity was three to four orders of magnitude greater in freshwater sediments than in the overlying water. Viable (most probable number) counts of denitrifiers provided sufficient resolution to distinguish between anoxic and surface waters, and between these and sediments, but did not correlate with differences between or within sediment cores. Activity within the sediment depended on the electrode potential (E h) profile, which in turn was related to the degree of turbulence and oxygen concentration in the overlying water. Sediments from the littoral zone or those in contact with oxygenated water were oxidizing to a depth of 5 to 10 mm and the E h then decreased rapidly. In these sediments nitrate reductase activity was often at its maximum at a depth of 10 to 15 mm, on the E h gradient, and coincided with a mean E h value of 210 mV. Under reducing conditions the E h gradient moved upwards and nitrate reductase activity was greatest at the sediment-water interface. These observations were supported by analyses of the nitrogen gas content of the sediments. Inhibition of the enzymes with chlorate indicated that approximately 60 % of the activity was dissimilatory in sediments where the E h was greater than +100 mV, and that this proportion increased to more than 90% when the E h fell below +50 mV. Although the evidence was not conclusive, there was also some indication that nitrate reductase activity in aerobic surface sediments was greater in the larger (> 250 μm) particle size fraction, which suggested that these particles might act as microsites for nitrate respiration.

A Microbiological Study of Sediments from the Cumbrian Lakes

Summary: The microbiology of the benthos of 16 lakes in Cumbria was studied. The lakes formed a series ranging from oligotrophic to eutrophic, and the results were compared with other surveys of their chemistry and biology. The sediments of the more productive lakes contained more organic matter, E h measurements indicated that they were more reduced, and the overlying water was deoxygenated to a greater degree. These results correlated with greater microbial activity, biomass and numbers in the sediments of the richer lakes, as measured by electron transport system activity, ATP and direct counts. The data obtained from the sediments were, however, more variable, and showed poorer agreement with the assumed ranking of the lakes than the results obtained from the water column in this and past surveys. The microbiology of the benthos suggested that the more productive lakes might be considered as a distinct group, but more detailed sampling and careful choice of indicator micro-organisms would be required to provide statistically significant evidence for this.

Some Differences in the Microbiology of Profundal and Littoral Lake Sediments

SUMMARY: The microbial populations of the littoral (shallow water) and profundal (deep water) surface sediments of Blelham Tarn and the South Basin of Windermere were examined. Microbial numbers (direct counts), biomass (ATP) and activity (electron transport system activity, CO2 evolution and [14C]glucose mineralization) were consistently higher in the profundal zone of both lakes, the difference being greater and average values higher in the more productive Blelham Tarn. The interstitial water of the profundal sediments contained higher concentrations of available substrates (carbohydrate, protein and amino acids). There were marked differences in the particle size distribution of the sediments with a greater proportion of small particles (< 45 μm in size) in the profundal samples. The smaller particles from both sites were colonized by a larger number of bacteria and contained higher electron transport system activity per gram dry weight. There was a greater diversity of benthic animals in the littoral zone of both lakes but larger numbers of ciliated protozoa were observed in the profundal sediments.

Microbiological Sources of Ammonia in Freshwater Lake Sediments

SUMMARY: During summer stratification ammonia is released from the profundal sediments of Blelham Tarn (English Lake District). The quantity of ammonia released exceeds the consumption of nitrate in the hypolimnion. Nitrate dissimilation may be a component in the generation of ammonia, but only during early summer when nitrate is still available. The remainder of the ammonia arises largely from the deamination of proteins, amino acids and urea. Population estimates of bacteria which produced ammonia from nitrate, amino acids and urea were of the same order of magnitude. Numbers of bacteria which produced ammonia from nitrate increased with sediment depth, and urea decomposers were more numerous in the profundal (deep water) sediments. While nitrate was available in the water column, surface sediments converted nitrate almost exclusively to nitrogen gas. After depletion of the nitrate, the release of ammonia from washed sediment particles was largely microbiological, whereas there was a significant chemical component to the release from intact sediment cores. Chemical binding of ammonia by the sediments was demonstrated and this hindered calculations of inorganic nitrogen metabolism based on changes in water chemistry. Trace additions of 14C-labelled protein, amino acids and urea to sediments showed that urea was turned over the most rapidly, but more reliable estimates of available protein in the sediments are required before decomposition rates can be treated with confidence.

Microbial Activity in Lake Sediments with Particular Reference to Electrode Potential Gradients

Summary: Microbial activity was determined along electrode potential gradients within sediments, and along sediment surfaces in a stratified eutrophic lake. There was evidence of a change from a tricarboxylic acid cycle-based metabolism to a fermentative one with decreasing electrode potential. A more detailed examination of the stratification of the microbial community showed that the activities of enzymes associated with energy metabolism (in this case electron transport) were highest on electrode potential gradients. This was observed within a sediment core on a millimetredepth scale, as well as over several metres at the surface of sediments, on a transect which ran from oxic littoral muds to the anoxic profundal zone. In contrast, microbial hydrolytic enzymes, such as protease and amylase, were most active at the sediment surface, where the highest concentrations of substrates might be expected.