Plant establishment during phytostabilization of legacy mine tailings in semiarid regions is challenging due to low pH, low organic carbon, low nutrients, and high toxic metal(loid) concentrations. Plant-associated bacterial communities are particularly important under these harsh conditions because of their beneficial services to plants. We hypothesize that bacterial colonization profiles on rhizoplane surfaces reflect deterministic processes that are governed by plant health and the root environment. The aim of this study was to identify associations between bacterial colonization patterns on buffalo grass (Buchloe dactyloides) rhizoplanes and both plant status (leaf chlorophyll and plant cover) and substrate biogeochemistry (pH, electrical conductivity, total organic carbon, total nitrogen, and rhizosphere microbial community). Buffalo grass plants from mesocosm- and field-scale phytostabilization trials conducted with tailings from the Iron King Mine and Humboldt Smelter Superfund Site in Dewey-Humboldt, Arizona, were analyzed. These tailings are extremely acidic and have arsenic and lead concentrations of 2-4 g kg-1 substrate. Bacterial communities on rhizoplanes and in rhizosphere-associated substrate were characterized using fluorescence in situ hybridization and 16S rRNA gene amplicon sequencing, respectively. The results indicated that the metabolic status of rhizoplane bacterial colonizers is significantly related to plant health. Principal component analysis revealed that root-surface Alphaproteobacteria relative abundance was associated most strongly with substrate pH and Gammaproteobacteria relative abundance associated strongly with substrate pH and plant cover. These factors also affected the phylogenetic profiles of the associated rhizosphere communities. In summary, rhizoplane bacterial colonization patterns are plant specific and influenced by plant status and rhizosphere biogeochemical conditions.
PMID: 18424534;PMCID: PMC2446573;Abstract:
Bacterial diversity in mine tailing microbial communities has not been thoroughly investigated despite the correlations that have been observed between the relative microbial diversity and the success of revegetation efforts at tailing sites. This study employed phylogenetic analyses of 16S rRNA genes to compare the bacterial communities present in highly disturbed, extremely (pH 2.7) and moderately (pH 5.7) acidic lead-zinc mine tailing samples from a semiarid environment with those from a vegetated off-site (OS) control sample (pH 8). Phylotype richness in these communities decreased from 42 in the OS control to 24 in the moderately acidic samples and 8 in the extremely acidic tailing samples. The clones in the extremely acidic tailing sample were most closely related to acidophiles, none of which were detected in the OS control sample. The comparison generated by this study between the bacteria present in extremely acidic tailing and that in moderately acidic tailing communities with those in an OS control soil provides a reference point from which to evaluate the successful restoration of mine tailing disposal sites by phytostabilization. Copyright © 2008, American Society for Microbiology. All Rights Reserved.
To explore the bacteria involved in the oxidation of arsenite (As(III)) under denitrifying conditions, three enrichment cultures (ECs) and one mixed culture (MC) were characterized that originated from anaerobic environmental samples. The oxidation of As(III) (0.5 mM) was dependent on NO(3) (-) addition and N(2) formation was dependent on As(III) addition. The ratio of N(2)-N formed to As(III) fed approximated the expected stoichiometry of 2.5. A 16S rRNA gene clone library analysis revealed three predominant phylotypes. The first, related to the genus Azoarcus from the division Betaproteobacteria, was found in the three ECs. The other two predominant phylotypes were closely related to the genera Acidovorax and Diaphorobacter within the Comamonadaceae family of Betaproteobacteria, and one of these was present in all of the cultures examined. FISH confirmed that Azoarcus accounted for a large fraction of bacteria present in the ECs. The Azoarcus clones had 96% sequence homology with Azoarcus sp. strain DAO1, an isolate previously reported to oxidize As(III) with nitrate. FISH analysis also confirmed that Comamonadaceae were present in all cultures. Pure cultures of Azoarcus and Diaphorobacter were isolated and shown to be responsible for nitrate-dependent As(III) oxidation. These results, taken as a whole, suggest that bacteria within the genus Azoarcus and the family Comamonadaceae are involved in the observed anoxic oxidation of As(III).
The uptake of arsenic by plants from contaminated soils presents a health hazard that may affect home gardeners neighboring contaminated environments. A controlled greenhouse study was conducted in parallel with a co-created citizen science program (home garden experiment) to characterize the uptake of arsenic by common homegrown vegetables near the Iron King Mine and Humboldt Smelter Superfund site in southern Arizona. The greenhouse and home garden arsenic soil concentrations varied considerably, ranging from 2.35 to 533 mg kg(-1). In the greenhouse experiment four vegetables were grown in three different soil treatments and in the home garden experiment a total of 63 home garden produce samples were obtained from 19 properties neighboring the site. All vegetables accumulated arsenic in both the greenhouse and home garden experiments, ranging from 0.01 to 23.0 mg kg(-1) dry weight. Bioconcentration factors were determined and show that arsenic uptake decreased in the order: Asteraceae>Brassicaceae>Amaranthaceae>Cucurbitaceae>Liliaceae>Solanaceae>Fabaceae. Certain members of the Asteraceae and Brassicaceae plant families have been previously identified as hyperaccumulator plants, and it can be inferred that members of these families have genetic and physiological capacity to accumulate, translocate, and resist high amounts of metals. Additionally, a significant linear correlation was observed between the amount of arsenic that accumulated in the edible portion of the plant and the arsenic soil concentration for the Asteraceae, Brassicaceae, Amaranthaceae, and Fabaceae families. The results suggest that home gardeners neighboring mining operations or mine tailings with elevated arsenic levels should be made aware that arsenic can accumulate considerably in certain vegetables, and in particular, it is recommended that gardeners limit consumption of vegetables from the Asteraceae and Brassicaceae plant families.
