Microbial associated plant growth and heavy metal accumulation to improve phytoextraction of contaminated soils

Abstract

Utilizing plants to remediate heavy metal contaminated soils, a process known as phytoextraction, offers many advantages but has yet to reach levels of efficiency that would make the strategy economically viable. Inoculation of the plant rhizosphere with microorganisms is an established route to improving phytoextraction efficiency. In general, microorganisms can improve phytoextraction by increasing the availability of heavy metals to the plant and by increasing plant biomass. This review uses a meta-analysis of the results from 103 microbialaugmented phytoextraction studies to examine if one of these microbial mechanisms has a greater potential to positively impact phytoextraction. Trends surrounding the use of heavy metal-accumulating versus non-heavy-metal-accumulating plants in phytoextraction are discussed. Microbially induced improvements in the accumulation of heavy metals in plant biomass, a focus of several studies, are always coincident with enhanced net phytoextraction. However, microbial treatments that improved plant biomass are more prevalent in the literature and account for a larger number of studies that reported improved phytoextraction, particularly in non-heavy-metal-accumulating plants. The experimental findings emerging from the literature that implicate specific microbial processes in improving phytoextraction are briefly reviewed and used to underline trends observed from the meta-analysis that indicate future directions regarding the use of microorganisms to improve phytoextraction efficiency.

Bacterial and fungal communities are differentially modified by melatonin in agricultural soils under abiotic stress

“…PCR products were examined by gel electrophoresis on a 1.5% agarose gel matrix and imaged by fluorescence under UV light. Bacterial and fungal community analyses by ARISA were conducted by Australian Genome Research Facility as per Wood et al (2016) (Wood et al, 2016). This involved the separation of amplicons, described as operation taxonomic units (OTUs), by capillary electrophoresis.…”

“…Data were normalized in the statistical software R version 3.3.2 (The R Foundation for Statistical Computing, Boston, USA), with bin sizes of 3 and 4 selected for bacteria and fungi respectively (Ramette, 2009, Butterly et al, 2016). The following analyses were conducted in Primer-E v6 (Quest Research Ltd., Auckland, New Zealand), with treatments considered as fixed factors and microbial responses analysed for each soil separately (Chow et al, 2013, Wood et al, 2016): 1) Bray-Curtis similarity for microbial communities; 2) SIMPER analysis to identify the contribution of individual Operational Taxonomic Units (OTUs) to (dis)similarity between replicates or different treatments; 3) DIVERSE to determine Shannon diversity index (H’) for sample data; 4) Permutational multivariate analysis of variance (PERMANOVA) to determine treatment effect (melatonin, stressors: cadmium and Salt) on microbial assemblages for each soil at various concentrations (High, Low and Zero). Monte Carlo statistical analysis was conducted to determine if individual treatment combinations had statistically significant effects on community compositions (Van Wijngaarden et al, 1995).…”

“…Bacterial and fungal biomasses were measured by quantitative PCR (qPCR) on DNA of the 16s rRNA and ITS region respectively. Bacterial communities were assessed using primer pairs 1114f (5’-CGG CAA CGA GCG CAA CCC-3’) −1275r (5’-CCA TTG TAG CAC GTG TGT AGC C-3’), and fungal communities were assessed using ITS1F (5’-TCC GTA GGT GAA CCT GCG G-3’) −5.8Sr (5’-CGC TGC GTT CTT CAT CG-3’) primer (Wood et al, 2016). Each treatment consisted of four biological replicates each quantified in technical triplicate using a CFX Connect Real-Time PCR Detection System (BioRad).…”

“…Soil microbial communities have an essential role in maintaining ecosystem health by direct exchange of nutrients and minerals with plants within the rhizosphere, as well as by providing nourishment to plants indirectly via nutrient cycling of organic matter (Wall and Virginia, 1999, Yao et al, 2000, Kirk et al, 2004, Wahid et al, 2016). Microbial community structures in soil can be altered by various abiotic stresses such as soil contaminants and salinity (Badri and Vivanco, 2009, Wood et al, 2016, Geisseler et al, 2017). Anthropogenic activities, such as phosphorus fertiliser treatment, may introduce the highly toxic heavy metal cadmium (e.g.…”

Competitive Traits Are More Important than Stress-Tolerance Traits in a Cadmium-Contaminated Rhizosphere: A Role for Trait Theory in Microbial Ecology

“…Understanding how rhizosphere-microbial communities assemble and are maintained temporally is a major goal in microbial ecology that is relevant to a wide range of disciplines including: plant-pathogen interactions (Pieterse et al, 2014 ), agriculture (Welbaum et al, 2004 ), community ecology (Fitzsimons and Miller, 2010 ), microbiome research (Costello et al, 2012 ), and phytoremediation (Thijs et al, 2016 ). In the field of phytoremediation, where plants are utilized to remove contaminants from soils, manipulation of the rhizosphere community via the addition of beneficial microorganisms has been used to improve remediation rates (De Souza et al, 1999 ; Ma et al, 2009 ; Liu et al, 2015 ; Wood et al, 2016a ). However, to fully exploit this potential, a greater understanding of factors driving community assembly in the rhizosphere during phytoremediation are needed.…”