Philip W. Ramsey

SOIL FUNGI ALTER INTERACTIONS BETWEEN THE INVADER CENTAUREA MACULOSA AND NORTH AMERICAN NATIVES

Soil microbes may affect the way exotic invasive plants interact with native neighbors. We investigated the effects of soil fungi on interactions between the invasive weed Centaurea maculosa (spotted knapweed) and six species native to the intermountain prairies of the northwestern United States. We also compared the effect of C. maculosa on the composition of the soil microbial community to that of the native species. In the field, fungicide (Benomyl) reduced AM mycorrhizal colonization of C. maculosa roots by >80%. Fungicide did not significantly reduce non-AM fungi. When grown alone, the biomass of C. maculosa was not affected by the fungicide application. However, depending on the combination of native competitor and fungicide, C. maculosa biomass varied from 10-fold decreases to 1.9-fold increases. In untreated soils, C. maculosa grew larger in the presence of Festuca idahoensis or Koeleria cristata than when alone. When fungicide was applied these positive effects of Festuca and Koeleria on C. maculosa did not occur. A third native grass, Pseudoroegneria spicata, had much stronger competitive effects on C. maculosa than Festuca or Koeleria, and fungicide reduced the competitive effects of Pseudoroegneria. Fungicide increased Centaurea biomass when competing with the forb Gallardia aristata. However, fungicide did not affect the way two other forbs; Achillea millefolium and Linum lewisii, interacted with C. maculosa. Rhizosphere microbial communities in the root zones of the three native bunchgrass species differed from that of C. maculosa. However, despite the strong effects of soil fungi in field interactions and differences in microbial community composition, soil biota from different plant rhizospheres did not affect the growth of C. maculosa in the absence of native competitors in greenhouse experiments. Our results suggest that successful invasions by exotic plant species can be affected by complex and often beneficial effects of local soil microbial communities. These effects were not manifest as simple direct effects, but become apparent only when native plants, invasive plants, and soil microbial communities were interacting at the same time.

Glomalin, an arbuscular-mycorrhizal fungal soil protein, responds to land-use change

Glomalin is a soil proteinaceous substance produced by arbuscular mycorrhizal fungi. Most of the information available concerning this protein has been collected in relation to its role in soil aggregation. In this study, we explored the distribution of glomalin across soil horizons, decomposition of glomalin, and relationship with soil C and N in an agricultural field, a native forest, and an afforested system. Glomalin was present in A, B, and C horizons in decreasing concentrations. Land-use type significantly affected glomalin concentrations (mg cm−3), with native forest soils having the highest concentrations of the three land-use types in both A and B horizons. In terms of glomalin stocks (Mg ha−1), calculated based on corrected horizon weights, the agricultural area was significantly lower than both afforested and native forest areas. As measured after a 413 day laboratory soil incubation, glomalin was least persistent in the A horizon of the afforested area.. In agricultural soils and native soils, ca. 50% of glomalin was still remaining after this incubation, indicating that some glomalin may be in the slow or recalcitrant soil C fraction. Comparison of glomalin decomposition with CO2-C respired during incubation indicates that glomalin makes a large contribution to active soil organic C pools. Soil C and N were highly correlated with glomalin across all soils and within each land-use type, indicating that glomalin may be under similar controls as soil C. Our results show that glomalin may be useful as an indicator of land-use change effects on deciduous forest soils.

Differences in Hyporheic-Zone Microbial Community Structure along a Heavy-Metal Contamination Gradient

ABSTRACT The hyporheic zone of a river is nonphotic, has steep chemical and redox gradients, and has a heterotrophic food web based on the consumption of organic carbon entrained from downwelling surface water or from upwelling groundwater. The microbial communities in the hyporheic zone are an important component of these heterotrophic food webs and perform essential functions in lotic ecosystems. Using a suite of methods (denaturing gradient gel electrophoresis, 16S rRNA phylogeny, phospholipid fatty acid analysis, direct microscopic enumeration, and quantitative PCR), we compared the microbial communities inhabiting the hyporheic zone of six different river sites that encompass a wide range of sediment metal loads resulting from large base-metal mining activity in the region. There was no correlation between sediment metal content and the total hyporheic microbial biomass present within each site. However, microbial community structure showed a significant linear relationship with the sediment metal loads. The abundances of four phylogenetic groups (groups I, II, III, and IV) most closely related to α-, β-, and γ-proteobacteria and the cyanobacteria, respectively, were determined. The sediment metal content gradient was positively correlated with group III abundance and negatively correlated with group II abundance. No correlation was apparent with regard to group I or IV abundance. This is the first documentation of a relationship between fluvially deposited heavy-metal contamination and hyporheic microbial community structure. The information presented here may be useful in predicting long-term effects of heavy-metal contamination in streams and provides a basis for further studies of metal effects on hyporheic microbial communities.

Severe plant invasions can increase mycorrhizal fungal abundance and diversity

Invasions by non-native plants can alter ecosystem functions and reduce native plant diversity, but relatively little is known about their effect on belowground microbial communities. We show that invasions by knapweed (Centaurea stoebe) and leafy spurge (Euphorbia esula, hereafter spurge)—but not cheatgrass (Bromus tectorum)—support a higher abundance and diversity of symbiotic arbuscular mycorrhizal fungi (AMF) than multi-species native plant communities. The higher AMF richness associated with knapweed and spurge is unlikely due to a co-invasion by AMF, because a separate sampling showed that individual native forbs hosted a similar AMF abundance and richness as exotic forbs. Native grasses associated with fewer AMF taxa, which could explain the reduced AMF richness in native, grass-dominated communities. The three invasive plant species harbored distinct AMF communities, and analyses of co-occurring native and invasive plants indicate that differences were partly driven by the invasive plants and were not the result of pre-invasion conditions. Our results suggest that invasions by mycotrophic plants that replace poorer hosts can increase AMF abundance and richness. The high AMF richness in monodominant plant invasions also indicates that the proposed positive relationship between above and belowground diversity is not always strong. Finally, the disparate responses among exotic plants and consistent results between grasses and forbs suggest that AMF respond more to plant functional group than plant provenance.

Structure and seasonal dynamics of hyporheic zone microbial communities in free-stone rivers of the western United States

The hyporheic zone of a river is characterized by being nonphotic, exhibiting chemical/redox gradients, and having a heterotrophic food web based on the consumption of organic carbon entrained from surface waters. Hyporheic microbial communities constitute the base of food webs in these environments and are important for maintaining a functioning lotic ecosystem. While microbial communities of rivers dominated by fine-grained sediments are relatively well studied, little is known about the structure and seasonal dynamics of microbial communities inhabiting the predominantly gravel and cobble hyporheic zones of rivers of the western United States. Here, we present the first molecular analysis of hyporheic microbial communities of three different stream types (based on mean base discharge, substratum type, and drainage area), in Montana. Utilizing 16S rDNA phylogeny, DGGE pattern analysis, and qPCR, we have analyzed the prokaryotic communities living on the 1.7 to 2.36 mm grain-size fraction of hyporheic sediments from three separate riffles in each stream. DGGE analysis showed clear seasonal community patterns, indicated similar community composition between different riffles within a stream (95.6–96.6% similarity), and allowed differentiation between communities in different streams. Each river supported a unique complement of species; however, several phylogenetic groups were conserved between all three streams including Pseudomonads and members of the genera Aquabacterium, Rhodoferax, Hyphomicrobium, and Pirellula. Each group showed pronounced seasonal trends in abundance, with peaks during the Fall. The Hyphomicrobium group was numerically dominant throughout the year in all three streams. This work provides a framework for investigating the effects of various environmental factors and anthropogenic effects on microbial communities inhabiting the hyporheic zone.

Relationship Between Communities and Processes; New Insights from a Field Study of a Contaminated Ecosystem

We used a 93-year-old mine waste contamination gradient in alluvial soil to explore the relationship between ecosystem level functioning and community structure in a chronically stressed ecosystem. The sensitivity of broad functional parameters (in situ soil respiration, microbial biomass, above and below ground plant biomass) and microbial diversity [phospholipid fatty acid (PLFA) abundance and richness] were compared. Functional responses were linear with respect to contaminants while thresholds were detected in the community structural response to contamination along the gradient. For example, in situ soil respiration was negatively and linearly correlated to contamination concentration (R = -0.783, P < 0.01), but changes in microbial community structure only became evident where contaminant concentrations were greater than 28 times above background levels. Our results suggest that functional redundancy does not prevent depression of ecosystem function in the long-term.

Seasonal dynamics of shallow-hyporheic-zone microbial community structure along a heavy-metal contamination gradient.

ABSTRACT Heavy metals contaminate numerous freshwater streams and rivers worldwide. Previous work by this group demonstrated a relationship between the structure of hyporheic microbial communities and the fluvial deposition of heavy metals along a contamination gradient during the fall season. Seasonal variation has been documented in microbial communities in numerous terrestrial and aquatic environments, including the hyporheic zone. The current study was designed to assess whether relationships between hyporheic microbial community structure and heavy-metal contamination vary seasonally by monitoring community structure along a heavy-metal contamination gradient for more than a year. No relationship between total bacterial abundance and heavy metals was observed (R2 = 0.02, P = 0.83). However, denaturing gradient gel electrophoresis pattern analysis indicated a strong and consistent linear relationship between the difference in microbial community composition (populations present) and the difference in the heavy metal content of hyporheic sediments throughout the year (R2 = 0.58, P < 0.001). Correlations between heavy-metal contamination and the abundance of four specific phylogenetic groups (most closely related to the α, β, and γ-proteobacteria and cyanobacteria) were apparent only during the fall and early winter, when the majority of organic matter is deposited into regional streams. These seasonal data suggest that the abundance of susceptible populations responds to heavy metals primarily during seasons when the potential for growth is highest.

Do native and invasive plants differ in their interactions with arbuscular mycorrhizal fungi? A meta‐analysis

Summary Divergent hypotheses have been proposed that suggest plant invasions either enhance or degrade the mutualism between plants and arbuscular mycorrhizal (AM) fungi, but their relative support remains unknown. We conducted a meta-analysis using 67 publications, involving 70 native and 55 invasive plant species to assess support for the enhanced mutualism hypothesis, the degraded mutualism hypothesis and an alternative hypothesis that factors other than invasive status (such as plant functional group) better predict AM function following invasion. We used multiple measurements to test these hypotheses: AM fungal colonization, growth responses to AM fungi and AM fungal-mediated shifts in competitive interactions among native and invasive plants. Additionally, we assessed whether invasive plants alter AM associations in native plants and whether native and invasive plants host different AM fungal abundances and communities. Arbuscular mycorrhizal fungal colonization (%) and average growth responses did not differ between native and invasive plants. However, growth responses (±) were dampened among invasive plants, and the positive correlation between AM fungal colonization and growth response in native plants was absent in invasive plants. Rather than plant invasive status, plant functional group was a significant explanatory factor; forbs were generally more colonized and exhibited positive growth responses (when grown alone and in competition), whereas grass responses were neutral to negative. Arbuscular mycorrhizal fungal abundance (measured by percentage colonization, extraradical hyphal and spore densities, as well as neutral lipid fatty acid and glomalin concentrations) did not differ between native and invasive plants, but invasive plants hosted different AM fungal communities in 78% of studies. AM fungal colonization of native plants was lower when grown with, or after, invasive plants, likely due to the prevalence of non-mycorrhizal plants in studies of neighbour and legacy effects. Synthesis. Neither the degraded nor the enhanced mutualism hypothesis was supported, suggesting that invasions do not select for directional shifts in AM associations. Instead, our results indicate that AM fungi are most likely to influence invasion trajectories when native and invasive plants belong to different functional groups.

Pedobacter nyackensis sp. nov., Pedobacter alluvionis sp. nov. and Pedobacter borealis sp. nov., isolated from Montana flood-plain sediment and forest soil

Three Gram-negative, rod-shaped, non-spore-forming eubacterial strains were isolated in western Montana, USA, and subjected to taxonomic studies. Strains NWG-II14(T) and NWER-II11(T) were isolated from hyporheic sediments of a large alluvial flood plain, whereas strain G-1(T) was isolated from a conifer forest soil. On the basis of 16S rRNA gene sequence similarity, strains NWG-II14(T), NWER-II11(T) and G-1(T) were shown to belong to the family Sphingobacteriaceae and are most closely related to various species of the genus Pedobacter. The results of molecular, physiological and biochemical tests allowed genotypic and phenotypic differentiation of these three strains from 23 Pedobacter species with validly published names. The three isolates therefore represent novel species, for which the names Pedobacter nyackensis sp. nov. (type strain NWG-II14(T) =DSM 19625(T) =LMG 24260(T)), Pedobacter alluvionis sp. nov. (type strain NWER-II11(T) =DSM 19624(T) =LMG 24258(T)) and Pedobacter borealis sp. nov. (type strain G-1(T) =DSM 19626(T) =LMG 24259(T)) are proposed.

Determining Rates of Change and Evaluating Group-Level Resiliency Differences in Hyporheic Microbial Communities in Response to Fluvial Heavy-Metal Deposition

ABSTRACT Prior field studies by our group have demonstrated a relationship between fluvial deposition of heavy metals and hyporheic-zone microbial community structure. Here, we determined the rates of change in hyporheic microbial communities in response to heavy-metal contamination and assessed group-level differences in resiliency in response to heavy metals. A controlled laboratory study was performed using 20 flowthrough river mesocosms and a repeated-measurement factorial design. A single hyporheic microbial community was exposed to five different levels of an environmentally relevant metal treatment (0, 4, 8, 16, and 30% sterilized contaminated sediments). Community-level responses were monitored at 1, 2, 4, 8, and 12 weeks via denaturing gradient gel electrophoresis and quantitative PCR using group-specific primer sets for indigenous populations most closely related to the α-, β-, and γ-proteobacteria. There was a consistent, strong curvilinear relationship between community composition and heavy-metal contamination (R2 = 0.83; P < 0.001), which was evident after only 7 days of metal exposure (i.e., short-term response). The abundance of each phylogenetic group was negatively affected by the heavy-metal treatments; however, each group recovered from the metal treatments to a different extent and at a unique rate during the course of the experiment. The structure of hyporheic microbial communities responded rapidly and at contamination levels an order of magnitude lower than those shown to elicit a response in aquatic macroinvertebrate assemblages. These studies indicate that hyporheic microbial communities are a sensitive and useful indicator of heavy-metal contamination in streams.