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

Abstract

Understanding how biotic and abiotic factors govern the assembly of rhizosphere-microbial communities is a long-standing goal in microbial ecology. In phytoremediation research, where plants are used to remediate heavy metal-contaminated soils, a deeper understanding of rhizosphere-microbial ecology is needed to fully exploit the potential of microbial-assisted phytoremediation. This study investigated whether Grime's competitor/stress-tolerator/ruderal (CSR) theory could be used to describe the impact of cadmium (Cd) and the presence of a Cd-accumulating plant, Carpobrotus rossii (Haw.) Schwantes, on the assembly of soil-bacterial communities using Illumina 16S rRNA profiling and the predictive metagenomic-profiling program, PICRUSt. Using predictions based on CSR theory, we hypothesized that Cd and the presence of a rhizosphere would affect community assembly. We predicted that the additional resource availability in the rhizosphere would enrich for competitive life strategists, while the presence of Cd would select for stress-tolerators. Traits identified as competitive followed CSR predictions, discriminating between rhizosphere and bulk-soil communities whilst stress-tolerance traits increased with Cd dose, but only in bulk-soil communities. These findings suggest that a bacterium's competitive attributes are critical to its ability to occupy and proliferate in a Cd-contaminated rhizosphere. Ruderal traits, which relate to community re-colonization potential, were synergistically decreased by the presence of the rhizosphere and Cd dose. Taken together this microcosm study suggests that the CSR theory is broadly applicable to microbial communities. Further work toward developing a simplified and robust strategy for microbial CSR classification will provide an ecologically meaningful framework to interpret community-level changes across a range of biomes.

Trait-based life-history strategies explain succession scenario for complex bacterial communities under varying disturbance

“…However, meta-analyses of microbial communities to infer CSR strategies can be challenging due to 83 the different factors related to experimental design and techniques employed across different studies, 84 which could heavily affect the observations in comparison. Recent studies on soil-bacterial 85 communities in cadmium-contaminated rhizospheres (Wood et al 2018) and tillage-disturbed fields 86 (Schmidt et al 2018) also suggested the applicability of the CSR approach to soil-microbial 87 communities. To our knowledge, the CSR framework has not yet been employed within a single study 88 to changes in microbial communities upon disturbance in any microbiome other than soil.…”

Soil microbial communities with greater investment in resource acquisition have lower growth yield

“…This tradeoff is reiterated by the absence of scenarios of communities excelling in both traits (figure 2, upper right). However, in certain communities lower resource acquisition costs were accompanied with lower growth yields (Figure 2, lower left), where, it is plausible that either or both of these traits trade off with some other unmeasured trait, likely stress tolerance 15,19,20 . In support of this interpretation, we previously found lower growth rate and yield in acidic soils ( Figure S1-2) 15 highlighting much higher maintenance costs of acid stress tolerance in such soils.…”

Defining trait-based microbial strategies with consequences for soil carbon cycling under climate change

“…This classification was mostly based on microbial substrate preferences and growth rates and has since been widely applied in various contexts (Fierer et al, 2012;Thomson et al, 2013). Several recent efforts have also applied C-S-R life history strategies to microbial systems, particularly in the context of anthropogenic environmental change (Fierer, 2017;Ho et al, 2013;Krause et al, 2014;Wood et al, 2018). Ho et al (2013) classified methane-oxidising bacteria into C-S-R life strategies based on activity, recovery from disturbances, substrate utilization patterns and stress tolerance.…”