RDX (Hexahydro-1,3,5-trinitro-1,3,5-triazine) Biodegradation in Aquifer Sediments under Manganese-Reducing Conditions

A shallow, RDX (hexahydro-1,3,5-trinitro-1,3,5-triazine)-contaminated aquifer at Naval Submarine Base Bangor has been characterized as predominantly manganese-reducing, anoxic with local pockets of oxic conditions. The potential contribution of microbial RDX degradation to localized decreases observed in aquifer RDX concentrations was assessed in sediment microcosms amended with [U-¹⁴C] RDX. Greater than 85% mineralization of ¹⁴C-RDX to ¹⁴CO₂ was observed in aquifer sediment microcosms under native, manganese-reducing, anoxic conditions. Significant increases in the mineralization of ¹⁴C-RDX to ¹⁴CO₂ were observed in anoxic microcosms under NO₃-amended or Mn(IV)-amended conditions. No evidence of ¹⁴C-RDX biodegradation was observed under oxic conditions. These results indicate that microbial degradation of RDX may contribute to natural attenuation of RDX in manganese-reducing aquifer systems.     

Breakdown of phytoplankton pigments in Baltic sediments: effects of anoxia and loss of deposit-feeding macrofauna

We examined the decay of chlorophyll a and the carotenoid fucoxanthin in oxic and anoxic sediment microcosms, with and without the deposit-feeding benthic amphipod Monoporeia affinis, over 57 days at 5 degrees C. Deep frozen phytoplankton from the Baltic Sea proper was added to all but a few microcosms. The range of chlorophyll a and fucoxanthin decay rate constants observed in microcosms with phytoplankton addition was 0.04-0.07 day(-1). The fastest pigment decay and build-up of chlorophyll breakdown products after phytoplankton addition were found in oxic treatments with amphipods. No effects of amphipods on pigment breakdown were found in anoxic treatments, or in treatments without phytoplankton addition. Greater losses of chlorophyll a in oxic (96%) than in anoxic (80%) treatments after 57 days indicates that preservation of sedimentary organic matter will be enhanced during periods of anoxia. Due to slow recruitment and recolonization in Baltic sediments, a single anoxic event may cause long-term (years) absence of significant macrobenthos. Anoxic events will thus not only reduce decay of plant pigments, and presumably other organic matter, while they last, but will also have longer-term effects, through elimination of macrofauna, which when present enhance organic matter decomposition.     

Mineralization of hexachlorocyclohexane in soil during solid-phase bioremediation

Soil containing hexachlorocyclohexane (HCH) was spiked with ¹⁴C--HCH and then subjected to bioremediation in bench-scale microcosms to determine the rate and extent of mineralization of the ¹⁴C-labeled HCH to ¹⁴CO₂. The soil was treated using two different DARAMEND amendments, D6386 and D6390. The amendments were previously found to enhance natural HCH bioremediation as determined by measuring the disappearance of parent compounds under either strictly oxic conditions (D6386), or cycled anoxic/oxic conditions (D6390). Within 80 days of the initiation of treatment, mineralization was observed in all of the strictly oxic microcosms. However, mineralization was negligible in the cycled anoxic/oxic microcosms throughout the 275-day study, even after cycling was ceased at 84 days and although significant removal (up to 51%) of indigenous -HCH (146 mg/kg) was detected by GC with electron capture detector. Of the amended, strictly oxic treatments, only one, in which 47% of the spiked ¹⁴C-HCH was recovered as ¹⁴CO₂, enhanced mineralization compared with an unamended treatment (in which 34% recovery was measured). Other oxic treatments involving higher amendment application rates or auxiliary carbon sources were inhibitory to mineralization. Thus, although HCH degradation occurs during the application of either oxic or cycled anoxic/oxic DARAMEND treatments, mineralization of -HCH may be inhibited depending on the amendment and treatment protocol.     

Activation of methanogenesis in arid biological soil crusts despite the presence of oxygen

Methanogenesis is traditionally thought to occur only in highly reduced, anoxic environments. Wetland and rice field soils are well known sources for atmospheric methane, while aerated soils are considered sinks. Although methanogens have been detected in low numbers in some aerated, and even in desert soils, it remains unclear whether they are active under natural oxic conditions, such as in biological soil crusts (BSCs) of arid regions. To answer this question we carried out a factorial experiment using microcosms under simulated natural conditions. The BSC on top of an arid soil was incubated under moist conditions in all possible combinations of flooding and drainage, light and dark, air and nitrogen headspace. In the light, oxygen was produced by photosynthesis. Methane production was detected in all microcosms, but rates were much lower when oxygen was present. In addition, the δ(13)C of the methane differed between the oxic/oxygenic and anoxic microcosms. While under anoxic conditions methane was mainly produced from acetate, it was almost entirely produced from H(2)/CO(2) under oxic/oxygenic conditions. Only two genera of methanogens were identified in the BSC-Methanosarcina and Methanocella; their abundance and activity in transcribing the mcrA gene (coding for methyl-CoM reductase) was higher under anoxic than oxic/oxygenic conditions, respectively. Both methanogens also actively transcribed the oxygen detoxifying gene catalase. Since methanotrophs were not detectable in the BSC, all the methane produced was released into the atmosphere. Our findings point to a formerly unknown participation of desert soils in the global methane cycle.     

Vibrio coralliilyticus Search Patterns across an Oxygen Gradient

The coral pathogen, Vibrio coralliilyticus shows specific chemotactic search pattern preference for oxic and anoxic conditions, with the newly identified 3-step flick search pattern dominating the patterns used in oxic conditions. We analyzed motile V. coralliilyticus cells for behavioral changes with varying oxygen concentrations to mimic the natural coral environment exhibited during light and dark conditions. Results showed that 3-step flicks were 1.4× (P = 0.006) more likely to occur in oxic conditions than anoxic conditions with mean values of 18 flicks (95% CI = 0.4, n = 53) identified in oxic regions compared to 13 (95% CI = 0.5, n = 38) at anoxic areas. In contrast, run and reverse search patterns were more frequent in anoxic regions with a mean value of 15 (95% CI = 0.7, n = 46), compared to a mean value of 10 (95% CI = 0.8, n = 29) at oxic regions. Straight swimming search patterns remained similar across oxic and anoxic regions with a mean value of 13 (95% CI = 0.7, n = oxic: 13, anoxic: 14). V. coralliilyticus remained motile in oxic and anoxic conditions, however, the 3-step flick search pattern occurred in oxic conditions. This result provides an approach to further investigate the 3-step flick.     

Survival of host-associated bacteroidales cells and their relationship with Enterococcus spp., Campylobacter jejuni, Salmonella enterica serovar Typhimurium, and adenovirus in freshwater microcosms as measured by propidium monoazide-quantitative PCR

The ideal host-associated genetic fecal marker would be capable of predicting the presence of specific pathogens of concern. Flowthrough freshwater microcosms containing mixed feces and inocula of the pathogens Campylobacter jejuni, Salmonella enterica serovar Typhimurium, and adenovirus were placed at ambient temperature in the presence and absence of diurnal sunlight. The total Enterococcus DNA increased during the early periods (23 h) under sunlight exposure, even though cultivable Enterococcus and DNA in intact cells, as measured by propidium monoazide (PMA), decreased with first-order kinetics during the entire period. We found a significant difference in the decay of host-associated Bacteroidales cells between sunlight exposure and dark conditions (P value < 0.05), whereas the persistence of host-associated Bacteroidales DNA was comparable. The 2-log reduction times of adenovirus were 72 h for sunlight exposure and 99 h for dark conditions with similar decay rate constants (P value = 0.13). The persistences of fecal Bacteroidales cells and Campylobacter cells exposed to sunlight were similar, and host-associated Bacteroidales DNA and waterborne pathogen DNA were degraded at comparable rates (P values > 0.05). Overall, the ratio of quantitative PCR (qPCR) cycle threshold (C(T)) values with and without PMA treatment was indicative of the time elapsed since inoculation of the microcosm with (i) fecal material from different animal sources based on host-associated Bacteroidales and (ii) pure cultures of bacterial pathogens. The use of both PMA-qPCR and qPCR may yield more realistic information about recent sources of fecal contamination and result in improved prediction of waterborne pathogens and assessment of health risk.     

Adherence and intracellular survival within human macrophages of Enterococcus faecalis isolates from coastal marine sediment

Enterococcus faecalis is part of the human intestinal microbiota and an important nosocomial pathogen. It can be found in the marine environment, where it is also employed as a fecal indicator. To assess the pathogenic potential of marine E. faecalis, four strains isolated from marine sediment were analyzed for their ability to survive in human macrophages. Escherichia coli DH5α was used as a negative control. The number of adherent and intracellular bacteria was determined 2.5 h after the infection (T₀) and after further 24h (T₂₄) by CFU and qPCR counts. At T₂₄ adherent and intracellular enterococcal CFU counts were increased for all strains, the increment in intracellular bacteria being particularly marked. No CFU of E. coli DH5α were detected. In contrast, qPCR counts of intracellular enterococcal and E. coli bacteria were similar at both time points. These findings suggest that whereas E. coli was killed within macrophages (no CFU, positive qPCR), the E. faecalis isolates not only escaped killing, but actually multiplied, as demonstrated by the increase in the viable cell population. These findings support earlier data by our group, further documenting that marine sediment can be a reservoir of pathogenic enterococci.     

A propidium monoazide–quantitative PCR method for the detection and quantification of viable Enterococcus faecalis in large-volume samples of marine waters

The development of rapid detection assays of cell viability is essential for monitoring the microbiological quality of water systems. Coupling propidium monoazide with quantitative PCR (PMA-qPCR) has been successfully applied in different studies for the detection and quantification of viable cells in small-volume samples (0.25–1.00 mL), but it has not been evaluated sufficiently in marine environments or in large-volume samples. In this study, we successfully integrated blue light-emitting diodes for photoactivating PMA and membrane filtration into the PMA-qPCR assay for the rapid detection and quantification of viable Enterococcus faecalis cells in 10-mL samples of marine waters. The assay was optimized in phosphate-buffered saline and seawater, reducing the qPCR signal of heat-killed E. faecalis cells by 4 log₁₀ and 3 log₁₀ units, respectively. Results suggest that high total dissolved solid concentration (32 g/L) in seawater can reduce PMA activity. Optimal PMA-qPCR standard curves with a 6-log dynamic range and detection limit of 10² cells/mL were generated for quantifying viable E. faecalis cells in marine waters. The developed assay was compared with the standard membrane filter (MF) method by quantifying viable E. faecalis cells in seawater samples exposed to solar radiation. The results of the developed PMA-qPCR assay did not match that of the standard MF method. This difference in the results reflects the different physiological states of E. faecalis cells in seawater. In conclusion, the developed assay is a rapid (～5 h) method for the quantification of viable E. faecalis cells in marine recreational waters, which should be further improved and tested in different seawater settings.     

Culture-Dependent and Independent Studies of Microbial Diversity in Highly Copper-Contaminated Chilean Marine Sediments

Cultivation and molecular-based approaches were used to study microbial diversity in two Chilean marine sediments contaminated with high (835 ppm) and very high concentrations of copper (1,533 ppm). The diversity of cultivable bacteria resistant to copper was studied at oxic and anoxic conditions, focusing on sulfate-, thiosulfate-, and iron-reducing bacteria. For both sediments, the cultivable bacteria isolated at oxic conditions were mostly affiliated to the genus Bacillus, while at anoxic conditions the majority of the cultivable bacteria found were closely related to members of the genera Desulfovibrio, Sphingomonas, and Virgibacillus. Copper resistance was between 100 and 400 ppm, with the exception of a strain affiliated to members of the genus Desulfuromonas, which was resistant up to 1,000 ppm of copper. In parallel, cloning and sequencing of 16S rRNA was performed to study the total bacterial diversity in the sediments. A weak correlation was observed between the isolated strains and the 16S rRNA operational taxonomic units detected. The presence of copper resistance genes (copA, cusA, and pcoA) was tested for all the strains isolated; only copA was detected in a few isolates, suggesting that other copper resistance mechanisms could be used by the bacteria in those highly copper-contaminated sediments.     

Redox Fluctuation Structures Microbial Communities in a Wet Tropical Soil

Frequent high-amplitude redox fluctuation may be a strong selective force on the phylogenetic and physiological composition of soil bacterial communities and may promote metabolic plasticity or redox tolerance mechanisms. To determine effects of fluctuating oxygen regimens, we incubated tropical soils under four treatments: aerobic, anaerobic, 12-h oxic/anoxic fluctuation, and 4-day oxic/anoxic fluctuation. Changes in soil bacterial community structure and diversity were monitored with terminal restriction fragment length polymorphism (T-RFLP) fingerprints. These profiles were correlated with gross N cycling rates, and a Web-based phylogenetic assignment tool was used to infer putative community composition from multiple fragment patterns. T-RFLP ordinations indicated that bacterial communities from 4-day oxic/anoxic incubations were most similar to field communities, whereas those incubated under consistently aerobic or anaerobic regimens developed distinctly different molecular profiles. Terminal fragments found in field soils persisted either in 4-day fluctuation/aerobic conditions or in anaerobic/12-h treatments but rarely in both. Only 3 of 179 total fragments were ubiquitous in all soils. Soil bacterial communities inferred from in silico phylogenetic assignment appeared to be dominated by Actinobacteria (especially Micrococcus and Streptomycetes), “Bacilli,” “Clostridia,” and Burkholderia and lost significant diversity under consistently or frequently anoxic incubations. Community patterns correlated well with redox-sensitive processes such as nitrification, dissimilatory nitrate reduction to ammonium (DNRA), and denitrification but did not predict patterns of more general functions such as N mineralization and consumption. The results suggest that this soil's indigenous bacteria are highly adapted to fluctuating redox regimens and generally possess physiological tolerance mechanisms which allow them to withstand unfavorable redox periods.     

Roles of TLR/MyD88/MAPK/NF-κB Signaling Pathways in the Regulation of Phagocytosis and Proinflammatory Cytokine Expression in Response to E. faecalis Infection

Enterococcus faecalis is a commensal bacterium residing in the gastrointestinal tract of mammals, but in certain situations it is also an opportunistic pathogen which can cause serious disease. Macrophages have been shown to play a critical role in controlling infections by commensal enterococci and also have an important role in mediating chromosomal instability and promoting colon cancer during high-level enterococcal colonization in genetically susceptible mice. However, the molecular mechanisms involved in the interaction of macrophages with enterococci during infection are not fully understood. In this study, using BMDM and RAW264.7 macrophages we show that enterococcal infection activates ERK, JNK and p38 MAPK as well as NF-κB, and drives polarization of macrophages towards the M1 phenotype. Inhibition of NF-κB activation significantly reduced the expression of TNF-α and IL-1β, as did the inhibition of ERK, JNK and p38 MAPK, although to differing extent. Enterococci-induced activation of these pathways and subsequent cytokine expression was contact dependent, modest compared to activation by E. coli and, required the adaptor protein MyD88. Phagocytosis of enterococci by macrophages was enhanced by preopsonization with E. faecalis antiserum and involved the ERK and JNK signaling pathways, with the adaptor protein MyD88 as an important mediator. This study of the interaction of macrophages with enterococci could provide a foundation for studying the pathogenesis of infection by this opportunistic pathogen and to developing new therapeutic approaches to combat enterococcal infection.     

Succession of bacterial community structure and diversity in a paddy soil oxygen gradient

Cultivation-independent techniques were applied to assess the succession and phylogenetic composition of bacterial communities in a vertical oxygen gradient in flooded, unplanted paddy soil microcosms. Microsensor measurements showed that within 6 h of flooding, oxygen was depleted from 200 microM at the floodwater-soil interface to undetectable amounts at a depth of approximately 2 mm and below. The gradient was quite stable over time, although the oxygen depletion was less pronounced 84 days than 6 h after flooding. Community fingerprint patterns were obtained by terminal restriction fragment length polymorphism (T-RFLP) analysis from the oxic, transition, and anoxic zones of triplicate soil microcosms at 0, 1 and 6 h, and 1, 2, 7, 21, 30, 42, 84, and 168 days after flooding. Correspondence analyses revealed that T-RFLP patterns obtained using either community DNA or RNA were affected by time and oxygen zone, and that there was a significant interaction between the effects of time and oxygen zone. The temporal dynamics of bacterial populations were resolved more clearly using RNA than using DNA. At the RNA level, successional community dynamics were most pronounced from 1 h to 2 days and less pronounced from 2 to 21 days after flooding, for both oxic and anoxic zones. No effect of time or oxygen zone on the community dynamics was observed from 21 to 168 days after flooding. Dominant early successional populations were identified by cloning and comparative sequence analysis of environmental 16S rRNA and 16S rRNA genes as members of the Betaproteobacteria (oxic zone) and the clostridial cluster I (anoxic zone). Dominant late successional populations belonged to the Verrucomicrobia and Nitrospira (detected mainly in the oxic zone), and to the Myxococcales (detected mainly in the anoxic zone). In conclusion, the bacterial community developed through successional stages, leading at the RNA level to almost stable community patterns within 21 days after flooding. This principal finding, in combination with the phylogenetic identity of early- and late-appearing populations, suggests that the community dynamics can be explained by the principles of r- and K-selection.     

Characterization of Monolaurin Resistance in Enterococcus faecalis

There is increasing concern regarding the presence of vancomycin-resistant enterococci in domestically farmed animals, which may act as reservoirs and vehicles of transmission for drug-resistant enterococci to humans, resulting in serious infections. In order to assess the potential for the use of monolaurin as a food preservative, it is important to understand both its target and potential mechanisms of resistance. A Tn917 mutant library of Enterococcus faecalis AR01/DGVS was screened for resistance (MIC, >100 μg/ml) to monolaurin. Three mutants were identified as resistant to monolaurin and were designated DGRM2, DGRM5, and DGRM12. The gene interrupted in all three mutants was identified as traB, which encodes an E. faecalis pheromone shutdown protein and whose complementation in trans restored monolaurin sensitivity in all three mutants. DGRM2 was selected for further characterization. E. faecalis DGRM2 showed increased resistance to gentamicin and chloramphenicol (inhibitors of protein synthesis), while no difference in the MIC was observed with the cell wall-active antibiotics penicillin and vancomycin. E. faecalis AR01/DGVS and DGRM2 were shown to have similar rates (30% cell lysis after 4 h) of cell autolytic activity when activated by monolaurin. Differences in cell surface hydrophobicity were observed between the wild type and the mutant, with the cell surface of the parent strain being significantly more hydrophobic. Analysis of the cell wall structure of DGRM2 by transmission electron microscopy revealed an increase in the apparent cell wall thickness and contraction of its cytoplasm. Taken together, these results suggest that the increased resistance of DGRM2 was due to a change in cell surface hydrophobicity, consequently limiting the diffusion of monolaurin to a potential target in the cytoplasmic membrane and/or cytoplasm of E. faecalis.     

Spatiotemporal distribution of bacteriochlorophylls in the meromictic Lake Suigetsu, Japan

The spatiotemporal distribution of chlorophyll pigments (chloropigments) in the water column of a meromictic lake, Lake Suigetsu (Fukui, Japan), was investigated. Water samples were collected from the central basin of Lake Suigetsu bimonthly between May 2008 and March 2010 at appropriate depths, including the oxic surface, oxic–anoxic interface, and anoxic bottom layers. Chlorophyll a, related to cyanobacteria and eukaryotic phytoplankton, was detected throughout the water column during the years of the study, whereas bacteriochlorophyll e, related to brown-colored green sulfur bacteria, was detected in the anoxic layers below the chemocline at a maximum concentration of 825 μg L^?1. The concentration of bacteriochlorophyll e was generally maximal at or just below the chemocline of the lake. The cellular content of bacteriochlorophyll e was estimated to be low in the upper part of the chemocline and tended to increase with increasing water depth. Bacteriochlorophyll a, which was presumably related to purple sulfur bacteria, was only detected at the chemocline during summer and autumn at concentrations of 5.4–16.3 μg L^?1. Our analysis of the chloropigment distribution for the two years of the study suggested that brown-colored green sulfur bacteria are the predominant phototroph in the anoxic layers of Lake Suigetsu, and that these play a significant role in the carbon and sulfur cycling of the lake, especially from spring to summer.     

Microbial reduction of Fe(III) in the presence of oxygen under low pH conditions

In acidic, coal mining lake sediments, facultatively anaerobic Acidiphilium species are probably involved in the reduction of Fe(III). Previous results indicate that these bacteria can co-respire O2 and Fe(III). In this study, we investigated the capacity of the sediment microbiota to reduce Fe(III) in the presence of O2 at pH 3. In sediment microcosms with 4% O2 in the headspace, the concentration of Fe(II) increased at a rate of 1.03 micromol (g wet sediment)-1 day-1 within the first 7 days of incubation which was similar to the rate obtained with controls incubated under anoxic conditions. However, in microcosms incubated under air, Fe(II) was consumed after a lag phase of 8 h with a rate of 2.66 micromol (g wet sediment)-1 day-1. Acidiphilium cryptum JF-5, isolated from this sediment, reduced soluble Fe(III) with either 4 or 21% O2 in the headspace, and concomitantly consumed O2. However, the rate of Fe(II) formation normalized for cell density decreased under oxic conditions. Schwertmannite, the predominant Fe(III)-mineral of this sediment, was also reduced by A. cryptum JF-5 under oxic conditions. The rate of Fe(II) formation by A. cryptum JF-5 decreased after transfer from preincubation under air in medium lacking Fe(III). Acidiphilium cryptum JF-5 did not form Fe(II) when preincubated under air and transferred to anoxic medium containing Fe(III) and chloramphenicol, an inhibitor of protein synthesis. These results indicate that: (i) the reduction of Fe(III) can occur at low O2 concentrations in acidic sediments; (ii) Fe(II) can be oxidized at O2 concentrations near saturation; and (iii) the enzyme(s) responsible for the reduction of Fe(III) in A. cryptum JF-5 are not constitutive.     

Abundant Trimethylornithine Lipids and Specific Gene Sequences Are Indicative of Planctomycete Importance at the Oxic/Anoxic Interface in Sphagnum-Dominated Northern Wetlands

Northern wetlands make up a substantial terrestrial carbon sink and are often dominated by decay-resistant Sphagnum mosses. Recent studies have shown that planctomycetes appear to be involved in degradation of Sphagnum-derived debris. Novel trimethylornithine (TMO) lipids have recently been characterized as abundant lipids in various Sphagnum wetland planctomycete isolates, but their occurrence in the environment has not yet been confirmed. We applied a combined intact polar lipid (IPL) and molecular analysis of peat cores collected from two northern wetlands (Saxnäs Mosse [Sweden] and Obukhovskoye [Russia]) in order to investigate the preferred niche and abundance of TMO-producing planctomycetes. TMOs were present throughout the profiles of Sphagnum bogs, but their concentration peaked at the oxic/anoxic interface, which coincided with a maximum abundance of planctomycete-specific 16S rRNA gene sequences. The sequences detected at the oxic/anoxic interface were affiliated with the Isosphaera group, while sequences present in the anoxic peat layers were related to an uncultured planctomycete group. Pyrosequencing-based analysis identified Planctomycetes as the major bacterial group at the oxic/anoxic interface at the Obukhovskoye peat (54% of total 16S rRNA gene sequence reads), followed by Acidobacteria (19% reads), while in the Saxnäs Mosse peat, Acidobacteria were dominant (46%), and Planctomycetes contributed to 6% of the total reads. The detection of abundant TMO lipids in planctomycetes isolated from peat bogs and the lack of TMO production by cultures of acidobacteria suggest that planctomycetes are the producers of TMOs in peat bogs. The higher accumulation of TMOs at the oxic/anoxic interface and the change in the planctomycete community with depth suggest that these IPLs could be synthesized as a response to changing redox conditions at the oxic/anoxic interface.     

Occurrence of Enterococci on Plants in a Wild Environment

Enterococci were obtained from 14% of nearly 2,200 flowers, 3.4% of non-floral structures of angiosperms, and from 8.3% of samples of soil, water, and lesser plants of the Great Smoky Mountains National Park, an area little influenced by the presence of man. The enterococci were recovered from one or more flowers or flower clusters of 1,515 samples in 47 taxa, but not from flowers of 67 taxa with 654 samples. The per cent of recovery was influenced adversely by dense forest cover and by increase in elevation, as compared with recovery from flowers in sunny locations in the lower elevations. The per cent recovery increased directly with rising seasonal temperature, with the maximal per cent of recovery occurring in September. In no instance did all samples of a species of flower or plant yield enterococci on culture, and with only three genera, Cacalia, Delphinium, and Mitchella, were the bacteria obtained from more than 50% of the samples. Approximately 11% of the cultures isolated were identified as Streptococcus faecalis, 64% as the soft curd producing, caseolytic variant of S. faecalis, 4% as S. faecalis var. zymogenes, and 20% as S. faecium. The per cent distribution of these species on plants was reasonably similar to the distribution within wild animals in the same environment. It was concluded that the enterococci occurring on plants arise commonly from the wild animals, and that they do not represent plant-specific species or variants of the enterococci.     

Sulfate reduction and thiosulfate transformations in a cyanobacterial mat during a diel oxygen cycle

Abstract Bacterial sulfate reduction and transformations of thiosulfate were studied with radiotracers in a Microcoleus chthonoplastes -dominated microbial mat growing in a hypersaline pond at the Red Sea. The study showed how a diel cycle of oxygen evolution affected respiration by sulfate-reducing bacteria and the metabolism of thiosulfate through oxidative and reductive pathways. Sulfate reduction occurred in both oxic and anoxic layers of the mat and varied diurnally, apparently according to temperature rather than to oxygen. Time course experiments showed that the radiotracer method underestimated sulfate reduction in the oxic zone due to rapid reoxidation of the produced sulfide. Extremely high reduction rates of up to 10 μmol cm⁻³ d⁻¹ were measured just below the euphotic zone. Although thiosulfate was simultaneously oxidized, reduced and disproportionated by bacteria in all layers of the mat, there was a shift from predominant oxidation in the oxic zone to predominant reduction below. Concurrent disproportionation of thiosulfate to sulfate and sulfide occurred in all zones and was an important pathway of the sulfur cycle in the mat.     

Adaptation of sympatric Achromatium spp. to different redox conditions as a mechanism for coexistence of functionally similar sulphur bacteria

Changes in the abundance of sympatric Achromatium spp. in response to the artificial manipulation of redox conditions in sediment microcosms was determined by fluorescence in situ hybridization (FISH). Adaptation to different redox conditions was shown to be one mechanism that supported the coexistence of functionally similar Achromatium spp. In sediment microcosms, in which the overlying water was oxygenated, Achromatium community size and composition remained unchanged over time. However, imposition of anoxic conditions induced changes in community structure. Anoxia caused a reduction in the relative abundance of Achromatium sp. RY8 (72 +/- 4% to 49 +/- 2%) and an increase in Achromatium sp. RY5 (19 +/- 5% to 32 +/- 3%) and a newly identified Achromatium sp., RYKS (14 +/- 4% to 27 +/- 2%). In anoxic microcosms supplemented with a single addition of nitrate at different initial concentrations the relative decline in Achromatium sp. RY8 was dependent on the initial nitrate concentration. In these experiments nitrate was rapidly removed. In contrast, when high levels of nitrate were maintained by periodic replacement of the overlying water with nitrate supplemented anoxic water, the composition of the Achromatium community remained stable over time. This suggested that all of the coexisting Achromatium spp. are obligate or facultative anaerobes, but, Achromatium sp. RY8 was more sensitive to sediment redox conditions than the other Achromatium species. Given the heterogeneous nature of sedimentary environments, redox-related niche differentiation may promote coexistence of sympatric Achromatium spp.     

Abundant and diverse arsenic-metabolizing microorganisms in peatlands treating arsenic-contaminated mining wastewaters

Mining operations produce large quantities of wastewater. At a mine site in Northern Finland, two natural peatlands are used for the treatment of mining-influenced waters with high concentrations of sulphate and potentially toxic arsenic (As). In the present study, As removal and the involved microbial processes in those treatment peatlands (TPs) were assessed. Arsenic-metabolizing microorganisms were abundant in peat soil from both TPs (up to 10⁸ cells g_dw⁻¹), with arsenate respirers being about 100 times more abundant than arsenite oxidizers. In uninhibited microcosm incubations, supplemented arsenite was oxidized under oxic conditions and supplemented arsenate was reduced under anoxic conditions, while little to no oxidation/reduction was observed in NaN₃-inhibited microcosms, indicating high As-turnover potential of peat microbes. Formation of thioarsenates was observed in anoxic microcosms. Sequencing of the functional genemarkers aioA (arsenite oxidizers), arrA (arsenate respirers) and arsC (detoxifying arsenate reducers) demonstrated high diversity of the As-metabolizing microbial community. The microbial community composition differed between the two TPs, which may have affected As removal efficiencies. In the present situation, arsenate reduction is likely the dominant net process and contributes substantially to As removal. Changes in TP usage (e.g. mine closure) with lowered water tables and heightened oxygen availability in peat might lead to re-oxidation and re-mobilization of bound arsenite.