Microbial Biomass and Activity in Lead-Contaminated Soil

Microbial community diversity, potential microbial activity, and metal resistance were determined in three soils whose lead contents ranged from 0.00039 to 48 mmol of Pb kg of soil⁻¹. Biomass levels were directly related to lead content. A molecular analysis of 16S rRNAs suggested that each soil contained a complex, diverse microbial community. A statistical analysis of the phospholipid fatty acids indicated that the community in the soil having the highest lead content was not related to the communities in the other soils. All of the soils contained active microbial populations that mineralized [¹⁴C]glucose. In all samples, 10 to 15% of the total culturable bacteria were Pb resistant and had MIC of Pb for growth of 100 to 150 μM.     

The microbial gene diversity along an elevation gradient of the Tibetan grassland

Tibet is one of the most threatened regions by climate warming, thus understanding how its microbial communities function may be of high importance for predicting microbial responses to climate changes. Here, we report a study to profile soil microbial structural genes, which infers functional roles of microbial communities, along four sites/elevations of a Tibetan mountainous grassland, aiming to explore the potential microbial responses to climate changes via a strategy of space-for-time substitution. Using a microarray-based metagenomics tool named GeoChip 4.0, we showed that microbial communities were distinct for most but not all of the sites. Substantial variations were apparent in stress, N and C-cycling genes, but they were in line with the functional roles of these genes. Cold shock genes were more abundant at higher elevations. Also, gdh converting ammonium into urea was more abundant at higher elevations, whereas ureC converting urea into ammonium was less abundant, which was consistent with soil ammonium contents. Significant correlations were observed between N-cycling genes (ureC, gdh and amoA) and nitrous oxide flux, suggesting that they contributed to community metabolism. Lastly, we found by Canonical correspondence analysis, Mantel tests and the similarity tests that soil pH, temperature, NH₄⁺–N and vegetation diversity accounted for the majority (81.4%) of microbial community variations, suggesting that these four attributes were major factors affecting soil microbial communities. On the basis of these observations, we predict that climate changes in the Tibetan grasslands are very likely to change soil microbial community functional structure, with particular impacts on microbial N-cycling genes and consequently microbe-mediated soil N dynamics.     

Loss in microbial diversity affects nitrogen cycling in soil

Microbial communities have a central role in ecosystem processes by driving the Earth''s biogeochemical cycles. However, the importance of microbial diversity for ecosystem functioning is still debated. Here, we experimentally manipulated the soil microbial community using a dilution approach to analyze the functional consequences of diversity loss. A trait-centered approach was embraced using the denitrifiers as model guild due to their role in nitrogen cycling, a major ecosystem service. How various diversity metrics related to richness, eveness and phylogenetic diversity of the soil denitrifier community were affected by the removal experiment was assessed by 454 sequencing. As expected, the diversity metrics indicated a decrease in diversity in the 1/10³ and 1/10⁵ dilution treatments compared with the undiluted one. However, the extent of dilution and the corresponding reduction in diversity were not commensurate, as a dilution of five orders of magnitude resulted in a 75% decrease in estimated richness. This reduction in denitrifier diversity resulted in a significantly lower potential denitrification activity in soil of up to 4–5 folds. Addition of wheat residues significantly increased differences in potential denitrification between diversity levels, indicating that the resource level can influence the shape of the microbial diversity–functioning relationship. This study shows that microbial diversity loss can alter terrestrial ecosystem processes, which suggests that the importance of functional redundancy in soil microbial communities has been overstated.     

Cross-Site Soil Microbial Communities under Tillage Regimes: Fungistasis and Microbial Biomarkers

The exploitation of soil ecosystem services by agricultural management strategies requires knowledge of microbial communities in different management regimes. Crop cover by no-till management protects the soil surface, reducing the risk of erosion and nutrient leaching, but might increase straw residue-borne and soilborne plant-pathogenic fungi. A cross-site study of soil microbial communities and Fusarium fungistasis was conducted on six long-term agricultural fields with no-till and moldboard-plowed treatments. Microbial communities were studied at the topsoil surface (0 to 5 cm) and bottom (10 to 20 cm) by general bacterial and actinobacterial terminal restriction fragment length polymorphism (T-RFLP) and phospholipid fatty acid (PLFA) analyses. Fusarium culmorum soil fungistasis describing soil receptivity to plant-pathogenic fungi was explored by using the surface layer method. Soil depth had a significant impact on general bacterial as well as actinobacterial communities and PLFA profiles in no-till treatment, with a clear spatial distinction of communities (P < 0.05), whereas the depth-related separation of microbial communities was not observed in plowed fields. The fungal biomass was higher in no-till surface soil than in plowed soil (P < 0.07). Soil total microbial biomass and fungal biomass correlated with fungistasis (P < 0.02 for the sum of PLFAs; P < 0.001 for PLFA 18:2ω6). Our cross-site study demonstrated that agricultural management strategies can have a major impact on soil microbial community structures, indicating that it is possible to influence the soil processes with management decisions. The interactions between plant-pathogenic fungi and soil microbial communities are multifaceted, and a high level of fungistasis could be linked to the high microbial biomass in soil but not to the specific management strategy.     

Association of Microbial Community Composition and Activity with Lead, Chromium, and Hydrocarbon Contamination

Microbial community composition and activity were characterized in soil contaminated with lead (Pb), chromium (Cr), and hydrocarbons. Contaminant levels were very heterogeneous and ranged from 50 to 16,700 mg of total petroleum hydrocarbons (TPH) kg of soil⁻¹, 3 to 3,300 mg of total Cr kg of soil⁻¹, and 1 to 17,100 mg of Pb kg of soil⁻¹. Microbial community compositions were estimated from the patterns of phospholipid fatty acids (PLFA); these were considerably different among the 14 soil samples. Statistical analyses suggested that the variation in PLFA was more correlated with soil hydrocarbons than with the levels of Cr and Pb. The metal sensitivity of the microbial community was determined by extracting bacteria from soil and measuring [³H]leucine incorporation as a function of metal concentration. Six soil samples collected in the spring of 1999 had IC₅₀ values (the heavy metal concentrations giving 50% reduction of microbial activity) of approximately 2.5 mM for CrO₄²⁻ and 0.01 mM for Pb²⁺. Much higher levels of Pb were required to inhibit [¹⁴C]glucose mineralization directly in soils. In microcosm experiments with these samples, microbial biomass and the ratio of microbial biomass to soil organic C were not correlated with the concentrations of hydrocarbons and heavy metals. However, microbial C respiration in samples with a higher level of hydrocarbons differed from the other soils no matter whether complex organic C (alfalfa) was added or not. The ratios of microbial C respiration to microbial biomass differed significantly among the soil samples (P < 0.05) and were relatively high in soils contaminated with hydrocarbons or heavy metals. Our results suggest that the soil microbial community was predominantly affected by hydrocarbons.     

Short-term effects of wildfire on microbial biomass and abundance in black pine plantation soils in Turkey

Measurement of soil microbial biomass and abundance offers a means of assessing the response of all microbial populations to changes in the soil environment after a fire. We examined the effects of wildfire on microbial biomass C and N, and abundance of bacteria and fungi 2 months after a fire in a pine plantation. Soil organic carbon (C_org), total nitrogen (N_tot), and electrical conductivity (EC) increased following the fire. In terms of microbial abundance, the overall results showed that burned forest soils had the most bacteria and fungi. Microbial biomass C and N from soil in the burned forest were not significantly different from their unburned forest counterparts. However, microbial indices indicated that fire affects soil microbial community structure by modifying the environmental conditions. The results also suggested that low-intensity fire promotes microorganism functional activity and improves the chemical characteristics of soils under humid climatic conditions.     

Responses of soil microbial communities to prescribed burning in two paired vegetation sites in southern China

Prescribed burning is a common site preparation practice for forest plantation in southern China. However, the effects of prescribed burning on soil microbial communities are poorly understood. This study examined changes in microbial community structure, measured by phospholipid fatty acids (PLFAs), after a single prescribed burning in two paired vegetation sites in southern China. The results showed that the total amount of PLFA (totPLFA) was similar under two vegetation types in the wet season but differed among vegetation type in the dry season, and was affected significantly by burning treatment only in the wet season. Bacterial PLFA (bactPLFA) and fungal PLFA (fungPLFA) in burned plots all decreased compared to the unburned plots in both seasons (P = 0.059). Fungi appeared more sensitive to prescribed burning than bacteria. Both G⁺ bacterial PLFA and G⁻ bacterial PLFA were decreased by the burning treatment in both dry and wet seasons. Principal component analysis of PLFAs showed that the burning treatment induced a shift in soil microbial community structure. The variation in soil microbial community structure was correlated significantly to soil organic carbon, total nitrogen, available phosphorus and exchangeable potassium. Our results suggest that prescribed burning results in short-term changes in soil microbial communities but the long-term effects of prescribed burning on soil microbial community remain unknown and merit further investigation.     

Fire and soil-plant nutrient relations in a pine-wiregrass savanna on the coastal plain of North Carolina

Summary Changes in soil and plant nutrient conditions were evaluated following various burn and clip treatments in a longleaf pine-wiregrass savanna in Bladen Co., N.C., USA. Ground fires were found to add substantial quantities of N, P, K, Ca, and Mg to the soil, though not necessarily in forms immediately available to plants. Less than 1% of the total nitrogen in the charred residue (ash) is present as nitrate or ammonium. Considerable quantities of all nutrients examined were lost to the atmosphere during burning. Green leaf tissue in recently burned areas was consistently higher in N, P, K, Ca, and Mg compared to unburned areas. Howerver, when compared to similar tissues from clipped plots, burned area tissues were significantly higher in N, Ca, and Mg only. Data presented here suggest that tissue age significantly affects nutrient content and must be considered in any analysis of tissue nutrient content following burning. Within 4–6 months following fire, burned-area tissue nutrient content decreases to concentrations found in the unburned area. Burning resulted in initial enrichment of available soil nutrients including PO₄, K⁺, Ca⁺⁺, and Mg⁺⁺, however, NO₃ ^-, and NH₄ ⁺ concentrations in burned soil were not significantly different from unbruned soil. Soil and plant nutrient changes in an area burned two years in succession indicate that repeated burning may diminish nutrient availability. Plant response to various nutrient enrichment treatments of the soil indicated that nitrogen is limiting growth in both burned and unburned soils and that burning may alter some factors other than nutrients which may retard plant growth in unburned areas.     

Influence of Soil Characteristics on the Diversity of Bacteria in the Southern Brazilian Atlantic Forest

The Brazilian Atlantic Forest is one of the 25 biodiversity hot spots in the world. Although the diversity of its fauna and flora has been studied fairly well, little is known of its microbial communities. In this work, we analyzed the Atlantic Forest ecosystem to determine its bacterial biodiversity, using 16S rRNA gene sequencing, and correlated changes in deduced taxonomic profiles with the physicochemical characteristics of the soil. DNAs were purified from soil samples, and the 16S rRNA gene was amplified to construct libraries. Comparison of 754 independent 16S rRNA gene sequences from 10 soil samples collected along a transect in an altitude gradient showed the prevalence of Acidobacteria (63%), followed by Proteobacteria (25.2%), Gemmatimonadetes (1.6%), Actinobacteria (1.2%), Bacteroidetes (1%), Chloroflexi (0.66%), Nitrospira (0.4%), Planctomycetes (0.4%), Firmicutes (0.26%), and OP10 (0.13%). Forty-eight sequences (6.5%) represented unidentified bacteria. The Shannon diversity indices of the samples varied from 4.12 to 3.57, indicating that the soils have a high level of diversity. Statistical analysis showed that the bacterial diversity is influenced by factors such as altitude, Ca²⁺/Mg²⁺ ratio, and Al³⁺ and phosphorus content, which also affected the diversity within the same lineage. In the samples analyzed, pH had no significant impact on diversity.The Brazilian Atlantic Forest is one of the 25 biodiversity hot spots in the world. Altogether, these hot spots contain more than 60% of the total terrestrial species of the planet (17). The Atlantic Forest is a dense ombrophilous forest with several variations, including coastal (3 to 50 m), submontane (50 to 500 m), montane (500 to 1,200 m), and high montane (1,200 to 1,400 m) forests, creating a vegetation gradient ranging from shrubs to well-developed montane forest (4). The Serra do Mar is a mountainous system that shelters the main remainder of the Atlantic Forest following the Brazilian east coast, from north to south along the coastal line, and it is divided into diverse sections of high and low blocks, which have regional denominations.The most important law-protected conservation area of the Brazilian Atlantic Forest is located in the Serra do Mar of the southern state of Paraná. This conservation area (∼5,000 km²) shelters 72% of the fauna and flora species that occur in Paraná and was declared a Biosphere Reserve by UNESCO in 1992. Much is known about the diversity of its fauna and flora, but little is known of its microbial diversity, particularly the soil microbial diversity and the soil characteristics that influence it.The soil microbial diversity is vast, and it is estimated that >99% of species remain unidentified (1, 28). Acidobacteria and Proteobacteria are the most abundant groups in soil (15). However, the Proteobacteria lineage is more diverse and stable than the Acidobacteria lineage, suggesting that the latter group is more susceptible to variation in soil properties and to disturbing factors (33). Seasonal, physical, and physicochemical factors can be relevant to the structure and diversity of microbial communities. For example, seasonal changes in vegetation and temperature led to replacement of dominant groups in a wheat field (25) and in grassland soils (16). The particle size also has an influence on the bacterial diversity of soils. The clay fraction has a more diverse bacterial community than do silt or sand fractions (23). Finally, analyses of communities from North and South American soils showed that pH plays a major role in bacterial diversity, with less diverse communities associated with a lower pH (9).Human activity can also change the microbial diversity of soils, both qualitatively and quantitatively. Analyses of microbial communities on coral atolls in the central Pacific Ocean under different degrees of human impact showed that the least-impacted atoll had autotrophs and heterotrophs equally distributed in the community, whereas the most-impacted atoll had a dominance of heterotrophs and about 10 times more microbial cells and virus-like particles in the water column, including a large percentage of potential pathogens (7). A comparison between bacterial communities in forest and pasture soil showed that there is a less diverse and more restricted community in pasture soils. The vegetation shift from forest to pasture resulted in changes to G+C% contents of soil bacterial DNA and amplified rRNA gene restriction analysis (ARDRA) profiles (18). Similar changes occurred with communities of soils submitted to agroindustrial treatments and pollutants (3, 30).In this work, we used a culture-independent approach based on 16S rRNA gene sequences to survey the bacterial community of the Atlantic Forest soils and determined the physicochemical factors affecting its bacterial biodiversity.     

Changes in Nitrogen-Fixing and Ammonia-Oxidizing Bacterial Communities in Soil of a Mixed Conifer Forest after Wildfire

This study was undertaken to examine the effects of forest fire on two important groups of N-cycling bacteria in soil, the nitrogen-fixing and ammonia-oxidizing bacteria. Sequence and terminal restriction fragment length polymorphism (T-RFLP) analysis of nifH and amoA PCR amplicons was performed on DNA samples from unburned, moderately burned, and severely burned soils of a mixed conifer forest. PCR results indicated that the soil biomass and proportion of nitrogen-fixing and ammonia-oxidizing species was less in soil from the fire-impacted sites than from the unburned sites. The number of dominant nifH sequence types was greater in fire-impacted soils, and nifH sequences that were most closely related to those from the spore-forming taxa Clostridium and Paenibacillus were more abundant in the burned soils. In T-RFLP patterns of the ammonia-oxidizing community, terminal restriction fragments (TRFs) representing amoA cluster 1, 2, or 4 Nitrosospira spp. were dominant (80 to 90%) in unburned soils, while TRFs representing amoA cluster 3A Nitrosospira spp. dominated (65 to 95%) in fire-impacted soils. The dominance of amoA cluster 3A Nitrosospira spp. sequence types was positively correlated with soil pH (5.6 to 7.5) and NH₃-N levels (0.002 to 0.976 ppm), both of which were higher in burned soils. The decreased microbial biomass and shift in nitrogen-fixing and ammonia-oxidizing communities were still evident in fire-impacted soils collected 14 months after the fire.     

Bacterial genome replication at subzero temperatures in permafrost

Microbial metabolic activity occurs at subzero temperatures in permafrost, an environment representing ∼25% of the global soil organic matter. Although much of the observed subzero microbial activity may be due to basal metabolism or macromolecular repair, there is also ample evidence for cellular growth. Unfortunately, most metabolic measurements or culture-based laboratory experiments cannot elucidate the specific microorganisms responsible for metabolic activities in native permafrost, nor, can bulk approaches determine whether different members of the microbial community modulate their responses as a function of changing subzero temperatures. Here, we report on the use of stable isotope probing with ¹³C-acetate to demonstrate bacterial genome replication in Alaskan permafrost at temperatures of 0 to −20 °C. We found that the majority (80%) of operational taxonomic units detected in permafrost microcosms were active and could synthesize ¹³C-labeled DNA when supplemented with ¹³C-acetate at temperatures of 0 to −20 °C during a 6-month incubation. The data indicated that some members of the bacterial community were active across all of the experimental temperatures, whereas many others only synthesized DNA within a narrow subzero temperature range. Phylogenetic analysis of ¹³C-labeled 16S rRNA genes revealed that the subzero active bacteria were members of the Acidobacteria, Actinobacteria, Chloroflexi, Gemmatimonadetes and Proteobacteria phyla and were distantly related to currently cultivated psychrophiles. These results imply that small subzero temperature changes may lead to changes in the active microbial community, which could have consequences for biogeochemical cycling in permanently frozen systems.     

The influence of goethite and gibbsite on soluble nutrient dynamics and microbial community composition

Iron and aluminum (oxyhydr)oxides are ubiquitous in the soil environment and have the potential to strongly affect the properties of dissolved organic matter. We examined the effect of oxide surfaces on soluble nutrient dynamics and microbial community composition using an incubation of forest floor material in the presence of (1) goethite and quartz, (2) gibbsite and quartz, and (3) quartz surfaces. Forest floor material was incubated over a period of 154 days. Aqueous extracts of the incubations were harvested on days 5, 10, 20, 30, 60, 90, and 154, and concentrations of P, N, PO₄ ^3?, NO₂ ^?, NO₃ ^?, and organic C were measured in the solutions. Microbial community composition was examined through pyrosequencing of bacterial and fungal small subunit ribosomal RNA genes on selected dates throughout the incubation. Results indicated that oxide surfaces exerted strong control on soluble nutrient dynamics and on the composition of the decomposer microbial community, while possibly having a small impact on system-level respiration. Goethite and gibbsite surfaces showed preferential adsorption of P-containing and high molar mass organic solutes, but not of N-containing compounds. On average, organic C concentrations were significantly lower in water extractable organic matter (WEOM) solutions from oxide treatments than from the control treatment (P = 0.0037). Microbial community composition varied both among treatments and with increasing time of incubation. Variation in bacterial and fungal community composition exhibited strong-to-moderate correlation with length of incubation, and several WEOM physiochemical characteristics including apparent (weight averaged) molar mass, pH and electrical conductivity. Additionally, variation in bacterial community composition among treatments was correlated with total P (r = 0.60, P < 0.0001), PO₄ ^3? (r = 0.79, P < 0.0001), and organic C (r = 0.36, P = 0.015) concentrations; while variation in fungal communities was correlated with organic C concentrations (r = ?0.48, P = 0.0008) but not with phosphorus concentrations. The relatively small impact of oxide surfaces on system-level microbial respiration of organic matter despite their significant effects on microbial community composition and WEOM dynamics lends additional support to the theory of microbial functional redundancy.     

Ecosystem Carbon Remains Low for Three Decades Following Fire and Constrains Soil CO2 Responses to Precipitation in Southwestern Ponderosa Pine Forests

The fire regime of ponderosa pine forests in the southwestern United States has shifted over the past century from historically frequent, low-intensity surface fires to infrequent, stand-replacing crown fires. We quantified plant and soil carbon (C) responses to this new fire regime and assessed interactions between changes in fire regime and changes in precipitation regime predicted by some climate models (specifically, an earlier monsoon rain season). We hypothesized that soil C pools and carbon dioxide (CO₂) efflux rates would decrease initially following stand-replacing fires (due to low plant C inputs and the loss of the soil surficial organic (O) horizon), but then increase with time-after-fire (as plant C inputs increase). Water availability often limits soil biological activity in these forests, but we predicted that low soil C availability following fire would constrain soil CO₂ efflux responses to precipitation. In a series of sites with histories of stand-replacing fires that burned between 2 and 34?years prior to sampling, burned patches had lower soil C pools and fluxes than adjacent unburned patches, but there was no evidence of a trend with time-after-fire. Burned forests had 7,500?g C m^?2 less live plant biomass C (P?<?0.001), 1,600?g C m^?2 less soil total C (P?<?0.001) and 90?g C m^?2 less soil labile C (P?<?0.001) than unburned forests. Lower soil labile C in burned patches was due to both a loss of O horizon mass with fire and lower labile C concentrations (g labile C kg^?1 soil total C) in the mineral soil. During the annual drought that precedes summer monsoon rains, both burned and unburned patches had soil CO₂ efflux rates ranging from 0.9 to 1.1?g CO₂-C m^?2 day^?1. During the monsoon season, soil CO₂ efflux in unburned patches increased to approximately 4.8?g CO₂-C m^?2 day^?1 and rates in paired burned patches (3.4?g CO₂-C m^?2 day^?1) were lower (P?<?0.001). We also used field irrigation to experimentally create an earlier and longer monsoon season, and soil CO₂ efflux rates at both burned and unburned plots increased initially in response to watering, but decreased to below control (plots without irrigation) rates within weeks. Watering did not significantly change cumulative growing season soil CO₂ efflux, supporting our prediction that C availability constrains soil CO₂ efflux responses to precipitation. This research advances our understanding of interactions among climate, fire, and C in southwestern forests, suggesting that climate-induced shifts toward more stand-replacing fires will decrease soil C for decades, such that a single fire can constrain future soil biological responses to precipitation regime changes.     

Culture-Dependent and -Independent Characterization of Microbial Communities Associated with a Shallow Submarine Hydrothermal System Occurring within a Coral Reef off Taketomi Island, Japan

Microbial communities in a shallow submarine hydrothermal system near Taketomi Island, Japan, were investigated using cultivation-based and molecular techniques. The main hydrothermal activity occurred in a craterlike basin (depth, ~23 m) on the coral reef seafloor. The vent fluid (maximum temperature, >52°C) contained 175 μM H₂S and gas bubbles mainly composed of CH₄ (69%) and N₂ (29%). A liquid serial dilution cultivation technique targeting a variety of metabolism types quantified each population in the vent fluid and in a white microbial mat located near the vent. The most abundant microorganisms cultivated from both the fluid and the mat were autotrophic sulfur oxidizers, including mesophilic Thiomicrospira spp. and thermophilic Sulfurivirga caldicuralii. Methane oxidizers were the second most abundant organisms in the fluid; one novel type I methanotroph exhibited optimum growth at 37°C, and another novel type I methanotroph exhibited optimum growth at 45°C. The number of hydrogen oxidizers cultivated only from the mat was less than the number of sulfur and methane oxidizers, although a novel mesophilic hydrogen-oxidizing member of the Epsilonproteobacteria was isolated. Various mesophilic to hyperthermophilic heterotrophs, including sulfate-reducing Desulfovibrio spp., iron-reducing Deferribacter sp., and sulfur-reducing Thermococcus spp., were also cultivated. Culture-independent 16S rRNA gene clone analysis of the vent fluid and mat revealed highly diverse archaeal communities. In the bacterial community, S. caldicuralii was identified as the predominant phylotype in the fluid (clonal frequency, 25%). Both bacterial clone libraries indicated that there were bacterial communities involved in sulfur, hydrogen, and methane oxidation and sulfate reduction. Our results indicate that there are unique microbial communities that are sustained by active chemosynthetic primary production rather than by photosynthetic production in a shallow hydrothermal system where sunlight is abundant.     

Convergent development of anodic bacterial communities in microbial fuel cells

Microbial fuel cells (MFCs) are often inoculated from a single wastewater source. The extent that the inoculum affects community development or power production is unknown. The stable anodic microbial communities in MFCs were examined using three inocula: a wastewater treatment plant sample known to produce consistent power densities, a second wastewater treatment plant sample, and an anaerobic bog sediment. The bog-inoculated MFCs initially produced higher power densities than the wastewater-inoculated MFCs, but after 20 cycles all MFCs on average converged to similar voltages (470±20 mV) and maximum power densities (590±170 mW m⁻²). The power output from replicate bog-inoculated MFCs was not significantly different, but one wastewater-inoculated MFC (UAJA3 (UAJA, University Area Joint Authority Wastewater Treatment Plant)) produced substantially less power. Denaturing gradient gel electrophoresis profiling showed a stable exoelectrogenic biofilm community in all samples after 11 cycles. After 16 cycles the predominance of Geobacter spp. in anode communities was identified using 16S rRNA gene clone libraries (58±10%), fluorescent in-situ hybridization (FISH) (63±6%) and pyrosequencing (81±4%). While the clone library analysis for the underperforming UAJA3 had a significantly lower percentage of Geobacter spp. sequences (36%), suggesting that a predominance of this microbe was needed for convergent power densities, the lower percentage of this species was not verified by FISH or pyrosequencing analyses. These results show that the predominance of Geobacter spp. in acetate-fed systems was consistent with good MFC performance and independent of the inoculum source.     

The Impact of Gamma Radiation on Sediment Microbial Processes

Microbial communities have the potential to control the biogeochemical fate of some radionuclides in contaminated land scenarios or in the vicinity of a geological repository for radioactive waste. However, there have been few studies of ionizing radiation effects on microbial communities in sediment systems. Here, acetate and lactate amended sediment microcosms irradiated with gamma radiation at 0.5 or 30 Gy h⁻¹ for 8 weeks all displayed NO₃⁻ and Fe(III) reduction, although the rate of Fe(III) reduction was decreased in 30-Gy h⁻¹ treatments. These systems were dominated by fermentation processes. Pyrosequencing indicated that the 30-Gy h⁻¹ treatment resulted in a community dominated by two Clostridial species. In systems containing no added electron donor, irradiation at either dose rate did not restrict NO₃⁻, Fe(III), or SO₄²⁻ reduction. Rather, Fe(III) reduction was stimulated in the 0.5-Gy h⁻¹-treated systems. In irradiated systems, there was a relative increase in the proportion of bacteria capable of Fe(III) reduction, with Geothrix fermentans and Geobacter sp. identified in the 0.5-Gy h⁻¹ and 30-Gy h⁻¹ treatments, respectively. These results indicate that biogeochemical processes will likely not be restricted by dose rates in such environments, and electron accepting processes may even be stimulated by radiation.     

The effects of fire on the thermal environment of sagebrush communities

Thermal heterogeneity provides options for organisms during extreme temperatures that can contribute to their fitness. Sagebrush (Artemisia spp.) communities exhibit vegetation heterogeneity that creates thermal variation at fine spatial scales. However, fire can change vegetation and thereby variation within the thermal environment of sagebrush communities. To describe spatial and temporal thermal variation of sagebrush communities following wildfire, we measured black bulb temperature (T_bb) at 144 random points dispersed within unburned and burned communities, for 24-h at each random point. We observed a wide thermal gradient in unburned (−7.3° to 63.3 °C) and burned (−4.6° to 64.8 °C) sagebrush communities. Moreover, unburned and burned sagebrush communities displayed high thermal heterogeneity relative to ambient temperature (T_air). Notably, T_bb varied by 47 °C in both unburned and burned communities when T_air was 20 °C. However, fire greatly reduced the buffering capacity and thermal refuge of Wyoming big sagebrush (A. tridentata wyomingensis) communities during low and high T_air. Furthermore, fire increased T_bb in Wyoming big sagebrush and mountain big sagebrush (A. t. vaseyana) during the mid-day hours. These results demonstrate how fire changes the thermal environment of big sagebrush communities and the importance of shrub structure which can provide thermal refuge for organisms in burned communities during extreme low and high T_air.     

Microbial community utilization of recalcitrant and simple carbon compounds: impact of oak-woodland plant communities

Little is known about how the structure of microbial communities impacts carbon cycling or how soil microbial community composition mediates plant effects on C-decomposition processes. We examined the degradation of four ¹³C-labeled compounds (starch, xylose, vanillin, and pine litter), quantified rates of associated enzyme activities, and identified microbial groups utilizing the ¹³C-labeled substrates in soils under oaks and in adjacent open grasslands. By quantifying increases in non-¹³C-labeled carbon in microbial biomarkers, we were also able to identify functional groups responsible for the metabolism of indigenous soil organic matter. Although microbial community composition differed between oak and grassland soils, the microbial groups responsible for starch, xylose, and vanillin degradation, as defined by ¹³C-PLFA, did not differ significantly between oak and grassland soils. Microbial groups responsible for pine litter and SOM-C degradation did differ between the two soils. Enhanced degradation of SOM resulting from substrate addition (priming) was greater in grassland soils, particularly in response to pine litter addition; under these conditions, fungal and Gram + biomarkers showed more incorporation of SOM-C than did Gram – biomarkers. In contrast, the oak soil microbial community primarily incorporated C from the added substrates. More ¹³C (from both simple and recalcitrant sources) was incorporated into the Gram – biomarkers than Gram + biomarkers despite the fact that the Gram + group generally comprised a greater portion of the bacterial biomass than did markers for the Gram – group. These experiments begin to identify components of the soil microbial community responsible for decomposition of different types of C-substrates. The results demonstrate that the presence of distinctly different plant communities did not alter the microbial community profile responsible for decomposition of relatively labile C-substrates but did alter the profiles of microbial communities responsible for decomposition of the more recalcitrant substrates, pine litter and indigenous soil organic matter.