c Biological nitrogen fixation is the primary supply of N to most ecosystems, yet there is considerable uncertainty about how Nfixing bacteria will respond to global change factors such as increasing atmospheric CO 2 and N deposition. Using the nifH gene as a molecular marker, we studied how the community structure of N-fixing soil bacteria from temperate pine, aspen, and sweet gum stands and a brackish tidal marsh responded to multiyear elevated CO 2 conditions. We also examined how N availability, specifically, N fertilization, interacted with elevated CO 2 to affect these communities in the temperate pine forest. Based on data from Sanger sequencing and quantitative PCR, the soil nifH composition in the three forest systems was dominated by species in the Geobacteraceae and, to a lesser extent, Alphaproteobacteria. The N-fixing-bacterial-community structure was subtly altered after 10 or more years of elevated atmospheric CO 2 , and the observed shifts differed in each biome. In the pine forest, N fertilization had a stronger effect on nifH community structure than elevated CO 2 and suppressed the diversity and abundance of N-fixing bacteria under elevated atmospheric CO 2 conditions. These results indicate that N-fixing bacteria have complex, interacting responses that will be important for understanding ecosystem productivity in a changing climate. N itrogen is the most common nutrient limiting productivity in terrestrial ecosystems and enters ecosystems predominantly through bacterial fixation. However, we lack a clear understanding of how N-fixing bacteria respond to climate change drivers or how conserved those responses might be across biomes in a geographic region. Nonagricultural biomes in the eastern United States that experience elevated atmospheric CO 2 and increasing N deposition include hardwood forests to the north, pine forests to the south, and brackish marsh areas along the eastern seaboard. Rising atmospheric CO 2 concentrations and shifting patterns of N deposition can interact and affect N fixation processes in soil (1). To determine if populations of N-fixing bacteria in soils of different biomes showed similarities in composition and in responses to elevated CO 2 , we conducted a systematic survey of soil N-fixing bacterial communities across four biomes in the eastern United States, utilizing long-term, free-air CO 2 enrichment (FACE) experiments (2). One of these field experiments combined elevated CO 2 and N fertilization treatments, allowing us to determine their interactive effects on the N-fixing community in a pine forest in the southeastern United States.Progressive N limitation theory proposes that ecosystems become more N limited with rising CO 2 , which suggests that the continued sequestration of CO 2 in terrestrial biomass will require greater N fixation inputs (3, 4). The increased ecosystem demand for N under elevated CO 2 has been documented after several years of whole-forest CO 2 enrichment (5, 6). N-fixing bacteria, which span many taxonomic groups with high levels of ...
Dormancy is thought to promote biodiversity within microbial communities, but how assembly of the active community responds to changes in environmental conditions is unclear. To measure the active and dormant communities of bacteria and fungi colonizing decomposing litter in maple forests, we targeted ribosomal genes and transcripts across a natural environmental gradient. Within bacterial and fungal communities, the active and dormant communities were phylogenetically distinct, but patterns of phylogenetic clustering varied. For bacteria, active communities were significantly more clustered than dormant communities, while the reverse was found for fungi. The proportion of operational taxonomic units (OTUs) classified as active and the degree of phylogenetic clustering of the active bacterial communities declined with increasing pH and decreasing C/N. No significant correlations were found for the fungal community. The opposing pattern of phylogenetic clustering in dormant and active communities and the differential response of active communities to environmental gradients suggest that dormancy differentially structures bacterial and fungal communities.
Bacillus thuringiensis subsp. kurstaki is applied extensively in North America to control the gypsy moth, Lymantria dispar. Since B. thuringiensis subsp. kurstaki shares many physical and biological properties with Bacillus anthracis, it is a reasonable surrogate for biodefense studies. A key question in biodefense is how long a biothreat agent will persist in the environment. There is some information in the literature on the persistence of Bacillus anthracis in laboratories and historical testing areas and for Bacillus thuringiensis in agricultural settings, but there is no information on the persistence of Bacillus spp. in the type of environment that would be encountered in a city or on a military installation. Since it is not feasible to release B. anthracis in a developed area, the controlled release of B. thuringiensis subsp. kurstaki for pest control was used to gain insight into the potential persistence of Bacillus spp. in outdoor urban environments. Persistence was evaluated in two locations: Fairfax County, VA, and Seattle, WA. Environmental samples were collected from multiple matrices and evaluated for the presence of viable B. thuringiensis subsp. kurstaki at times ranging from less than 1 day to 4 years after spraying. Real-time PCR and culture were used for analysis. B. thuringiensis subsp. kurstaki was found to persist in urban environments for at least 4 years. It was most frequently detected in soils and less frequently detected in wipes, grass, foliage, and water. The collective results indicate that certain species of Bacillus may persist for years following their dispersal in urban environments.
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