The rate of release of Fe(I1) from anoxic lake sediments was lower in the presence than in the absence of nitrate. The reduction of Fe(II1) by the sediments had a temperature optimum of 30 "C and was inhibited by HgC12, suggesting that the process was largely biological in nature.Of the iron sources tested with cultures of anaerobic iron-reducing bacteria, FeC13 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,, C105, MnO,, Mn203 and 02. 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(II1) reduction but > 30% of the activity appeared to be associated with extracellular components. Although Fe(II1) 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(II1). It is assumed that the organism used the Fe(II1) as a hydrogen sink. A second organism, an anaerobic facultative chemolithotroph, appeared to conserve energy by the reduction of Fe(II1). 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.
The addition of Fe(II1) to anoxic lake sediments decreased the quantity of volatile fatty acids which accumulated. Similarly, the addition of several substrates to the sediments stimulated the rate of reduction of the naturally occurring Fe(II1). Of the substrates used, malate caused the greatest stimulation, and subsequently a malate-fermenting Vibrio was isolated from the sediment. This organism also reduced NO, and Mn(IV), the addition of which to the growth medium significantly lowered the rate of Fe(1I) formation. The presence of Fe(II1) in the medium increased the molar growth yield by 28% and this did not appear to be associated with the presence of particulate matter or changes in pH and E,,. The addition of Fe(II1) to the growth medium also produced a minor shift in the fermentation end-products, with a decrease in the quantity of ethanol formed and a concomitant increase in volatile fatty acids, both of which were of the same order as the amount of Fe(II1) reduced. It would appear, therefore, that the presence of Fe(II1) might permit a slight diversion of metabolism to more energetically favourable end products. The scale of iron reduction, both in the field and the laboratory, was very small compared with the reducing potential of the available substrates. ~ ~~~Abbreiiation: VFA, volatile fatty acid.0022-I 287/84/000 1 -1 282 $02.00 0 1984 SG M
The incidence of antibiotic resistance was determined in over 2000 bacteria which were divided into the following groups: faecal streptococci, coliforms (excluding Escherichia coli), E. coli, Pseudomonas spp. and aquatic bacteria (i.e. bacteria predominant in the lake water which were excluded from the previous four categories). The isolates were obtained from the water of Windermere (English Lake District) and from a sewage effluent which entered the lake. With the exception of the faecal streptococci, the incidence of antibiotic resistance was higher in the bacteria isolated from the lake water than in those from the effluent, and ranked according to groups Pseudomonas spp. greater than E. coli greater than aquatic bacteria greater than coliforms greater than faecal streptococci. The highest incidence of multiple resistance was found among the pseudomonads. When corrected for the relative size of each population the pool of antibiotic resistance in the aquatic bacteria was by far the largest. The incidence of antibiotic resistance in aquatic bacteria isolated from Windermere was, however, lower than in those isolated from two remote upland tarns. This finding may have been due to differences in the species composition of the three sites except that the same results were obtained when only fluorescent pseudomonads were tested. The upland tarns were not totally isolated from man and other animals but did not receive any sewage or other effluents and therefore the results were surprising. Possible explanations include a lack of susceptibility in aquatic bacteria and increased resistance associated with growth in nutrient poor environments.
Factors affecting methanogenesis in the sediments of a eutrophic lake were studied during late summer, a period during which CH, 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 OC. Addition of H, or formic acid to sediment slurries stimulated CH, production to a greater extent than did acetic or pyruvic acid. Analysis of the kinetics of the conversion of H, to CH, suggested that the sediments were severely limited in H,, the concentration being considerably less than 2.5 pmol l-', the K , for the process. Methanogenesis was not stimulated by the addition of trace quantities of Ni2+, Co2+, MOO:-or Fe2+ ions but was inhibited by 0-5 mmol SO:-1-*. Under natural conditions the sediments were also limited in SO:-and sulphate reducers acted as net H, donors to the methanogens; addition of SO:-allowed the sulphate reducers to compete effectively for H,. The addition of 20 mmol Na,MoO, 1-I to sediments inhibited methanogenesis but this was not due entirely to its effect in the H, transfer from sulphate reducers; it also inhibited CO, uptake by sediments and the production of CH, from CH,COOH and CO, by cultures of methanogens. It is therefore inadvisable to use MOO:-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 H, concentration, were injected with trace quantities of H14C05 and I4CH3COOH. The results suggested that more than 75% of the CH, was derived from CO, and the remainder from CH,COOH. The overall rates of methanogenesis in the small cores agreed well with results from the field.
It is more difficult to obtain a reliable assessment of antibiotic resistance in populations of aquatic bacteria than in those populations which are well characterized (e.g. bacteria of medical and veterinary significance). Factors which influence the results include the bacterial taxa involved, their site of origin and the methods and media used to isolate and subculture the bacteria, and to perform the sensitivity tests. Examples of these effects are provided. The resistance profiles obtained with populations of aquatic pseudomonads depend on the species composition of the population. Resistance patterns in aquatic bacteria varied with the site from which they were isolated; a higher incidence of resistance was recorded along shorelines and in sheltered bays than in the open water. The inclusion of antibiotics in the media employed for primary isolation increased the number of individual and multiple resistances recorded. A similar effect was observed with increased inoculum size in the sensitivity disc method but this could be reversed by raising the incubation temperature. The medium used to conduct the test also affected the results and many aquatic bacteria failed to grow on media such as Iso-Sensitest Agar. It is recommended that the sensitivity disc method is adopted for aquatic bacteria because it permits interpretation of a wider range of response. Comparison of the incidence of antibiotic resistance in different habitats will remain meaningless, however, until comprehensive methods for the identification of bacteria are developed and the techniques used for sensitivity testing are standardized.
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