Metabolomic Analysis Reveals Contributions of Citric and Citramalic Acids to Rare Earth Bioleaching by a Paecilomyces Fungus

Abstract

Conventional methods for extracting rare earth elements from monazite ore require high energy inputs and produce environmentally damaging waste streams. Bioleaching offers a potentially more environmentally friendly alternative extraction process. In order to better understand bioleaching mechanisms, we conducted an exo-metabolomic analysis of a previously isolated rare earth bioleaching fungus from the genus Paecilomyces (GenBank accession numbers KM874779 and KM 874781) to identify contributions of compounds exuded by this fungus to bioleaching activity. Exuded compounds were compared under two growth conditions: growth with monazite ore as the only phosphate source, and growth with a soluble phosphate source (K 2 HPO 4 ) added. Overall metabolite profiling, in combination with glucose consumption and biomass accumulation data, reflected a lag in growth when this organism was grown with only monazite. We analyzed the relationships between metabolite concentrations, rare earth solubilization, and growth conditions, and identified several metabolites potentially associated with bioleaching. Further investigation using laboratory prepared solutions of 17 of these metabolites indicated statistically significant leaching contributions from both citric and citramalic acids. These contributions (16.4 and 15.0 mg/L total rare earths solubilized) accounted for a portion, but not all, of the leaching achieved with direct bioleaching (42 ± 15 mg/L final rare earth concentration). Additionally, citramalic acid released significantly less of the radioactive element thorium than did citric acid (0.25 ± 0.01 mg/L compared to 1.18 ± 0.01 mg/L), suggesting that citramalic acid may have preferable leaching properties for a monazite bioleaching process.

The smallest space miners: principles of space biomining

“…(Franz et al 1991;Bosecker 1997;Ambreen et al 2002;Adeleke et al 2010), Aspergillus spp, (Brisson et al 2016;Din et al 2020) and Paecilomyces spp. (Brisson et al 2016(Brisson et al , 2020. Bacterial species include Acinetobacter, Pseudomonas and Bacillus spp.…”

“…(c) During redoxolysis, metal solubilization is promoted by its enzymatic oxidation or reduction, while in (d) bioaccumulation the soluble metal ions can be transported through the cell membrane and accumulate as solid particle within the cell or in vacuoles (Asghari et al 2013 ; Srichandan et al 2019 ). The organic acids identified to be associated with bioleaching include citric acid, gluconic acid, oxalic acid, citramalic acid, acetic acid, succinic acid and itaconic acid (Gadd 1999 ; Adeleke et al 2010 ; Amin et al 2014 ; Brisson et al 2016 , 2020 ). Fungal bioleaching species include, but is not limited to, Penicillium spp.…”

“…(Reed et al 2016 ; Rozas et al 2017 ; Barnett et al 2018 ; Giese et al 2019 ). Brisson and colleagues (Brisson et al 2020 ) recently performed a thorough metabolomic study which suggested that bioleaching of a given element could be mediated by specific organic acids, rather than organic acids in general. (ii) The second group of organotrophic bioleaching microorganisms include species producing hydrogen cyanide (HCN, also called cyanogenic microorganisms), that use glycine for its production.…”

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“…Similar to siderophores, the production of organic acids such as aliphatic (citrate and oxalate) and aromatic (gluconic, malic, phthalate and salicylate) acids by microbes has been used as a method to leach metals of economic interest such as REE from diverse sources (Figure 8.1) (Barnett et al, 2018;Brisson et al, 2020). For instance, Brisson et al (2020) demonstrated that the fungus Paecilomyces could bioleach up to 42 mg/L of REE from monazite ore using organic acids (i.e., citric and citramalic acids). Similarly, gluconic acids produced by the bacterium Gluconobacter oxydans effectively leached REE (in the order of Y .…”

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“…Fungus from the genus Paecilomyces produces several metabolites potentially associated with bioleaching, and citric and citramalic acids present a significant contribution to REEs dissolution, 6.4 and 15.0 mg l −1 total REEs solubilized respectively. Furthermore, citramalic acid exhibits more interesting properties to those of citric acid due to the lower radioactive thorium release [32].…”