Temporal Dynamics of Soil Virus and Bacterial Populations in Agricultural and Early Plant Successional Soils.

As reported in many aquatic environments, recent studies in terrestrial ecosystems implicate a role for viruses in shaping the structure, function, and evolution of prokaryotic soil communities. However, given the heterogeneity of soil and the physical constraints (i.e., pore-scale hydrology and solid-phase adsorption of phage and host cells) on the mobility of viruses and bacteria, phage-host interactions likely differ from those in aquatic systems. In this study, temporal changes in the population dynamics of viruses and bacteria in soils under different land management practices were examined. The results showed that bacterial abundance was significantly and positively correlated to both virus and inducible prophage abundance. Bacterial and viral abundance were also correlated with soil organic carbon and nitrogen content as well as with C:N ratio. The seasonal variability in viral abundance increased with soil organic carbon content. The prokaryotic community structure was influenced more by land use than by seasonal variation though considerable variation was evident in the early plant successional and grassland sites. The free extracellular viral communities were also separated by land use, and the forest soil viral assemblage exhibiting the most seasonal variability was more distinct from the other sites. Viral assemblages from the agricultural soils exhibited the least seasonal variability. Similar patterns were observed for inducible prophage viral assemblages. Seasonal variability of viral assemblages was greater in mitomycin-C (mitC) induced prophages than in extracellular viruses irrespective of land use and management. Taken together, the data suggest that soil viral production and decay are likely balanced but there was clear evidence that the structure of viral assemblages is influenced by land use and by season.

Dynamics of autochthonous soil viral communities parallels dynamics of host communities under nutrient stimulation.

Viruses are highly abundant in soils with their numbers exceeding those of cooccurring bacterial cells by 10- to over 1000-fold. Water and organic matter content influence the magnitude of the viral-to-bacterial ratio in soils; thus, ecosystem type and land use shape interactions between viral and host microbial communities in soils. Less understood are the shorter term interactions between viral and host communities that ultimately maintain the large viral standing stock within soils. This study examined short-term dynamics of viral and bacterial communities in soils to determine whether the growth of soil bacterial communities results in the production of soil viruses, and if viral community responses occur within specific populations. In microcosms amended with different carbon sources, increases in viral abundance (VA) accompanied increases in bacterial abundance (BA) and bacterial respiration rate (BRR). The timing and intensity of increases in BA, VA and BRR were different across C sources suggesting differences in the predominant mode of viral replication within growth-stimulated bacterial populations. Moreover, compositional changes occurred in soil bacterial and viral communities indicating that new viral production arose from a subset of host populations. To our knowledge, these are the first observations of soil viral populations responding to short-term changes in soil bacterial communities.

Microbial sulfur transformations in sediments from Subglacial Lake Whillans.

Diverse microbial assemblages inhabit subglacial aquatic environments. While few of these environments have been sampled, data reveal that subglacial organisms gain energy for growth from reduced minerals containing nitrogen, iron, and sulfur. Here we investigate the role of microbially mediated sulfur transformations in sediments from Subglacial Lake Whillans (SLW), Antarctica, by examining key genes involved in dissimilatory sulfur oxidation and reduction. The presence of sulfur transformation genes throughout the top 34 cm of SLW sediments changes with depth. SLW surficial sediments were dominated by genes related to known sulfur-oxidizing chemoautotrophs. Sequences encoding the adenosine-5'-phosphosulfate (APS) reductase gene, involved in both dissimilatory sulfate reduction and sulfur oxidation, were present in all samples and clustered into 16 distinct operational taxonomic units. The majority of APS reductase sequences (74%) clustered with known sulfur oxidizers including those within the "Sideroxydans" and Thiobacillus genera. Reverse-acting dissimilatory sulfite reductase (rDSR) and 16S rRNA gene sequences further support dominance of "Sideroxydans" and Thiobacillus phylotypes in the top 2 cm of SLW sediments. The SLW microbial community has the genetic potential for sulfate reduction which is supported by experimentally measured low rates (1.4 pmol cm(-3)d(-1)) of biologically mediated sulfate reduction and the presence of APS reductase and DSR gene sequences related to Desulfobacteraceae and Desulfotomaculum. Our results also infer the presence of sulfur oxidation, which can be a significant energetic pathway for chemosynthetic biosynthesis in SLW sediments. The water in SLW ultimately flows into the Ross Sea where intermediates from subglacial sulfur transformations can influence the flux of solutes to the Southern Ocean.

Direct assessment of viral diversity in soils by random PCR amplification of polymorphic DNA.

Viruses are the most abundant and diverse biological entities within soils, yet their ecological impact is largely unknown. Defining how soil viral communities change with perturbation or across environments will contribute to understanding the larger ecological significance of soil viruses. A new approach to examining the composition of soil viral communities based on random PCR amplification of polymorphic DNA (RAPD-PCR) was developed. A key methodological improvement was the use of viral metagenomic sequence data for the design of RAPD-PCR primers. This metagenomically informed approach to primer design enabled the optimization of RAPD-PCR sensitivity for examining changes in soil viral communities. Initial application of RAPD-PCR viral fingerprinting to soil viral communities demonstrated that the composition of autochthonous soil viral assemblages noticeably changed over a distance of meters along a transect of Antarctic soils and across soils subjected to different land uses. For Antarctic soils, viral assemblages segregated upslope from the edge of dry valley lakes. In the case of temperate soils at the Kellogg Biological Station, viral communities clustered according to land use treatment. In both environments, soil viral communities changed along with environmental factors known to shape the composition of bacterial host communities. Overall, this work demonstrates that RAPD-PCR fingerprinting is an inexpensive, high-throughput means for addressing first-order questions of viral community dynamics within environmental samples and thus fills a methodological gap between narrow single-gene approaches and comprehensive shotgun metagenomic sequencing for the analysis of viral community diversity.

Acyl-homoserine lactones can induce virus production in lysogenic bacteria: an alternative paradigm for prophage induction.

Prophage typically are induced to a lytic cycle under stressful environmental conditions or when the host's survival is threatened. However, stress-independent, spontaneous induction also occurs in nature and may be cell density dependent, but the in vivo signal(s) that can trigger induction is unknown. In the present study, we report that acyl-homoserine lactones (AHL), the essential signaling molecules of quorum sensing in many gram-negative bacteria, can trigger phage production in soil and groundwater bacteria. This phenomenon also was operative in a lambda lysogen of Escherichia coli. In model coculture systems, we monitored the real-time AHL production from Pseudomonas aeruginosa PAO1 using an AHL bioluminescent sensor and demonstrated that lambda-prophage induction in E. coli was correlated with AHL production. As a working model in E. coli, we show that the induction responses of lambda with AHL remained unaffected when recA was deleted, suggesting that this mechanism does not involve an SOS response. In the same lambda lysogen we also demonstrated that sdiA, the AHL receptor, and rcsA, a positive transcriptional regulator of exopolysaccharide synthesis, are involved in the AHL-mediated induction process. These findings relate viral reproduction to chemical signals associated with high host cell abundance, suggesting an alternative paradigm for prophage induction.

Prevalence of lysogeny among soil bacteria and presence of 16S rRNA and trzN genes in viral-community DNA.

Bacteriophages are very abundant in the biosphere, and viral infection is believed to affect the activity and genetic diversity of bacterial communities in aquatic environments. Lysogenic conversion, for example, can improve host fitness and lead to phage-mediated horizontal gene transfer. However, little is known about lysogeny and transduction in the soil environment. In this study we employed atrazine-impregnated Bio-Sep beads (a cell immobilization matrix) to sample active microbiota from soils with prior pesticide exposure history. Once recovered from soil, the bead communities were induced with mitomycin C (MC), and viral and bacterial abundances were determined to evaluate the incidence of inducible prophage in soil bacteria. The inducible fraction calculated within bead communities was high (ca. 85%) relative to other studies in aquatic and sedimentary environments. Moreover, the bacterial genes encoding 16S rRNA and trzN, a chlorohydrolase gene responsible for dehalogenation of atrazine, were detected by PCR in the viral DNA fraction purified from MC-induced bead communities. A diverse collection of actinobacterial 16S rRNA gene sequences occurred within the viral DNA fraction of induced, water-equilibrated beads. Similar results were observed in induced atrazine-equilibrated beads, where 77% of the cloned sequences were derived from actinobacterial lineages. Heterogeneous 16S rRNA gene sequences consisting of fragments from two different taxa were detected in the clone libraries. The results suggest that lysogeny is a prevalent reproductive strategy among soil bacteriophages and that the potential for horizontal gene transfer via transduction is significant in soil microbial communities.

Metagenomic characterization of Chesapeake Bay virioplankton.

Viruses are ubiquitous and abundant throughout the biosphere. In marine systems, virus-mediated processes can have significant impacts on microbial diversity and on global biogeocehmical cycling. However, viral genetic diversity remains poorly characterized. To address this shortcoming, a metagenomic library was constructed from Chesapeake Bay virioplankton. The resulting sequences constitute the largest collection of long-read double-stranded DNA (dsDNA) viral metagenome data reported to date. BLAST homology comparisons showed that Chesapeake Bay virioplankton contained a high proportion of unknown (homologous only to environmental sequences) and novel (no significant homolog) sequences. This analysis suggests that dsDNA viruses are likely one of the largest reservoirs of unknown genetic diversity in the biosphere. The taxonomic origin of BLAST homologs to viral library sequences agreed well with reported abundances of cooccurring bacterial subphyla within the estuary and indicated that cyanophages were abundant. However, the low proportion of Siphophage homologs contradicts a previous assertion that this family comprises most bacteriophage diversity. Identification and analyses of cyanobacterial homologs of the psbA gene illustrated the value of metagenomic studies of virioplankton. The phylogeny of inferred PsbA protein sequences suggested that Chesapeake Bay cyanophage strains are endemic in that environment. The ratio of psbA homologous sequences to total cyanophage sequences in the metagenome indicated that the psbA gene may be nearly universal in Chesapeake Bay cyanophage genomes. Furthermore, the low frequency of psbD homologs in the library supports the prediction that Chesapeake Bay cyanophage populations are dominated by Podoviridae.

Molecular phylogenetic exploration of bacterial diversity in a Bakreshwar (India) hot spring and culture of Shewanella-related thermophiles.

The bacterial diversity of a hot spring in Bakreshwar, India, was investigated by a culture-independent approach. 16S ribosomal DNA clones derived from the sediment samples were found to be associated with gamma-Proteobacteria, cyanobacteria, and green nonsulfur and low-GC gram-positive bacteria. The first of the above phylotypes cobranches with Shewanella, a well-known iron reducer. This phylogenetic correlation has been exploited to develop culture conditions for thermophilic iron-reducing microorganisms.