The Alpine Cushion Plant Silene acaulis as Foundation Species: A Bug’s-Eye View to Facilitation and Microclimate

Alpine ecosystems are important globally with high levels of endemic and rare species. Given that they will be highly impacted by climate change, understanding biotic factors that maintain diversity is critical. Silene acaulis is a common alpine nurse plant shown to positively influence the diversity and abundance of organisms–predominantly other plant species. The hypothesis that cushion or nurse plants in general are important to multiple trophic levels has been proposed but rarely tested. Alpine arthropod diversity is also largely understudied worldwide, and the plant-arthropod interactions reported are mostly negative, that is,. herbivory. Plant and arthropod diversity and abundance were sampled on S. acaulis and at paired adjacent microsites with other non-cushion forming vegetation present on Whistler Mountain, B.C., Canada to examine the relative trophic effects of cushion plants. Plant species richness and abundance but not Simpson’s diversity index was higher on cushion microsites relative to other vegetation. Arthropod richness, abundance, and diversity were all higher on cushion microsites relative to other vegetated sites. On a microclimatic scale, S. acaulis ameliorated stressful conditions for plants and invertebrates living inside it, but the highest levels of arthropod diversity were observed on cushions with tall plant growth. Hence, alpine cushion plants can be foundation species not only for other plant species but other trophic levels, and these impacts are expressed through both direct and indirect effects associated with altered environmental conditions and localized productivity. Whilst this case study tests a limited subset of the membership of alpine animal communities, it clearly demonstrates that cushion-forming plant species are an important consideration in understanding resilience to global changes for many organisms in addition to other plants

Metagenomic investigation of Dehalococcoidia used for bioremediation of groundwater and soil contaminated with chlorinated ethenes and ethanes

Chlorinated aliphatic hydrocarbons (CAHs) such as vinyl chloride (VC) and perchloroethene (PCE) are persistent, toxic groundwater and soil pollutants. Bioremediation using mixed microbial cultures such as KB-1 and WBC-2 has proven to be a technically viable and economically feasible remediation strategy. These cultures include members of the Dehalococcoides and Dehalogenimonas, genera from the class Dehalococcoidia, who use reductive dehalogenase enzymes (RDases) to dechlorinate CAHs in an obligatory respiratory process. Metagenomic DNA sequencing was conducted on the KB-1 and WBC-2 enrichment cultures to describe the diversity of microorganisms living in the cultures, and to close genomes of key organisms. Ten new complete genomes of Dehalococcoidia are presented here. Their reductive dehalogenase genes and their evolutionary history are described. Dehalococcoides mccartyi in KB-1 was used as an example to study mobile DNA, with a particular focus on the possible exchange of reductive dehalogenase genes. Extrachromosomal circular elements were discovered in D. mccartyi, and found to replicate in a circular intermediate state separate from the genome. Simultaneously, the VC reductase gene (vcrA)-containing genomic island was also found to periodically replicate in a circular intermediate state. Twelve new prophages and their interaction with CRISPR systems carried by D. mccartyi were described. Additionally, a reductive dehalogenase from Dehalogenimonas was identified and biochemically characterized using a combination of blue-native polyacrylamide gel electrophoresis followed by enzymatic assays and peptide sequencing. In summary, this work has nearly doubled the number of Dehalococcoidia genomes available in NCBI, has led to the discovery of new dehalogenases, new types of mobile DNA, and has begun to elucidate a mechanism for lateral gene transfer in D. mccartyi. Together this knowledge will help better manage contaminated sites and design more effective bioremediation approaches.Ph.D

Female <i>Silene acaulis</i> in bloom on Whistler Mountain.

<p>Photo taken 7-21-2010, field of view, 25×15 cm.</p

Three space ordination plots from detrended correspondence analyses for plant and arthropod richness sampled at three alpine sites on cushions and paired open non-vegetated microsites.

<p>Black points represent cushion microsites while open grey points show open microsites.</p

Rarefaction curves for plant and arthropod species sampled on alpine cushion plants and paired open non-cushion plant vegetation.

<p>Mao Tau estimator and +/−1 s.e. are shown.</p

Detrended correspondence analyses for plant and arthropod richness patterns.

<p>Bold denotes significant correlations between factors and ordination axes at p≤0.05, and n/a indicates no relationship. See text for description of factors.</p

Microclimatic profiles on alpine cushion plants and paired open non-cushion vegetation microsites (n = 12) throughout the 2010 growing season.

<p>The mean relative humidity (%) and temperatures (°C) for cushions are shown by the light grey lines (mean ±2 σ). Rain events are shown as the amount of precipitation (mm).</p

The mean community composition estimates for plants and arthropods associated with cushion plants and open non-cushion vegetated microsites.

<p>The mean +/−1 s.e. are denoted (significant Tukey post hoc contrasts denoted by different letters). Please see text for full details of response variables.</p

A description of the study sites selected on Whistler Mountain used to test for the effects by <i>Silene acaulis</i> on the plant and invertebrate communities.

<p>Vegetation and arthropod sampling were conducted at sites 1–3 whilst the 4^th site was used for microclimatic instrumentation.</p

A summary of the GLMMs used to test the importance of treatment (cushion-open) and physical factors on estimates of alpine plant community composition.

<p>Richness is the number of species, abundance the total number of individuals, and diversity the Simpson’s index. Tukey post hoc contrasts were used to assess categorical, within factor level differences (i.e. C  =  cushion or Op  =  open) and regressions used for continuous (+ indicates significant positive relationship). The degrees of freedom listed refer to the specific factor (DF_model = 14). Bold denotes significance at p<0.05. The mean Rii is reported +/−1 s.e. to show strength of effect.</p