Rhizosphere Bacteria and Phytostabilization Success: The Association Between Bacteria, Plant Establishment and Metal(loid) Immobilization in Metalliferous Mine Tailings

Abstract

Phytostabilization offers a less expensive alternative to traditional cap and plant methods for containing metalliferous mine tailings to prevent wind erosion and contamination of nearby communities and the environment. However, plant establishment during phytostabilization of pyritic legacy mine tailings in semiarid regions is challenging due to particularly extreme conditions including low pH, low organic carbon, low nutrients, and high toxic metal(loid) concentrations. Microorganisms drive major biogeochemical cycles in soils, however, the roles microorganisms play at the root – soil interface during phytostabilization, particularly in relation to plant health and metal immobilization, are not yet fully understood. The aims of this dissertation are to focus on bacterial communities associated with the roots of buffalo grass used in the phytostabilization of pyritic metalliferous mine tailings to: i) characterize bacterial diversity and communities of rhizosphere and bulk substrate, ii) delineate associations between rhizoplane bacterial colonization patterns and environmental and plant status parameters, and iii) develop an in situ method to visually assess associations between roots, bacteria, and metals. Key findings indicate that after addition of a compost amendment to alleviate the plant-growth inhibiting characteristics of mine tailings, rhizosphere and bulk substrate contain a diverse, plant-growth supporting bacterial community. As substrate re-acidifies due to compost erosion, an emergence of an iron (Fe)- and sulfur (S)-oxidizer and Fe-reducer dominated, less diverse community develops in the bulk and rhizosphere substrate, thus posing a threat to successful plant establishment. However, even at low pH, some plant-growth-promoting bacteria are still evident in the rhizosphere. On the rhizoplane (root surface), the relative abundance of metabolically active bacteria was positively correlated with plant health, verifying the strong association between plant health and bacteria. Furthermore, pH showed a strong association with the relative abundance of Alphaproteobacteria and Gammaproteobacteria on the rhizoplane. In relation to microbe-metal interactions on the root surface, results showed that Actinobacteria and Alphaproteobacteria colocalized with Fe-plaque and arsenic (As) contaminant on the root surface, indicating their potential role in adsorbing or cycling of these metal(loid)s. Developing a more thorough understanding of bacteria-root-metal interactions in relation to plant health and metal immobilization can help to improve phytostabilization efforts and success.