Studies were conducted in opposing gradients of oxygen and sulfide in microslide capillaries to (i) characterize the chemical microenvironment preferred by Candidatus Arcobacter sulfidicus, a highly motile, sulfur-oxidizing bacterium that produces sulfur in filamentous form, and (ii) to develop a model describing the mechanism of filamentous-sulfur formation. The highly motile microorganisms are microaerophilic, with swarms effectively aggregating within oxic-anoxic interfaces by exhibiting a chemotactic response. The position of the band was found to be largely independent of the sulfide concentration as it always formed at the oxic-anoxic interface. Flux calculations based on steady state gradients of oxygen and sulfide indicate that sulfide is incompletely oxidized to sulfur, in line with the formation of filamentous sulfur by these organisms. It is proposed that Candidatus Arcobacter sulfidicus effectively competes with other sulfur-oxidizing bacteria in the environment by being able to tolerate higher concentrations of hydrogen sulfide (1-2 mM) and by possessing the ability to grow at very low oxygen concentrations (1-10 muM). The formation of mat-like structures from filamentous sulfur appears to be a population mediated effort allowing these organisms to effectively colonize environments characterized by high sulfide, low oxygen and dynamic fluid movement.

Water samples were collected at a fringing coral reef in overlying water, in bottom water between corals and in crevices under coral colonies, and analyzed for nutrient concentrations, bacterial numbers and production. We found decreasing bacterial densities from overlying water through bottom water into crevices (range 9 to 2 X 105 ml-l). Bacterial specific growth was enhanced in reef crevices (range 0.005 to 0.04 h-'). Although bactenal growth was enhanced, bacterial numbers were reduced, showing a transfer of bacterial biomass into the reef. The differences in bacterial numbers and grnwth between water types depended on water movement and bottom relief Nutrients were enhanced in reef crevices as a result of mineralization. Mineralization of bacterla removed by filterfeeders could contribute 11 and 21 '70 to the increase in N and P, respectively, in coral reef crevices.

The community structure of sulfate-reducing bacteria in littoral and profundal sediments of the oligotrophic Lake Stechlin (Germany) was investigated. A collection of 32 strains was isolated from the highest positive dilutions of most-probable-number series, and their partial 16S rRNA gene sequences and genomic fingerprints based on ERIC (enterobacterial repetitive intergenic consensus)-PCR were analyzed. The strains fell into eight distinct phylogenetic lineages, and the majority (70%) showed a close affiliation to the genus Desulfovibrio. Most of the remaining strains (22%) were related to the gram-positive Sporomusa and Desulfotomaculum groups. A high redundancy of 16S rRNA gene sequences was found within several of the phylogenetic lineages. This low phylogenetic diversity was most pronounced for the subset of strains isolated from oxic sediment layers. ERIC-PCR revealed that most of the strains with identical 16S rRNA gene sequences were genetically different. Since strains with identical 16S rRNA gene sequences but different genomic fingerprints also differed considerably with respect to their physiological capabilities, the high diversity detected in the present work is very likely of ecological relevance. Our results indicate that a high diversity of sulfate-reducing bacterial strains can be recovered from the natural environment using the established cultivation media.

The depth distribution and diversity of sulphate-reducing bacteria (SRB) was analysed in the upper intertidal zone of a sandy marine sediment of the Dutch island Schiermonnikoog. The upper centimetre of the sediment included the oxic-anoxic interface and was cut into five slices. With each slice, most probable number (MPN) dilution series were set up in microtitre plates using five different substrates. In the deeper sediment layers, up to 1 x 10(8) cm(-3) lactate-utilizing SRB were counted, corresponding to 23% of the total bacterial count. From the highest positive dilutions of the MPN series, 27 strains of SRB were isolated in pure culture. Sequencing of a 580 bp fragment of the 16S rDNA revealed that 21 isolates had identical sequences, also identical with that of the previously described species Desulfomicrobium apsheronum. However, the diversity of the isolates was higher with respect to their physiological properties: a total of 11 different phenotypes could be distinguished. Genomic fingerprinting by enterobacterial repetitive intergenic consensus (ERIC) polymerase chain reaction (PCR) revealed an even higher diversity of 22 different genotypes. A culture-independent analysis by PCR and denaturing-gradient gel electrophoresis (DGGE) revealed that the partial 16S rDNA sequence of the isolated D. apsheronum strains constituted a significant fraction of the Desulfovibrionaceae. The high subspecies diversity suggests that this abundant aggregate-forming species may have evolved adaptations to different ecological niches in the oxic sediment layers.

Isolations of 3-chlorobenzoate (3CBA)-degrading aerobic bacteria under reduced O 2 partial pressures yielded organisms which metabolized 3CBA via the gentisate or the protocatechuate pathway rather than via the catechol route. The 3CBA metabolism of one of these isolates, L6, which was identified as an Alcaligenes species, was studied in more detail. Resting-cell suspensions of L6 pregrown on 3CBA oxidized all known aromatic intermediates of both the gentisate and the protocatechuate pathways. Neither growth on nor respiration of catechol could be detected. Chloride production from 3CBA by L6 was strictly oxygen dependent. Cell-free extracts of 3CBA-grown L6 cells exhibited no catechol dioxygenase activity but possessed protocatechuate 3,4-dioxygenase, gentisate dioxygenase, and maleylpyruvate isomerase activities instead. In continuous culture with 3CBA as the sole growth substrate, strain L6 demonstrated an increased oxygen affinity with decreasing steady-state oxygen concentrations.

Denitrifying activity in a sediment from the freshwater part of a polluted estuary in northwest Europe was quantified using two independent approaches. High-resolution N 2 O microprofiles were recorded in sediment cores to which acetylene was added to the overlying water and injected laterally into the sediment. The vertical distribution of the rate of denitrification supported by nitrate uptake from the overlying water was then derived from the time series N 2 O concentration profiles. The rates obtained for the core incubations were compared to the rates predicted by a forward reactive transport model, which included rate expression for denitrification calibrated with potential rate measurements obtained in flowthrough reactors containing undisturbed, 1-cmthick sediment slices. The two approaches yielded comparable rate profiles, with a near-surface, 2-to 3-mm narrow zone of denitrification and maximum in situ rates on the order of 200 to 300 nmol cm ؊3 h ؊1 . The maximum in situ rates were about twofold lower than the maximum potential rate for the 0-to 1-cm depth interval of the sediment, indicating that in situ denitrification was nitrate limited. The experimentally and model-derived rates of denitrification implied that there was nitrate uptake by the sediment at a rate that was on the order of 50 (؎ 10) nmol cm ؊2 h ؊1 , which agreed well with direct nitrate flux measurements for core incubations. Reactive transport model calculations showed that benthic uptake of nitrate at the site is particularly sensitive to the nitrate concentration in the overlying water and the maximum potential rate of denitrification in the sediment.Anthropogenic inputs of nitrogen are a major cause of eutrophication in aquatic environments (39). Denitrification, the bacterial dissimilatory reduction of nitrate to gaseous products, counteracts eutrophication by removing inorganic nitrogen and releasing it to the atmosphere and by decomposing organic matter (34). The emissions resulting from denitrification, however, include nitrous oxide, a powerful greenhouse gas (8). Because of its environmental importance, denitrification has been the subject of numerous studies (for reviews, see references 12, 16, and 34). Oxygen and nitrate concentrations, temperature, pH, and the availability of suitable electron donor substrates (mainly organic carbon compounds) are considered the key factors controlling the occurrence and rate of denitrification (6,22,38).In estuarine and coastal environments, the most intense denitrification often occurs in sediments, where reduced solute transport rates enable the establishment of anaerobic conditions. A number of methods have been developed to quantify denitrification and related nitrogen transformations in sediments; these methods include, among others, benthic flux measurement (17), isotope pairing techniques (32), and microprofiling using nitrous oxide (31) or nitrate microsensors (9,20). The available data show that there are large spatial and temporal variations in denitrification rates in nearsho...