Effects of physico-chemical parameters on the bacterial and fungal communities during agricultural waste composting

The goal of this study was to identify and prioritize some of the physico-chemical parameters that contributed to bacterial and fungal community compositions during agricultural waste composting. Relationships between those parameters and microbial community compositions determined by PCR-DGGE were simultaneously evaluated by redundancy analysis (RDA). The results showed that the temporal variation of bacterial community composition was significantly related to water soluble carbon (WSC), ammonium and nitrate (P<0.05), while the most variation in distribution of fungal community composition was statistically explained by pile temperature, WSC, and moisture content (P<0.05). Significant amounts of the variation (54.9% and 56.0% for bacterial and fungal species data, respectively) were explained by those parameters, suggesting that those parameters were the most likely ones to influence, or be influenced by the bacterial and fungal species. Variation partitioning analyses indicated that WSC and pile temperature showed predominant effect on the bacterial and fungal community composition, respectively.     

Analysis of methanotrophic communities in landfill biofilters using diagnostic microarray

Biofilters operated for the microbial oxidation of landfill methane at two sites in Northern Germany were analysed for the composition of their methanotrophic community by means of diagnostic microarray targeting the pmoA gene of methanotrophs. The gas emitted from site Francop (FR) contained the typical principal components (CH4, CO2, N2) only, while the gas at the second site Müggenburger Strasse (MU) was additionally charged with non-methane volatile organic compounds (NMVOCs). Methane oxidation activity measured at 22 degrees C varied between 7 and 103 microg CH4 (g dw)(-1) h(-1) at site FR and between 0.9 and 21 microg CH4 (g dw)(-1) h(-1) at site MU, depending on the depth considered. The calculated size of the active methanotrophic population varied between 3 x 10(9) and 5 x 10(11) cells (g dw)(-1) for biofilter FR and 4 x 10(8) to 1 x 10(10) cells (g dw)(-1) for biofilter MU. The methanotrophic community in both biofilters as well as the methanotrophs present in the landfill gas at site FR was strongly dominated by type II organisms, presumably as a result of high methane loads, low copper concentration and low nitrogen availability. Within each biofilter, community composition differed markedly with depth, reflecting either the different conditions of diffusive oxygen supply or the properties of the two layers of materials used in the filters or both. The two biofilter communities differed significantly. Type I methanotrophs were detected in biofilter FR but not in biofilter MU. The type II community in biofilter FR was dominated by Methylocystis species, whereas the biofilter at site MU hosted a high abundance of Methylosinus species while showing less overall methanotroph diversity. It is speculated that the differing composition of the type II population at site MU is driven by the presence of NMVOCs in the landfill gas fed to the biofilter, selecting for organisms capable of co-oxidative degradation of these compounds.     

不同pH条件下除臭生物滤池微生物种群结构的分子生态分析

采用分子生物学方法研究生物滤池中不同pH环境下的微生物种群的基因多样性,通过扩增细菌16SrRNA基因的V3可变区,结合应用变性梯度凝胶电泳(DGGE)技术分析除臭生物滤池的生物种群的结构变化,并回收主要的DNA片段,采用PCR测序及T载体克隆测序,明确优势菌群的系统发育地位。结果表明,除臭生物滤池在不同的pH条件下,微生物的多样性及其丰度存在较大差别,强酸性对微生物具有较高的选择作用,与中性条件相比,微生物种群多样性相对较低。同时在滤池的不同层次上也表现出明显的空间分布多样性差异,序列比对显示硫氧化细菌在除臭过程中占有优势地位。为更好的处理恶臭气体提供可靠的科学依据,同时也为生物除臭的应用提供一定的理论基础     

Community structure in a methanotroph biofilter as revealed by phospholipid fatty acid analysis

The microbial community structure of two biofilters used for the oxidation of methane and organic trace gases generated in landfills was analysed by phospholipid fatty acid composition. Community structure varied with biofilter depth, reflecting varying conditions of substrate supply as well as of organic carbon content, nutrient status and osmotic stress determined by the different materials used for the individual biofilter layers. Both biofilters were dominated by type II methanotrophs. In the biofilter charged with landfill gas containing significant amounts of trace organics, fatty acid 18:1omega7c constituted 87% of the methanotrophic PLFA, while the recognised signature fatty acids 16:1omega8 and 18:1omega8, which were well represented in the other biofilter, were entirely absent. This indicates the development of a highly specific methanotrophic population, presumably as a result of the adaption to continuous organic trace gas exposure.     

Assessment of microbial populations in methyl ethyl ketone degrading biofilters by denaturing gradient gel electrophoresis

Denaturing gradient gel electrophoresis (DGGE) analysis of polymerase chain reaction-amplified genes coding for 16S rRNA was used to assess differences in bacterial community structure as a function of spatial location along the height of two biofilters used to treat a model waste gas stream containing methyl ethyl ketone (MEK). One of the laboratory-scale biofilters was operated as a conventional continuous-flow biofilter (CFB) and the other was operated as a sequencing batch biofilter (SBB). Both biofilters, inoculated with an identical starting culture and operated over a period lasting more than 300 days, received the same influent MEK concentration and same mass of MEK on a daily basis. The systems differed, however, in terms of the fraction of time during which contaminated air was supplied and the overall operating strategy employed. DGGE analysis indicated that microbial community structures differed as a function of height in each of the biofilters. The DGGE banding patterns also differed between the two biofilters, suggesting that operating strategies imposed on the biofilters imparted a sufficiently large selective pressure to influence microbial community structures. This may explain, in part, the superior performance of the SBB over the CFB during model transient loading conditions, and it may open new possibilities for purposely manipulating the microbial populations in biofilters treating gas-phase contaminants in a manner that leads to more favorable treatment performance.Electronic Supplementary Material Supplementary material is available in the online version of this article at     

Petrochemical wastewater odor treatment by biofiltration

The treatment of odorous pollutants by microorganisms on packed waste straw and cortex was investigated at the wastewater treatment plant of the Shanghai petrochemical factory. The removal efficiency of H₂S, NH₃ and VOCs (volatile organic compounds) reached 98%, 91% and 90%, respectively after operation for one month at an empty bed retention time (EBRT) of 120 s. The heterotrophic bacteria were found to be the dominant microorganism in the biofilter, while fungi and actinomycetes were also present. The bacteria were mostly identified as the members of the genus Bacillus (62.5% of cultured bacteria). The single strand conformation polymorphism (SSCP) results revealed that the genus Bacillus and Pseudomonas were the predominant bacteria. The microbial diversity gradually increased as the treatment progressed, which indicated that the microbial community in the biofilter became more stable upon pollutant removal. The scanning electron microscopy (SEM) was performed to evaluate the microorganism growth on the media. It was found that the waste straw and cortex were suitable for microorganism attachment and growth, and may have potential application in odor treatment.     

Nitrosomonas Nm143-like ammonia oxidizers and Nitrospira marina-like nitrite oxidizers dominate the nitrifier community in a marine aquaculture biofilm

Zero-discharge marine aquaculture systems are an environmentally friendly alternative to conventional aquaculture. In these systems, water is purified and recycled via microbial biofilters. Here, quantitative data on nitrifier community structure of a trickling filter biofilm associated with a recirculating marine aquaculture system are presented. Repeated rounds of the full-cycle rRNA approach were necessary to optimize DNA extraction and the probe set for FISH to obtain a reliable and comprehensive picture of the ammonia-oxidizing community. Analysis of the ammonia monooxygenase gene (amoA) confirmed the results. The most abundant ammonia-oxidizing bacteria (AOB) were members of the Nitrosomonas sp. Nm143-lineage (6.7% of the bacterial biovolume), followed by Nitrosomonas marina-like AOB (2.2% of the bacterial biovolume). Both were outnumbered by nitrite-oxidizing bacteria of the Nitrospira marina-lineage (15.7% of the bacterial biovolume). Although more than eight other nitrifying populations were detected, including Crenarchaeota closely related to the ammonia-oxidizer 'Nitrosopumilus maritimus', their collective abundance was below 1% of the total biofilm volume; their contribution to nitrification in the biofilter is therefore likely to be negligible.     

Characterisation of hexane-degrading microorganisms in a biofilter by stable isotope-based fatty acid analysis, FISH and cultivation

The hexane-degrading bacterial community of a biofilter was characterised by a combination of stable isotope-based phospholipid fatty acid analyses, fluorescence in situ hybridisation and cultivation. About 70 bacterial strains were isolated from a full-scale biofilter used for treatment of hexane containing waste gas of an oil mill. The isolation approach led to 16 bacterial groups, which were identified as members of the Alpha-, Beta- and Gammaproteobacteria, Actinobacteria and Firmicutes. Three groups showed good growth on hexane as the sole source of carbon. These groups were allocated to the genera Gordonia and Sphingomonas and to the Nevskia-branch of the Gammaproteobacteria. Actively degrading populations in the filter material were characterised by incubation of filter material samples with deuterated hexane and subsequent phospholipid fatty acid analysis. Significant labelling of the fatty acids 16:1 cis10, 18:1 cis9 and 18:0 10methyl affiliated the hexane-degrading activity of the biofilter with the isolates of the genus Gordonia. In vitro growth on hexane and in situ labelling of characteristic fatty acids confirmed the central role of these organisms in the hexane degradation within the full-scale biofilter.     

废气处理生物滤器微生物生态学研究进展

废气生物处理是一项新兴的气体污染控制技术,已成为当前环境领域的研究热点.本文概述了生物滤器系统(包括生物过滤器系统和生物滴滤器系统)结构及其去除气体污染物的原理,重点阐述了生物滤器微生物的分离、鉴定及其降解特性,微生物丰度、活性与微生物群落结构多样性之间的相关性,运行条件对微生物群落的影响,微生物群落结构时空变化规律,生物膜形成机理和模型等方面的研究进展,并对今后废气处理生物滤器微生物生态学研究方向进行了展望.     

Characterization of Humus Microbial Communities in Adjacent Forest Types That Differ in Nitrogen Availability

To address the link between soil microbial community composition and soil processes, we investigated the microbial communities in forest floors of two forest types that differ substantially in nitrogen availability. Cedar-hemlock (CH) and hemlock-amabilis fir (HA) forests are both common on northern Vancouver Island, B.C., occurring adjacently across the landscape. CH forest floors have low nitrogen availability and HA high nitrogen availability. Total microbial biomass was assessed using chloroform fumigation-extraction and community composition was assessed using several cultivation-independent approaches: denaturing gradient gel electrophoresis (DGGE) of the bacterial communities, ribosomal intergenic spacer analysis (RISA) of the bacterial and fungal communities, and phospholipid fatty acid (PLFA) profiles of the whole microbial community. We did not detect differences in the bacterial communities of each forest type using DGGE and RISA, but differences in the fungal communities were detected using RISA. PLFA analysis detected subtle differences in overall composition of the microbial community between the forest types, as well as in particular groups of organisms. Fungal PLFAs were more abundant in the nitrogen-poor CH forests. Bacteria were proportionally more abundant in HA forests than CH in the lower humus layer, and Gram-positive bacteria were proportionally more abundant in HA forests irrespective of layer. Bacterial and fungal communities were distinct in the F, upper humus, and lower humus layers of the forest floor and total biomass decreased in deeper layers. These results indicate that there are distinct patterns in forest floor microbial community composition at the landscape scale, which may be important for understanding nutrient availability to forest vegetation.     

Sample pooling obscures diversity patterns in intertidal ciliate community composition and structure

The aim of this study was to assess the effect of sample pooling on the portrayal of ciliate community structure and composition in intertidal sediment samples. Molecular ciliate community profiles were obtained from nine biological replicates distributed in three discrete sampling plots and from samples that were pooled prior to RNA extraction using terminal restriction fragment polymorphism (T-RFLP) analyses of SSU rRNA. Comparing the individual replicates of one sampling plot with each other, we found a differential variability among the individual biological replicates. T-RFLP profiles of pooled samples displayed a significantly different community composition compared with the cumulative individual biological replicate samples. We conclude that sample pooling obscures diversity patterns in ciliate and possibly also other microbial eukaryote studies. However, differences between pooled samples and replicates were less pronounced when community structure was analyzed. We found that the most abundant T-RFLP peaks were generally shared between biological replicates and pooled samples. Assuming that the most abundant taxa in an ecosystem under study are also the ones driving ecosystem processes, sample pooling may still be effective for the analyses of ecological key players.     

Seasonal changes in an alpine soil bacterial community in the colorado rocky mountains

The period when the snowpack melts in late spring is a dynamic time for alpine ecosystems. The large winter microbial community begins to turn over rapidly, releasing nutrients to plants. Past studies have shown that the soil microbial community in alpine dry meadows of the Colorado Rocky Mountains changes in biomass, function, broad-level structure, and fungal diversity between winter and early summer. However, little specific information exists on the diversity of the alpine bacterial community or how it changes during this ecologically important period. We constructed clone libraries of 16S ribosomal DNA from alpine soil collected in winter, spring, and summer. We also cultivated bacteria from the alpine soil and measured the seasonal abundance of selected cultured isolates in hybridization experiments. The uncultured bacterial communities changed between seasons in diversity and abundance within taxa. The Acidobacterium division was most abundant in the spring. The winter community had the highest proportion of Actinobacteria and members of the Cytophaga/Flexibacter/Bacteroides (CFB) division. The summer community had the highest proportion of the Verrucomicrobium division and of beta-PROTEOBACTERIA: As a whole, alpha-Proteobacteria were equally abundant in all seasons, although seasonal changes may have occurred within this group. A number of sequences from currently uncultivated divisions were found, including two novel candidate divisions. The cultured isolates belonged to the alpha-, beta-, and gamma-Proteobacteria, the Actinobacteria, and the CFB groups. The only uncultured sequences that were closely related to the isolates were from winter and spring libraries. Hybridization experiments showed that actinobacterial and beta-proteobacterial isolates were most abundant during winter, while the alpha- and gamma-proteobacterial isolates tested did not vary significantly. While the cultures and clone libraries produced generally distinct groups of organisms, the two approaches gave consistent accounts of seasonal changes in microbial diversity.     

Removal of benzene in a hybrid bioreactor

A novel hybrid bioreactor was designed to remove volatile organic compounds from wastewater and its performance was investigated. The bioreactor was composed of a biofilter section and a bubble column bioreactor section. Benzene was used as a model compound and the influent benzene was removed by immobilized cells in a bubble column bioreactor. Gas phase benzene stripped by air injection was removed in a biofilter. When the superficial air flow rate was 21.1 m h⁻¹ (0.76 min of residence time in a biofilter), up to 2.2 ppm of benzene in gas phase was removed completely in a biofilter and the maximum removal rate was 4.71 mg day⁻¹ cm⁻³. The concentration profile of benzene along the biofilter column was dependent on the superficial air flow rate and the degree of microbial adaptation. Air flow rate and residence time were found to be the most important operation parameters for the hybrid bioreactor. By manipulating these operational parameters, the removal efficiency and capacity of the hybrid bioreactor could be enhanced. The organic load on the hybrid bioreactor could be shared by the biofilter and bubble column bioreactors and the fluctuation of load on the hybrid bioreactor could be absorbed by changing the distribution of benzene between biofilter and bubble column bioreactors. The maximum removal capacity of the hybrid bioreactor in the experimental range was obtained when the biofilter took 50.3% of influent benzene while 100% of removal efficiency was achieved when the biofilter took 72.3% of influent benzene.     

Link between spatial structure of microbial communities and degradation of a complex mixture of volatile organic compounds in peat biofilters

AIMS: To investigate the relationships between the operation of the volatile organic compound (VOC) removal biofilter and the structure of microbial communities, and to study the impact on degradation activities and the structuring of microbial communities of biofilter malfunctions related to the qualitative composition of the polluted air. METHODS AND RESULTS: A microbiological study and a measurement of biodegradation activities were simultaneously carried out on two identical peat-packed columns, seeded with two different inocula, treating polluted air containing 11 VOCs. For both reactors, the spatial structure of the microbial communities was investigated by means of single-strand conformation polymorphism (SSCP) analysis. For both reactors, stratification of degradation activities in function of depth was observed. Oxygenated compounds were removed at the top of the column and aromatics at the bottom. Comparison of SSCP patterns clearly showed a shift in community structure in function of depth inside both biofilters. This distribution of biodegradation activities correlates with the spatialization of microbial density and diversity. Although the operating conditions of both reactors were identical and the biodegradation activities similar, the composition of microflora differed for biofilters A and B. Subdivision of biofilter B into two independent parts supplied with polluted air containing the complex VOC mixture showed that the microflora having colonized the bottom of biofilter B retained their potential for degrading oxygenated compounds. CONCLUSIONS: This work highlights the spatialization of biodegradation functions in a biofilter treating a complex mixture of VOCs. This distribution of biodegradation activities correlates with the spatialization of microbial density and diversity. SIGNIFICANCE AND IMPACT OF THE STUDY: This vertical structure of microbial communities must be taken into consideration when dealing with the malfunctioning of bioreactors. These results are also useful information about changes in microbial communities following natural or anthropogenic alterations in different ecosystems (soils and sediments) where structuring of microbial communities according to depth has been observed.     

Development of compost maturity and Actinobacteria populations during full-scale composting of organic household waste

AIMS: This study investigates changes in microbiological and physicochemical parameters during large-scale, thermophilic composting of a single batch of municipal organic waste. The inter-relationships between the microbial biomass and community structure as well as several physicochemical parameters and estimates of maturation were evaluated. METHODS AND RESULTS: Analyses of signature fatty acids with the phospholipid fatty acid and ester-linked methods showed that the total microbial biomass was highest during the early thermophilic phase. The contribution of signature 10Me fatty acids from Actinobacteria indicated a relatively constant proportion around 10% of the microbial community. However, analyses of the Actinobacteria species composition with a PCR-denaturing gradient gel electrophoresis approach targeting 16S rRNA genes demonstrated clear shifts in the community structure. CONCLUSIONS: This study demonstrates that compost quality, particularly maturity, is linked to the composition of the microbial community structure, but further studies in other full-scale systems are needed to validate the generality of these findings. SIGNIFICANCE AND IMPACT OF THE STUDY: The combination of signature lipid and nucleic acid-based analyses greatly expands the specificity and the scope for assessing the microbial community composition in composts. The results presented in this study give new information on how the development of the compost microbial community is connected to curing and maturation in the later stages of composting, and emphasizes the role of Actinobacteria in this respect.     

Spatial Variation in the Bacterial and Denitrifying Bacterial Community in a Biofilter Treating Subsurface Agricultural Drainage

Denitrifying biofilters can remove agricultural nitrates from subsurface drainage, reducing nitrate pollution that contributes to coastal hypoxic zones. The performance and reliability of natural and engineered systems dependent upon microbially mediated processes, such as the denitrifying biofilters, can be affected by the spatial structure of their microbial communities. Furthermore, our understanding of the relationship between microbial community composition and function is influenced by the spatial distribution of samples. In this study we characterized the spatial structure of bacterial communities in a denitrifying biofilter in central Illinois. Bacterial communities were assessed using automated ribosomal intergenic spacer analysis for bacteria and terminal restriction fragment length polymorphism of nosZ for denitrifying bacteria. Non-metric multidimensional scaling and analysis of similarity (ANOSIM) analyses indicated that bacteria showed statistically significant spatial structure by depth and transect, while denitrifying bacteria did not exhibit significant spatial structure. For determination of spatial patterns, we developed a package of automated functions for the R statistical environment that allows directional analysis of microbial community composition data using either ANOSIM or Mantel statistics. Applying this package to the biofilter data, the flow path correlation range for the bacterial community was 6.4 m at the shallower, periodically inundated depth and 10.7 m at the deeper, continually submerged depth. These spatial structures suggest a strong influence of hydrology on the microbial community composition in these denitrifying biofilters. Understanding such spatial structure can also guide optimal sample collection strategies for microbial community analyses.     

Prokaryotic and eukaryotic community structure affected by the presence of an acid mine drainage from an abandoned gold mine

The acid mine drainage that originates in the abandoned gold mine in San Luis, Argentina, is released into La Carolina stream. The aim of this study was to determine the influence of this mine drainage on the physicochemical parameters of the area studied and on both prokaryotic and eukaryotic community structure. In addition, specific relationships between microbial taxonomic groups and physicochemical parameters were established. The drainage that flows into La Carolina stream acidifies the stream and increases its sulfate, Zn, Cd and Te concentrations. Microbial analysis showed that prokaryotic community structure is mainly affected by pH values. Actinobacteria and Gammaproteobacteria were abundant in samples characterized by low pH values, while Nitrospirae, Chloroflexi, Deltaproteobacteria, Thaumarchaeota and Euryarchaeota were associated with high concentrations of heavy metals. Otherwise, Alphaproteobacteria was present in samples taken in sunlit areas. Regarding eukaryotic community structure, the sunlight had the greatest impact. Inside the mine, in the absence of light, fungi and protists members were the most abundant microorganisms, while those samples taken in the presence of light displayed algae (green algae and diatoms) as the most abundant ones. After receiving the mine drainage, the stream showed a decrease in the diatom abundance and green algae predominated.

     

Use of length heterogeneity PCR and fatty acid methyl ester profiles to characterize microbial communities in soil

In length heterogeneity PCR (LH-PCR) a fluorescently labeled primer is used to determine the relative amounts of amplified sequences originating from different microorganisms. Labeled fragments are separated by gel electrophoresis and detected by laser-induced fluorescence with an automated gene sequencer. We used LH-PCR to evaluate the composition of the soil microbial community. Four soils, which differed in terms of soil type and/or crop management practice, were studied. Previous data for microbial biomass, nitrogen and carbon contents, and nitrogen mineralization rates suggested that the microbial characteristics of these soils were different. One site received two different treatments: no-till and conventional till perennial ryegrass. The other sites were no-till continuous grass plots at separate locations with different soil types. Community composition was characterized by assessing the natural length heterogeneity in eubacterial sequences amplified from the 5' domain of the 16S rRNA gene and by determining fatty acid methyl ester (FAME) profiles. We found that LH-PCR results were reproducible. Both methods distinguished the three sites. The most abundant bacterial community members, based on cloned LH-PCR products, were members of the beta subclass of the class Proteobacteria, the Cytophaga-Flexibacter-Bacteriodes group, and the high-G+C-content gram-positive bacterial group. Strong correlations were found between LH-PCR results and FAME results. We found that the LH-PCR method is an efficient, reliable, and highly reproducible method that should be a useful tool in future assessments of microbial community composition.     

Phylogenetic characterization of the heterotrophic bacterial communities inhabiting a marine recirculating aquaculture system

Aims: The aim of the present work was to characterize the heterotrophic bacterial community of a marine recirculating aquaculture system (RAS). Methods and Results: An experimental RAS was sampled for the rearing water (RW) and inside the biofilter. Samples were analysed for bacterial abundances, community structure and composition by using a combination of culture‐dependent and ‐independent techniques. The most represented species detected among biofilter clones was Pseudomonas stutzeri, while Ruegeria spp. and Roseobacter spp. were more abundant among isolates. In comparison, the genera Roseobacter and Ruegeria were well represented in both the biofilter and the RW samples. A variety of possible bacterial pathogens (e.g. Vibrio spp., Erwinia spp. and Coxiella spp.) were also identified in this study. Conclusions: Results revealed that the bacterial community in the RW was quite different to that associated with the biofilter. Moreover, data obtained suggest that the whole bacterial community can be involved in maintaining an effective and a stable rearing environment (shelter effect). Significance and Impact of the Study: Improving the reliability and the sustainability of RAS depends on the correct management of the bacterial populations inside it. This study furnishes more accurate information on the bacterial populations and better clarifies the existing relationships between the bacterial flora in the RW and that associated with the biofilter.     

The biogeography of abundant and rare bacterioplankton in the lakes and reservoirs of China

Bacteria play key roles in the ecology of both aquatic and terrestrial ecosystems; however, little is known about their diversity and biogeography, especially in the rare microbial biosphere of inland freshwater ecosystems. Here we investigated aspects of the community ecology and geographical distribution of abundant and rare bacterioplankton using high-throughput sequencing and examined the relative influence of local environmental variables and regional (spatial) factors on their geographical distribution patterns in 42 lakes and reservoirs across China. Our results showed that the geographical patterns of abundant and rare bacterial subcommunities were generally similar, and both of them showed a significant distance–decay relationship. This suggests that the rare bacterial biosphere is not a random assembly, as some authors have assumed, and that its distribution is most likely subject to the same ecological processes that control abundant taxa. However, we identified some differences between the abundant and rare groups as both groups of bacteria showed a significant positive relationship between sites occupancy and abundance, but the abundant bacteria exhibited a weaker distance–decay relationship than the rare bacteria. Our results implied that rare subcommunities were mostly governed by local environmental variables, whereas the abundant subcommunities were mainly affected by regional factors. In addition, both local and regional variables that were significantly related to the spatial variation of abundant bacterial community composition were different to those of rare ones, suggesting that abundant and rare bacteria may have discrepant ecological niches and may play different roles in natural ecosystems.