Diazotrophic community structure and function in two successional stages of biological soil crusts from the Colorado Plateau and Chihuahuan Desert

Effects of altered temperature and precipitation on desert protozoa associated with biological soil crusts

Biological soil crusts are diverse assemblages of bacteria, cyanobacteria, algae, fungi, lichens, and mosses that cover much of arid land soils. The objective of this study was to quantify protozoa associated with biological soil crusts and test the response of protozoa to increased temperature and precipitation as is predicted by some global climate models. Protozoa were more abundant when associated with cyanobacteria/lichen crusts than with cyanobacteria crusts alone. Amoebae, flagellates, and ciliates originating from the Colorado Plateau desert (cool desert, primarily winter precipitation) declined 50-, 10-, and 100-fold, respectively, when moved in field mesocosms to the Chihuahuan Desert (hot desert, primarily summer rain). However, this was not observed in protozoa collected from the Chihuahuan Desert and moved to the Sonoran desert (hot desert, also summer rain, but warmer than Chihuahuan Desert). Protozoa in culture began to encyst at 37 degrees C. Cysts survived the upper end of daily temperatures (37-55 degrees C), and could be stimulated to excyst if temperatures were reduced to 15 degrees C or lower. Results from this study suggest that cool desert protozoa are influenced negatively by increased summer precipitation during excessive summer temperatures, and that desert protozoa may be adapted to a specific desert's temperature and precipitation regime.

Sphingomonas mucosissima sp. nov. and Sphingomonas desiccabilis sp. nov., from biological soil crusts in the Colorado Plateau, USA

Two bacterial strains, CP173-2(T) and CP1D(T), were isolated from biological soil crusts (BSCs) collected in the Colorado Plateau, USA. Both strains were pigmented, Gram-negative, non-motile rods and produced abundant mucus. They contained C(16 : 0), C(18 : 1)omega7c and C(14 : 0) 2-OH as the predominant cellular fatty acids, ubiquinone-10 as the isoprenoid quinone and sphingoglycolipid. Based on the above characteristics, the isolates were assigned to the family Sphingomonadaceae; 16 rRNA gene signature nucleotides placed them within the genus Sphingomonas. Strains CP173-2(T) and CP1D(T) had a 16S rRNA gene sequence similarity of 96.7 % with each other and 91.6-98.9 % sequence similarity with other species in the genus, indicating that they represent two separate, and possibly novel, species. The closest species to strains CP173-2(T) and CP1D(T) were, respectively, Sphingomonas dokdonensis (98.9 % gene sequence similarity) and Sphingomonas panni (97.9 %). However, strain CP173-2(T) exhibited a DNA-DNA relatedness of only 32.5 % with the type strain of S. dokdonensis. Similarly, the DNA-DNA relatedness between strain CP1D(T) and the type strain of S. panni was only 18 %. Phenotypic characterization supported this low relatedness. On the basis of this evidence, we propose that the new strains represent two novel species, for which the names Sphingomonas mucosissima sp. nov. (with type strain CP173-2(T)=ATCC BAA-1239(T)=DSM 17494(T)) and Sphingomonas desiccabilis sp. nov. (with type strain CP1D(T)=ATCC BAA-1041(T)=DSM 16792(T)) are proposed.

Three distinct clades of cultured heterocystous cyanobacteria constitute the dominant N2-fixing members of biological soil crusts of the Colorado Plateau, USA

The identity of the numerically dominant N(2)-fixing bacteria in biological soil crusts of the Colorado Plateau region and two outlying areas was determined using multiple approaches, to link the environmental diversity of nifH gene sequences to cultured bacterial isolates from the regions. Of the nifH sequence-types detected in soil crusts of the Colorado Plateau, 89% (421/473) were most closely related to nifH signature sequences from cyanobacteria of the order Nostocales. N(2)-fixing cyanobacterial strains were cultured from crusts and their morphotypes, 16S rRNA gene and nifH gene sequences were characterized. The numerically dominant diazotrophs in the Colorado Plateau crusts fell within three clades of heterocystous cyanobacteria. Two clades are well-represented by phylogenetically and morphologically coherent strains, corresponding to the descriptions of Nostoc commune and Scytonema hyalinum, which are widely recognized as important N(2)-fixing components of soil crusts. A third, previously-overlooked clade was represented by a phylogenetically coherent but morphologically diverse group of strains that encompass the morphogenera Tolypothrix and Spirirestis. Many of the strains in each of these groups contained at least two nifH copies that represent different clusters in the nifH environmental survey.

Endolithic photosynthetic communities within ancient and recent travertine deposits in Yellowstone National Park

Molecular and culture based methods were used to survey endolithic, photosynthetic communities from hot spring-formed travertine rocks of various ages, ranging from<10 to greater than 300,000 years. Much of this travertine contained a 1-3-mm-thick greenish band composed mainly of cyanobacteria 1-5 mm below the rock surface. The travertine rocks experienced desiccation in summer and freezing in winter. A total of 83 environmental 16S rRNA gene sequences were obtained from clone libraries and denaturing gradient gel electrophoresis. Small subunit rRNA gene sequences and cell morphology were determined for 36 cyanobacterial culture isolates from these samples. Phylogenetic analysis showed that the 16S rRNA gene sequences fell into 15 distinct clusters, including several novel lineages of cyanobacteria.

Watering, fertilization, and slurry inoculation promote recovery of biological crust function in degraded soils

Biological soil crusts are very sensitive to human-induced disturbances and are in a degraded state in many areas throughout their range. Given their importance in the functioning of arid and semiarid ecosystems, restoring these crusts may contribute to the recovery of ecosystem functionality in degraded areas. We conducted a factorial microcosm experiment to evaluate the effects of inoculation type (discrete fragments vs slurry), fertilization (control vs addition of composted sewage sludge), and watering frequency (two vs five times per week) on the cyanobacterial composition, nitrogen fixation, chlorophyll content, and net CO2 exchange rate of biological soil crusts inoculated on a semiarid degraded soil from SE Spain. Six months after the inoculation, the highest rates of nitrogen fixation and chlorophyll a content were found when the biological crusts were inoculated as slurry, composted sewage sludge was added, and the microcosms were watered five times per week. Net CO2 exchange rate increased when biological crusts were inoculated as slurry and the microcosms were watered five times per week. Denaturing gradient gel electrophoresis fingerprints and phylogenetic analyses indicated that most of the cyanobacterial species already present in the inoculated crust had the capability to spread and colonize the surface of the surrounding soil. These analyses showed that cyanobacterial communities were less diverse when the microcosms were watered five times per week, and that watering frequency (followed in importance by the addition of composted sewage sludge and inoculation type) was the treatment that most strongly influenced their composition. Our results suggest that the inoculation of biological soil crusts in the form of slurry combined with the addition of composted sewage sludge could be a suitable technique to accelerate the recovery of the composition and functioning of biological soil crusts in drylands.

Biogeographical distribution and ecological ranges of benthic cyanobacteria in East Antarctic lakes

For the first time, the cyanobacterial diversity from microbial mats in lakes of Eastern Antarctica was investigated using microscopic and molecular approaches. The present study assessed the biogeographical distribution of cyanobacteria in Antarctica. Five samples were taken from four lakes spanning a range of different ecological environments in Larsemann Hills, Vestfold Hills and Rauer Islands to evaluate the influence of lake characteristics on the cyanobacterial diversity. Seventeen morphospecies and 28 16S rRNA gene-based operational taxonomic units belonging to the Oscillatoriales, Nostocales and Chroococcales were identified. The internal transcribed spacer was evaluated to complement the 16S rRNA gene data and showed similar but more clear-cut tendencies. The molecular approach suggested that potential Antarctic endemic species, including a previously undiscovered diversity, are more abundant than has been estimated by morphological methods. Moreover, operational taxonomic units, also found outside Antarctica, were more widespread over the continent than potential endemics. The cyanobacterial diversity of the most saline lakes was found to differ from the others, and correlations between the sampling depth and the cyanobacterial communities can also be drawn. Comparison with database sequences illustrated the ubiquity of several cyanobacterial operational taxonomic units and their remarkable range of tolerance to harsh environmental conditions.

The community and phylogenetic diversity of biological soil crusts in the Colorado Plateau studied by molecular fingerprinting and intensive cultivation

We studied the bacterial communities in biological soil crusts (BSCs) from the Colorado Plateau by enrichment and cultivation, and by statistically analyzed denaturing gradient gel electrophoresis (DGGE) fingerprinting of environmental 16S rRNA genes, and phylogenetic analyses. Three 500-m-long transects, tens of km apart, consisting of 10 equally spaced samples each, were analyzed. BSC communities consistently displayed less richness (10-32 detectable DGGE bands per sample) and Shannon diversity (2.1-3.3) than typical soil communities, with apparent dominance by few members. In spite of some degree of small-scale patchiness, significant differences in diversity and community structure among transects was detectable, probably related to the degree of crust successional maturity. Phylogenetic surveys indicated that the cyanobacterium Microcoleus vaginatus was dominant, with M. steenstrupii second among phototrophs. Among the 48 genera of nonphototrophs detected, Actinobacteria (particularly Streptomyces spp.) were very common and diverse, with 18 genera and an average contribution to the total 16S rDNA amplificate of 11.8%. beta-Proteobacteria and Bacteriodetes contributed around 10% each; Low-GC Gram-positives, alpha-Proteobacteria, Thermomicrobiales, and Acidobacteria were common (2-5%). However, the second largest contribution was made by deep-branching unaffiliated alleles (12.6%), with some of them representing candidate bacterial divisions. Many of the novel strains isolated are likely new taxa, and some were representatives of the phylotypes detected in the field. The mucoid or filamentous nature of many of these isolates speaks for their role in crust formation.

Detection and quantification of the nifH gene in shoot and root of cucumber plants

A real-time polymerase chain reaction (PCR) method was applied to quantify the nifH gene pool in cucumber shoot and root and to evaluate how nitrogen (N) supply and plant age affect the nifH gene pool. In shoots, the relative abundance of the nifH gene was affected neither by different stages of plant growth nor by N supply. In roots, higher numbers of diazotrophic bacteria were found compared with that in the shoot. The nifH gene pool in roots significantly increased with plant age, and unexpectedly, the pool size was positively correlated with N supply. The relative abundance of nifH gene copy numbers in roots was also positively correlated (r = 0.96) with total N uptake of the plant. The data suggest that real-time PCR-based nifH gene quantification in combination with N-content analysis can be used as an efficient way to perform further studies to evaluate the direct contribution of the N2-fixing plant-colonizing plant growth promoting bacteria to plant N nutrition.Key words: real-time PCR, biological nitrogen fixation, cucumber, N nutrition, plant growth promoting bacteria.

A molecular dawn for biogeochemistry

Biogeochemistry is at the dawn of an era in which molecular advances enable the discovery of novel microorganisms having unforeseen metabolic capabilities, revealing new insight into the underlying processes regulating elemental cycles at local to global scales. Traditionally, biogeochemical inquiry began by studying a process of interest, and then focusing downward to uncover the microorganisms and metabolic pathways mediating that process. With the ability to sequence functional genes from the environment, molecular approaches now enable the flow of inquiry in the opposite direction. Here, we argue that a focus on functional genes, the microorganisms in which they reside, and the interaction of those organisms with the broader microbial community could transform our understanding of many globally important biogeochemical processes.

Composition and function of biological soil crust communities along topographic gradients in grasslands of central interior British Columbia (Chilcotin) and southwestern Yukon (Kluane)

Grasslands in the rainshadow of the Chilcotin (British Columbia) and St. Elias (Kluane, Yukon) mountain ranges of western Canada are characterized by widely spaced clumps of bunchgrass and sage, between which can be found lichen-dominated biological soil crusts (BSC). Our examination of Chilcotin and Kluane grasslands showed differential BSC development along topographic gradients, favoring those sites with lower levels of soil disturbance. Lichen species richness was greatest in upper topographic positions, that is, on valley side terraces in the Chilcotin and esker slopes in Kluane. Common BSC lichens in both grasslands included Acarospora schleicheri, Caloplaca tominii , Collema tenax , Diploschistes muscorum , Fulgensia bracteata , Phaeorrhiza nimbosa , Placidium squamulosum , and Psora decipiens . Chilcotin BSC additionally contained many Cladonia species (e.g., C. carneola , C. chlorophaea , C. pyxidata ), particularly where vegetation had encroached on BSC. The potential for nitrogen fixation by Collema -dominated crusts in Kluane was examined using acetylene reduction assays (ARA) and soil surface microclimate monitoring. ARA activity was highly dependent upon the duration of wetting events, reaching C2H4 levels up to 63 μmol·m–2·h–1after 40 h of hydration. Given the abundance of Collema-dominated crusts in Kluane and the optimal conditions for ARA activity that are reached during wetting–drying transitions, we hypothesized that BSC communities potentially make an important contribution to ecosystem nitrogen budgets. Enrichment in total and mineralizable N, as well as 15N natural abundance values, was consistent with N fixation making an important contribution to soil N pools in these ecosystems. Both Chilcotin and Kluane BSC had similar spongy microstructures that contrasted with the platy microstructures of the underlying surface mineral soils, but only the latter site showed micromorphological evidence of burial of mosses and other BSC components by continuing loess deposition. BSC may have performed similar roles in analogous steppe-like ecosystems that existed under full-glacial conditions in the unglaciated areas of eastern Beringia in Alaska and Yukon.

Microbial biogeography: putting microorganisms on the map

We review the biogeography of microorganisms in light of the biogeography of macroorganisms. A large body of research supports the idea that free-living microbial taxa exhibit biogeographic patterns. Current evidence confirms that, as proposed by the Baas-Becking hypothesis, 'the environment selects' and is, in part, responsible for spatial variation in microbial diversity. However, recent studies also dispute the idea that 'everything is everywhere'. We also consider how the processes that generate and maintain biogeographic patterns in macroorganisms could operate in the microbial world.

The prokaryotic diversity of biological soil crusts in the Sonoran Desert (Organ Pipe Cactus National Monument, AZ)

We studied prokaryotic community structure and composition in biological soil crusts (BSCs) from the Sonoran Desert, and their variability over space and time, using statistically analyzed, PCR-based molecular surveys of environmental 16S rRNA genes. Four sites, tens of km apart, were sampled, 3 times over a 1 year period, collecting 10 duplicate samples every 50 m in each site. Denaturing gradient gel electrophoresis (DGGE) revealed communities much less diverse than those of typical soil assemblages, displaying dominance of some bacterial types. No differences in crust microbial diversity or composition were detected between crusts under plant canopies and those in plant interspaces, indicating a likely crust independence from higher plant resources. However, statistically significant variability with space and time could be detected, and samples within a site were more similar than samples between sites. Both temporal and spatial variability in community composition involved non-dominant members of the community. Extensive sequencing and phylogenetic analysis revealed a large array of bacterial types, many novel. The most common included members of Cyanobacteria, Proteobacteria, Actinobacteria and Acidobacteria. Bacteriodetes, Chloroflexi and Gemmatimonadetes were not seen in high numbers, but were present in all sites, and Deinococci were also detected. Archaea were present, but as minor components. Sonoran BSC communities were distinct in rough compositional character from those in bulk arid soils or agricultural soils, and contained reoccurring, uncultured microbes.

Nickel mine spoils revegetation attempts: effect of pioneer plants on two functional bacterial communities involved in the N-cycle

Nickel mine spoils in New Caledonia represent an extreme environment, rich in nickel and strongly deficient in elementary elements such as carbon and nitrogen. To rehabilitate these sites, revegetation attempts are performed with endemic plant species establishing dinitrogen-fixation symbiosis (Gymnostoma webbianum and Serianthes calycina). As this biological fixation process provides the major source of available nitrogen in this extreme environment, it could be expected that nitrogen cycling would be stimulated. To study the revegetation effect on mine spoils, the effect of the two pioneer plants on the structure and activity of two functional bacterial communities involved in the N-cycle was investigated. nifH and narG genes were used as molecular markers for dinitrogen-fixers and dissimilatory nitrate reducers respectively. In order to assess the influence of the plants on both communities, nine clone libraries were constructed for each targeted gene. Libraries containing 602 and 513 nifH and narG clones, respectively, were screened by restriction fragment length polymorphism (RFLP) analysis. One hundred and forty-one and 78 representative clones from at least all RFLP families containing more than one clone were sequenced from nifH and narG clone libraries respectively. Both pioneer plants modified the diversity and activity of the two functional communities. However, distinct effects were observed depending on the plant species and the community considered. Serianthes calycina strongly selected a diazotroph phylotype and restored the potential activity of both communities. In contrast, G. webbianum selected no particular phylotype and only restored a fixing activity.

The metagenomics of soil

Phylogenetic surveys of soil ecosystems have shown that the number of prokaryotic species found in a single sample exceeds that of known cultured prokaryotes. Soil metagenomics, which comprises isolation of soil DNA and the production and screening of clone libraries, can provide a cultivation-independent assessment of the largely untapped genetic reservoir of soil microbial communities. This approach has already led to the identification of novel biomolecules. However, owing to the complexity and heterogeneity of the biotic and abiotic components of soil ecosystems, the construction and screening of soil-based libraries is difficult and challenging. This review describes how to construct complex libraries from soil samples, and how to use these libraries to unravel functions of soil microbial communities.

Application of a newly developed ARB software-integrated tool for in silico terminal restriction fragment length polymorphism analysis reveals the dominance of a novel pmoA cluster in a forest soil

TRF-CUT, an ARB-implemented tool, was developed to predict in silico the terminal restriction fragments of aligned small-subunit rRNA gene or functional gene sequences. Application of this new tool to perform directed terminal restriction fragment length polymorphism analysis of pmoA products obtained from a forest soil revealed that novel cluster I methanotrophic bacteria were dominant.

Dyadobacter crusticola sp. nov., from biological soil crusts in the Colorado Plateau, USA, and an emended description of the genus Dyadobacter Chelius and Triplett 2000

Bacterial strain CP183-8T was isolated from biological soil crusts collected in the Colorado Plateau, USA. Cells of this strain were aerobic, non-motile, Gram-negative, psychrotolerant and formed beaded chains in the stationary growth phase. They contained C16 : 1 ω5c and C16 : 1 ω7c as major fatty acids. 16S rRNA gene sequence analysis assigned the strain to the genus Dyadobacter. However, it shared a sequence similarity of only 95·88 % with the type strain of Dyadobacter fermentans, NS114T. Because it also exhibited a significant number of phenotypic and chemotaxonomic differences from D. fermentans, it is described as a novel second species in the genus Dyadobacter, with the name Dyadobacter crusticola sp. nov. The type strain is CP183-8T (=DSM 16708T=ATCC BAA-1036T).

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 NH 3 -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.