Heterotrophic Populations Research Articles

Quantitative steps in symbiogenesis and the evolution of homeostasis.

The merging of two independent populations of heterotrophs and autotrophs into a single population of mixotrophs has occurred frequently in evolutionary history. It is an example of a wide class of related phenomena, known as symbiogenesis. The physiological basis is almost always (reciprocal) syntrophy, where each species uses the products of the other species. Symbiogenesis can repeat itself after specialization on particular assimilatory substrates. We discuss quantitative aspects and delineate eight steps from two free-living interacting populations to a single fully integrated endosymbiotic one. The whole process of gradual interlocking of the two populations could be mimicked by incremental changes of particular parameter values. The role of products gradually changes from an ecological to a physiological one. We found conditions where the free-living, epibiotic and endobiotic populations of symbionts can co-exist, as well as conditions where the endobiotic symbionts outcompete other symbionts. Our population dynamical analyses give new insights into the evolution of cellular homeostasis. We show how structural biomass with a constant chemical composition can evolve in a chemically varying environment if the parameters for the formation of products satisfy simple constraints. No additional regulation mechanisms are required for homeostasis within the context of the dynamic energy budget (DEB) theory for the uptake and use of substrates by organisms. The DEB model appears to be dosed under endosymbiosis. This means that when each free-living partner follows DEB rules for substrate uptake and use, and they become engaged in an endosymbiotic relationship, a gradual transition to a single fully integrated system is possible that again follows DEB rules for substrate uptake and use.

Fate of coliform bacteria in composted beef cattle feedlot manure.

The link between livestock production, manure management, and human health has received much public attention in recent years. Composting is often promoted as a means of sanitizing manure to ensure that pathogenic bacteria are not spread to a wider environment during land application. In a two-year study (1998 and 1999) in southern Alberta, we examined the fate of coliform bacteria during windrow composting of cattle (Bos taurus) manure from feedlot pens bedded with cereal straw or wood chips. Numbers of total coliforms (TC) and Escherichia coli declined as the composting period progressed. In 1998, TC levels (mean of both bedding types) were log10 7.86 cells g(-1) dry wt. for raw manure on Day 0, log10 3.38 cells g(-1) by Day 7, and log10 1.69 cells g(-1) by Day 14. More than 99.9% of TC and E. coli was eliminated in the first 7 d when average windrow temperatures ranged from 33.5 to 41.5 degrees C. The type of bedding did not influence the numbers of TC or E. coli. Dessication probably played a minor role in coliform elimination, since water loss was low (< 0.07 kg kg(-1)) in the first 7 d of composting. However, total aerobic heterotroph populations remained high (> 7.0 log10 CFU g(-1) dry wt., where CFU is colony forming units) throughout the composting period, possibly causing an antagonistic effect. Land application of compost, with its nondetectable levels of E. coli compared with raw manure, should minimize environmental risk in areas of intensive livestock production.

Changes in community properties during microbial succession

Succession is one of the oldest concepts in community ecology and various theoretical and empirical studies have addressed succession and it's underlying mechanisms, particularly in plant communities (Odum 1969, Connell and Slatyer 1977, Finegan 1984). The development of algal periphyton on submerged rocks in streams has also been viewed as an example of succession in autotrophic communities (Stevenson 1983, Johnson et al. 1997). More recently, the formation of bacterial communities on submerged surfaces (biofilms) has been viewed in the context of primary ecological succession (Jackson et al. 2001), and studies have used molecular techniques to examine the changes in bacterial community structure that occur during biofilm development (Santegoeds et al. 1998, Jackson et al. 2001). Briefly, when a fresh surface (glass slides, rocks etc.) is submerged in an aquatic habitat it is rapidly colonized by bacterial populations, some of which will grow and persist as part of the attached biofilm community. These initial heterotrophic populations are dependent upon the availability of dissolved organic carbon (DOC) from the overlying water, i.e. it is an open system with little internal cycling of nutrients or carbon. Over time, new populations of bacteria arrive through immigration from the water column, whereas other populations are lost from the community, either through cell death and losses due to sloughing, or because conditions in the biofilm change (largely because of microbial activity and growth) and those bacteria are no longer adapted to biofilm life. The latter case recalls the facilitation model of Connell and Slatyer (1977) in that early colonizing bacteria change the environmental conditions in the biofilm, facilitating the later dominance of different populations (indeed, those authors suggested that the facilitation mechanism may be particularly applicable when organisms are colonizing a new substrate, i.e. during primary succession, which is likely to be the case during biofilm formation). In the later stages of biofilm development, populations of heterotrophic bacteria are more dependent upon autotrophic production occurring within the biofilm (such as from algae or cyanobacteria), i.e. the community becomes more of a closed system. Based on the results of a recent study of epilithic biofilm development, Jackson et al. (2001) suggested certain conceptual changes that might occur in community properties during bacterial biofilm succession (Fig. 1). Initial colonization of a fresh surface leads to a rapid increase in species richness (S), which subsequently declines as some of the colonizing populations are less competitive and do not survive in the biofilm community. However, competition becomes less important in structuring the bacterial assemblage as the biofilm ages and new resources and habitats become available (such as through the accumulation of metabolic waste products that can serve as growth substrates for other organisms, the emergence of anoxic pockets permitting the growth of anaerobic bacteria, and the development of a three dimensional architecture allowing greater area for attachment and growth). Thus, S might increase again as the biofilm community matures, and community structure is driven by resource diversity rather than by competition. This model can however be extended, and a number of changes made to more accurately explain community changes. Firstly, the term resource diversity is vague. Originally the increase in resource diversity throughout community development was intended to convey both the appearance of new substrates for microbial growth, and a switch from a two dimensional community to a three dimensional one (so an increase in both habitat space and variety). An alternative term would be the number of ecological niches, and what was originally described as an increase in resource diversity can be more succinctly thought of as an increase in the amount of niche space (R, following MacArthurs' (1972) idea of a resource spectrum, and assuming that resource spectrum length is equivalent to niche space). Thus, as the biofilm develops, the increased heterogeneity of the community facilitates a continuous increase in the potential number

Odours from pulp mill effluent treatment ponds: the origin of significant levels of geosmin and 2-methylisoborneol (MIB)

Pulp and paper mills are well known for their sharp, sulphurous stack emissions, but the secondary treatment units also can be significant contributors to local odour. This study investigated the source(s) of earthy/musty emissions from a mixed hardwood pulp mill in response to a high local odour. Samples from five sites in the mill over five months were analyzed for earthy/musty volatile organic compounds (VOCs), examined microscopically, and plated for bacteria and moulds. In all cases, activated sludge showed substantial geosmin levels and to a lesser extent 2-methylisoborneol (MIB) at 2000–9000 times their odour threshold concentrations (OTCs). These VOCs were lower or absent upstream and downstream, suggesting that they were produced within the bioreactor. Geosmin and MIB were highest in late summer and declined over winter, and correlated with different operating parameters. Geosmin was most closely coupled with temperature and MIB with nitrogen uptake. Cyanobacteria were present in all sludge samples, but actinomycetes were not found. Gram-negative bacteria and one fungal species isolated from the bioreactor and secondary outfall tested negative for geosmin or MIB. We conclude: (i) geosmin and MIB contribute significantly to airborne odours from this mill, but are diluted below OTC levels at the river; (ii) these VOCs are generated by biota in the activated sludge; and (iii) cyanobacteria are likely primary source(s). The growth of cyanobacteria in activated sludge represents a loss of energy to the heterotrophic population; thus earthy/musty odours may represent a diagnostic for less than optimal conditions.

Spatial and temporal variation of drinking water quality in ten small Quebec utilities

A comparative study relating to distributed water quality was undertaken in 10 small municipal drinking water utilities in Quebec. All of these utilities apply direct chlorination to surface water or groundwater under the direct influence of surface water without any previous treatment. These utilities were divided into two groups: four utilities that had never or rarely served water violating the provincial drinking water microbiological standards (relating to fecal and (or) total coliform bacteria) and six utilities that very often infringed upon said standards. The objective of this study was to identify key parameters responsible for the differences between the two groups of utilities, to explore the capacity of studied utilities to simultaneously and effectively handle the acute disease risk associated with microorganisms and the chronic health hazard linked to chlorination by-products and to identify the parameters upon which it may be possible to act to achieve better water quality in each of the two utility groups. The study includes comparisons of characteristics of water quality at the source, chlorination conditions in the plant, and water quality from the entrance to the extremity of the distribution system. Results show that the differences between the two groups of utilities are associated essentially with maintained chlorine residuals and heterotrophic plate count bacteria populations in corresponding distribution systems and, to a lesser extent, to the applied chlorine doses. Subsequent multivariate analyses allowed identification of variables (i.e., factors) upon which utility managers may act to improve the quality of distributed water in each group of utilities. For the group of utilities that had very little or no violation, these factors are related to disinfection levels, whereas for the group that often violated quality standards, raw water natural organic matter content reduction through source water protection and raised chlorine doses and residuals appear to be the factors that may lead to better microbiological quality of distributed water. Key words: drinking water, water quality, distribution systems, small utilities, Quebec.

Weathering of a subarctic oil spill over 25 years: the Caribou-Poker Creeks Research Watershed experiment

A small experimental oil spill conducted in an open black spruce forest within the Caribou-Poker Creeks Research Watershed (CPCRW), 48 km north of Fairbanks, AK, in the winter of 1976 was designed to examine the effects of crude oil spills in permafrost terrain. No clean-up was attempted, and the site now provides an opportunity to follow the natural weathering of spilled oil under these conditions. In summer 2001, more than 25 years after the spill, we sampled soils from the spill plot and a nearby reference plot to determine how the oil had weathered, and to assess microbial populations and activity. All samples collected from the oiled plot contained substantial amounts of methylene chloride extractable oil, between 4% and 66% by weight. Using 17α(H)21β(H)hopane as a conserved internal marker within the oil, we determined that while some heavily oiled samples were almost unchanged since the spill, others had lost more than 80% of their initial hydrocarbon. Evaporation, biodegradation and photooxidation all seem to have played important roles in this process, but to varying degrees in different samples. Assays of culturable populations of total heterotrophs and crude oil emulsifiers, and mineralization potentials for hexadecane and phenanthrene, indicate that the microbial population in the oiled soils has remained acclimated to degrade hydrocarbons. We conclude that natural weathering processes will eventually lead to the removal of much of the hydrocarbon from these heavily oiled subarctic soils; however, the combination of low rates of nutrient turnover, a short thaw season, and high hydrocarbon concentrations will result in the persistence of oil residue for many more decades. Finding an environmentally appropriate cleanup technology for sites like this remains an important challenge for future research.

Population dynamics in an aerobic submerged fixed bed reactor (ASFBR) process

In this study, an aerobic submerged fixed bed reactor's (ASFBR) population dynamics has been studied in order to know its behavior in different conditions of organic load and oxygen concentration. The reactor was fed with synthetic wastewater. Tested variables and applied values were: 1) Variations in organic load (OL): 16-65 g COD/m2/d. 2) Variations in influent's COD concentration: 40-400 g COD/m3. 3) Variations in specific air flow (SAF): 15-127 m3air/kgCOD. Biofilm samples were taken at the top of the reactor. This study showed important variations in the composition and abundance of the microfauna depending on the experimental conditions. Variations in influent concentration had no significant effect on the abundance of the studied groups. However, differences depending on organic load and aeration conditions were observed. Organic load influenced every group studied but with different results. Sessile cilliates, metazoa and flagellates were abundant in low load, while crawling ones were in high load. Aeration intensity influenced most of the groups except Peranema and Vorticella spp. Despite obtaining good yields, not many protozoa, typical of biofilms under conventional processes, were found. Thus, a great variety of microorganisms, such as many classes of sessile and crawling cilliates, were not found. Important nitrifying activity was obtained at 20 cm depth in a bed. From this point, the heterotrophic and nitrifying populations exist but are inactive.

Who eats what? Classifying microbial populations based on diurnal profiles of rRNA levels

Identifying the relationships between various bacterial populations and the substrates they consume is central to the understanding of population dynamics and to the development of process control in activated sludge. However, linking a heterotrophic population to its activity in situ is difficult because ribosomal RNA (rRNA) techniques, while allowing the rapid identification of populations, provide little information about their heterotrophic activity. Activated sludge models describe biodegradation kinetics by classifying substrates into two types: readily and slowly degradable substrates. Assuming that bacterial populations specialize in degrading one type of substrate, their growth rate should be affected differently if the COD loading rate varies diurnally as for a municipal activated sludge system. Modeling results suggested that the growth rates of populations consuming readily degradable substrates vary according to variations in COD loading rate. On the other hand, the growth rates of populations consuming slowly degradable substrates do not change despite the variation in COD loading rate. Since the cellular rRNA level is positively correlated with the growth rate, we hypothesized that the rRNA levels of some populations in municipal activated sludge should increase throughout the day, while they should stay constant for other populations. This hypothesis was verified by monitoring the rRNA level of Acinetobacter (a model population consuming readily degradable substrates) and Gordonia (a model population consuming slowly degradable substrates) in the mixed liquor of a full-scale municipal activated sludge reactor for three weeks.

Sequenced aeration in a membrane bioreactor: Specific nitrogen removal rates

AbstractThe aim of this work was to quantify the activity of autotrophic and heterotrophic populations present in a submerged membrane bioreactor submitted to a sequenced aeration, ensuring alternate nitrification and denitrification (BIOSEP® process). Specific nitrification and denitrification rates were determined over a fifteen‐month running period, with and without physico‐chemical phosphorus removal. The results show (i) zero‐order kinetics; (ii) a specific activity independent of MLVSS concentration; and (iii) an evolution of the nitrification kinetics directly linked to an evolution of the nitrifying population within the unit. The study confirmed that denitrification kinetics depend on the concentration and on the nature of wastewater organic matter.

COMPARISON OF MEDIA AND A LIQUID DETERGENT EXTRACTION STEP ON BACTERIAL RECOVERY FROM ANIMAL FEEDS

The objective of this study was to evaluate the recovery of Tryptic soy agar (TSA) or low nutrient R2 bacterial populations from fresh feeds and animal feeds held in long-term cold storage after extraction with a detergent-based solution combined with repeated centrifugation. Repeated centrifugation after exposure to a detergent-based solution resulted in recoveries generally no more than 10% of the microbial population in the original feed. Feed heterotrophic populations were significantly correlated (P<0.01) with total aerobic populations enumerated from the feed and feed extracts, as well as heterotrophic populations from feed extracts. Similarly, TSA bacterial populations from feed were significantly correlated with TSA populations from extracts.

Respirometric needs of heterotrophic populations developed in an immersed membrane bioreactor working in sequenced aeration

The aim of this work is first to determine the respirometric needs of heterotrophic populations developed in a submerged membrane bioreactor (MBR) working in sequenced aeration (BIOSEP ® process). The oxygen needs were quantified in known conditions of substrate nature and concentration. The respirometric parameters were measured in a particular system, called Respir’Eaux. Samples of the biological suspension were taken from the BIOSEP ® process and introduced into the Respir’Eaux in which the oxygen supply was controlled. First experiments, carried out without any substrate addition, showed that: (i) endogenous needs remained approximately constant during the 5 months of study, showing an apparent stability of the culture; (ii) exogenous needs were very variable, supposedly because the culture was subjected to very variable biodegradable substrate material flows. With the aim of better underlining the importance of the nature and the concentration of the available substrate, experiments were performed from biomass samples subjected to the addition of specific nutrients (acetate and calibrated fractions of municipal wastewater). The results showed that: (i) oxygen needs depend directly on the nature and the quantity of added substrate; (ii) oxygen needs depend on the acclimation of the micro-organisms to the added substrate; (iii) in the experimental conditions investigated, oxygen needs demonstrate the complete oxidation of the soluble organic fraction of the wastewater by the MBR suspension, although this part may contain recalcitrant compounds for conventional biological culture.

Effect of inorganic N on the population, in vitro colonization and morphology of Acetobacter diazotrophicus (syn. Gluconacetobacter diazotrophicus )

We investigated whether Acetobacter diazotrophicus (syn.Gluconacetobacter diazotrophicus) could be recovered only from sugarcane plants either with low or no application of fertiliser N. We report here the enrichment and enumeration of A. diazotrophicus from high N-fertilised samples where high heterotrophic populations reduce the numbers of A. diazotrophicus ultimately diminshing its isolation frequency as reported earlier. The growth medium of micropropagated sugarcane seedlings of the varieties Co 8021, Co 86249, Co 86010, Co 86032, and Co 87025 was amended with potassium nitrate, ammonium nitrate, ammonium chloride and urea. The colonisation and AR activity of A. diazotrophicus were affected in the presence of high levels (25 mM) of ammonium chloride and ammonium nitrate but remained unaffected in low levels of N (i.e 1/10th of MS liquid medium) and with high levels of potassium nitrate (25 mM) and urea (500 ppm). A. diazotrophicus was detected in the inoculated plants both at low and high levels of N based on the amplification of a specific 16S rRNA gene fragment using PCR based method targeting a stretch of 445 bp with primers AC and DI. High levels of N in the growth medium induced morphological changes on A. diazotrophicus cells resulting in long pleomorphic cells. The percentage of pleomorphic cells was in the decending order from NH4NO3, NH4Cl, KNO3, and urea. These changes were more prominent in ammonium chloride and ammonium nitrate than potassium nitrate, urea and N free medium. The morphological changes and the increased heterotrophic populations may play a role on the survival ofA. diazotrophicus in high N-fertilised samples/environments.

Nitrifying and heterotrophic population dynamics in biofilm reactors: effects of hydraulic retention time and the presence of organic carbon

Two biofilm reactors operated with hydraulic retention times of 0.8 and 5.0 h were used to study the links between population dynamics and reactor operation performance during a shift in process operation from pure nitrification to combined nitrification and organic carbon removal. The ammonium and the organic carbon loads were identical for both reactors. The composition and dynamics of the microbial consortia were quantified by fluorescence in situ hybridization (FISH) with rRNA-targeted oligonucleotide probes combined with confocal laser scanning microscopy, and digital image analysis. In contrast to past research, after addition of acetate as organic carbon nitrification performance decreased more drastically in the reactor with longer hydraulic retention time. FISH analysis showed that this effect was caused by the unexpected formation of a heterotrophic microorganism layer on top of the nitrifying biofilm that limited nitrifiers oxygen supply. Our results demonstrate that extension of the hydraulic retention time might be insufficient to improve combined nitrification and organic carbon removal in biofilm reactors.

Dynamics of yard trimmings composting as determined by dehydrogenase activity, ATP content, arginine ammonification, and nitrification potential

Microbial activities, numbers, and biomass are key parameters that can be used to elucidate the dynamics of the composting process. In the present study, four different biochemical parameters (dehydrogenase activity, ATP content, arginine ammonification, and nitrification potential) were measured (1) to monitor the dynamics of yard trimmings composting; and (2) to relate these parameters to changes in microbial numbers, physico-chemical properties, and maturity (stability) of yard trimmings compost. The initial yard trimmings was a mixture of leaves, grass clippings, and shredded bark (1:1:1 v/v). Three windrows, each having a dimension of 1.5 m (height)×4 m (length)×0.6 m in (width) were established. The temperature profile showed a rapid self-heating of the compost mass from ambient temperature of 20 to 70 °C in the first 24 h of composting. This thermophilic temperature was sustained until day 14 and then dropped to ambient level towards the end of composting (day 63). The maturation of yard trimmings compost was accompanied by a decline in pile temperature to ambient level, increases in bulk density, (NO 3 −+NO 2 −)–N, population sizes of actinomycetes and fungi, drop in C:N ratio to 20:1, and decreases and stabilisation of biochemical properties (dehydrogenase activity, ATP content, and nitrification potential) to low levels. Dehydrogenase activity, ATP content, and nitrification potential depended on the changes in C and N content during composting. These parameters were correlated with the populations of either total aerobic heterotrophs or actinomycetes, indicating the usefulness of these parameters as indicators of microbial activity and dynamics of the composting process. Arginine ammonification was dependent on the change of neither C nor N. It also did not correlate with microbial numbers, suggesting that this parameter may not be suitable for evaluating microbial activity and dynamics of yard trimmings composting.

Enhanced phenanthrene biodegradation in soil by slender oat root exudates and root debris.

To investigate the mechanisms by which slender oat (Avena barbata Pott ex Link) enhances phenanthrene biodegradation, we analyzed the impacts of root exudates and root debris on phenanthrene biodegradation and degrader community dynamics. Accelerated phenanthrene biodegradation rates occurred in soils amended with slender oat root exudates as well as combined root debris + root exudate as compared with unamended controls. Root exudates significantly enhanced phenanthrene biodegradation in rhizosphere soils, either by increasing contaminant bioavailability and/or increasing microbial population size and activity. A modified most probable number (MPN) method was used to determine quantitative shifts in heterotrophic and phenanthrene degrader communities. During the first 4 to 6 d of treatment, heterotrophic populations increased in all amended soils. Both root debris-amended and exudate-amended soil then maintained larger phenanthrene degrader populations than in control soils later in the experiment after much of the phenanthrene had been utilized. Thus, root amendments had a greater impact over time on phenanthrene degraders than heterotrophs resulting in selective maintenance of degrader populations in amended soils compared with controls.

Soil microbial community responses to fly ash amendment as revealed by analyses of whole soils and bacterial isolates

Due to its silty texture and plant nutrient content, coal fly ash may prove a valuable amendment to coarse-textured soils. Its effects on soil chemical and physical properties in the field have been studied, but little is known regarding effects on soil microbial communities. In this study, field plots were amended with fly ash at rates of 0 or 505 Mg fly ash ha −1 and subsequently cropped to a fallow–corn–wheat rotation or continuous fescue. Twenty months later, microbial responses to the fly ash were assessed by analyzing the fatty acid composition and carbon substrate utilization potential of microbial communities and aerobic heterotrophic bacteria isolated from the field plots cropped to wheat and fescue. Differences in whole-soil fatty acid profiles from amended and non-amended soils were found. Soils amended with fly ash were enriched in fatty acid 16:1 ω5 c, and elevated quantities of 17:0 cy and 16:1 ω7 c were present in fly ash-amended soils cropped to fescue and wheat, respectively. Fatty acid profiles also were affected by cropping system. Extracts from wheat-cropped soils were enriched in 17:1 ω7 c, while those from fescue plots had greater amounts of 18:2 ω6 c and 18:1 ω9 c. Carbon substrate utilization patterns of microbial communities were affected by cropping system but not by fly ash amendment; communities from soils cropped to wheat utilized more carbon substrates than did communities from fescue-cropped soils at the soil dilution tested. Studies of bacterial isolates revealed that Arthrobacter species dominated the culturable, aerobic heterotrophic population, accounting for 25–42% of the total number of isolates recovered from the field plots. Percentages of unidentified isolates also were significant and ranged from 27 to 45% of isolate totals. Effects of fly ash on soil isolates were detected within species of Arthrobacter, with reduced numbers of A. protophormiae in soils amended with fly ash relative to non-amended soils. Overall, the structure of the culturable, aerobic heterotrophic population did not reflect that of the soil community, as fatty acids reported to be markers for Gram-positive organisms were not the major community fatty acids. Enhanced crop growth and soil texture, pH, and nutrient content as a result of fly ash amendment may explain why no detrimental effects to the microbial community were found. Instead, whole-soil fatty acid data indicates that fly ash amendment may benefit fungi and Gram-negative bacteria relative to other components of the soil microbial community.

Effects of mercury contamination on the culturable heterotrophic, functional and genetic diversity of the bacterial community in soil.

This study investigates the effect of mercury contamination on the culturable heterotrophic, functional and genetic diversity of the bacterial community in soil. The changes in diversity were monitored in soil microcosms, enriched with 25 &mgr;g Hg(II) g(-1) soil, over a period of 3 months. The culturable heterotrophic diversity was investigated by colony morphology and colony appearance on solid LB medium. Functional diversity was analysed as sole carbon utilisation patterns in ECOplates. Genetic diversity was measured as bands on denaturing gradient gel electrophoresis (DGGE) gels obtained by purification of total soil DNA and amplification of bacterial 16S rDNA fragments by polymerase chain reaction. Concentrations of bioavailable and total mercury were measured throughout the experiment. The effect on the culturable heterotrophic and genetic diversity was very similar, showing an immediate decrease after mercury addition but then slowly increasing throughout the entire experimental period. Pre-exposure levels were not reached within the time span of this investigation. The DGGE band pattern indicated that a shift in the community structure was responsible for recovered diversity. When analysed by Shannon-Weaver indices, functional diversity was found to increase almost immediately after mercury addition and to remain at a level higher than the control soil for the rest of the experiment. The fraction of culturable heterotrophic bacteria increased from 1% to 10% of the total bacterial number as a result of mercury addition, and the mercury-resistant population increased to represent the entire heterotrophic population.

Effects of mercury contamination on the culturable heterotrophic, functional and genetic diversity of the bacterial community in soil

This study investigates the effect of mercury contamination on the culturable heterotrophic, functional and genetic diversity of the bacterial community in soil. The changes in diversity were monitored in soil microcosms, enriched with 25 μg Hg(II) g −1 soil, over a period of 3 months. The culturable heterotrophic diversity was investigated by colony morphology and colony appearance on solid LB medium. Functional diversity was analysed as sole carbon utilisation patterns in ECOplates. Genetic diversity was measured as bands on denaturing gradient gel electrophoresis (DGGE) gels obtained by purification of total soil DNA and amplification of bacterial 16S rDNA fragments by polymerase chain reaction. Concentrations of bioavailable and total mercury were measured throughout the experiment. The effect on the culturable heterotrophic and genetic diversity was very similar, showing an immediate decrease after mercury addition but then slowly increasing throughout the entire experimental period. Pre-exposure levels were not reached within the time span of this investigation. The DGGE band pattern indicated that a shift in the community structure was responsible for recovered diversity. When analysed by Shannon–Weaver indices, functional diversity was found to increase almost immediately after mercury addition and to remain at a level higher than the control soil for the rest of the experiment. The fraction of culturable heterotrophic bacteria increased from 1% to 10% of the total bacterial number as a result of mercury addition, and the mercury-resistant population increased to represent the entire heterotrophic population.

Effects of oil spills on microbial heterotrophs in Antarctic soils

Oil spillage on the moist coastal soils of the Ross Sea region of Antarctica can impact on populations of microbial heterotrophs in these soils, as determined by viable plate counts and a most probable number technique. Elevated numbers of culturable hydrocarbon degraders, bacteria and fungi were detected in surface and subsurface soils from oil-contaminated sites, compared with nearby control sites. Culturable yeasts were not detected in soil from coastal control sites, yet reached >105 organisms g–1 dry weight in contaminated soils. The presence of hydrocarbons in soils resulted in a shift in the genera of culturable filamentous fungi. Chrysosporium dominated control soils, yet Phialophora was more abundant in oil-contaminated soils. Hydrocarbon degraders are most likely bacteria; however, fungi could play a role in degradation of hydrocarbons or their metabolites. Depleted levels of nitrate detected in some contaminated soils and decreased pH may be the result of growth of hydrocarbon degraders. Numbers and diversity of culturable microbes from Antarctic soil varied depending on whether a pristine site or a human-impacted (in this case, by fuel spills) site is studied.

Nitrification of high strength ammonia wastewaters: comparative study of immobilisation media

Due to legislative pressures, sludge production and processing in the UK will increase substantially in the future resulting in a supernatant liquid high in ammonia (500–1000 mg 1 −1) and “hard” COD (∼500 mg 1 −1). A small footprint reactor is required to effectively nitrify this effluent, and the aim of this work was to compare a number of immobilisation media under a variety of conditions in order to determine which media held the most promise for future development. Laboratory-scale continuously stirred tank reactors containing freely suspended and immobilised biomass were operated with a high-strength synthetic ammonia wastewater (500 mg N l −1) to determine the nitrification rates at various temperatures, and ammonia and COD loadings. COD : NH 3 ratios in sludge liquors vary widely depending on the treatment processes employed, and therefore ratios of 1 : 1 and 2 : 1 were tested as being fairly typical. The freely suspended nitrifiers were washed out of the reactors at a 1 d hydraulic retention time (HRT), whereas the reactors containing adsorption particles (Linpor and Kaldnes) and PVA-encapsulated nitrifiers continued partially nitrifying down to 12 h, and oxygen addition enhanced nitrification. A decrease in temperature from 25 to 16°C only caused a small (10%) decrease in nitrification in the immobilised cell reactors, demonstrating that nitrification was mass transfer rather than kinetically controlled. A reduction in nitrification occurred when glucose (500 mg l −1) was added to the feed due to the growth of a heterotrophic population. The adsorbed biomass reactors lost 35% of nitrification compared to only 7% with PVA, and it appears that the colonisation of PVA by heterotrophs is more difficult than for Linpor and Kaldnes. Respiration rates for all particles increased with time in the reactors, and nitrifiers immobilised in PVA retained approximately 40% of their viability after immobilisation. Volumetric nitrification rates were generally higher for the PVA reactor than for Linpor and Kaldnes, and were: suspended biomass reactor: 0.36; Linpor: 0.57; Kaldnes: 0.53 and PVA: 0.70 kg N m −3-reactor d −1 for a 25% reactor fill. These equate to 2.28, 4.24 and 3.97 g N m −2-media d −1 for Linpor, Kaldnes and PVA respectively, hence other reactor fill rates for Kaldnes warrant further investigation. However, the PVA particles with the highest nitrification rates under all conditions showed promise as an immobilisation medium, and are amenable to further optimisation for the nitrification of high-strength ammonia wastewaters.