Improvement of the Uranium Sequestration Ability of a Chlamydomonas sp. (ChlSP Strain) Isolated From Extreme Uranium Mine Tailings Through Selection for Potential Bioremediation Application

Abstract

The extraction and processing of uranium (U) have polluted large areas worldwide, rendering anthropogenic extreme environments inhospitable to most species. Noticeably, these sites are of great interest for taxonomical and applied bioprospection of extremotolerant species successfully adapted to U tailings contamination. As an example, in this work we have studied a microalgae species that inhabits extreme U tailings ponds at the Saelices mining site (Salamanca, Spain), characterized as acidic (pH between 3 and 4), radioactive (around 4 μSv h−1) and contaminated with metals, mainly U (from 25 to 48 mg L−1) and zinc (from 17 to 87 mg L−1). After isolation of the extremotolerant ChlSP strain, morphological characterization and internal transcribed spacer (ITS)-5.8S gene sequences placed it in the Chlamydomonadaceae, but BLAST analyses identity values, against the nucleotide datasets at the NCBI database, were very low (<92%). We subjected the ChlSP strain to an artificial selection protocol to increase the U uptake and investigated its response to selection. The ancestral strain ChlSP showed a U-uptake capacity of ≈4.30 mg U g−1 of dry biomass (DB). However, the artificially selected strain ChlSG was able to take up a total of ≈6.34 mg U g−1 DB, close to the theoretical maximum response (≈7.9 mg U g−1 DB). The selected ChlSG strain showed two possible U-uptake mechanisms: the greatest proportion by biosorption onto cell walls (ca. 90%), and only a very small quantity, ~0.46 mg g−1 DB, irreversibly bound by bioaccumulation. Additionally, the kinetics of the U-uptake process were characterized during a microalgae growth curve; ChlSG cells removed close to 4 mg L−1 of U in 24 days. These findings open up promising prospects for sustainable management of U tailings waters based on newly evolved extremotolerants and outline the potential of artificial selection in the improvement of desired features in microalgae by experimental adaptation and selection.

Evidence of microalgal isotopic fractionation through enrichment of depleted uranium

“…We performed two independent experiments, each with one of the strains of interest, the Chlamydomonas (ChlGS) and Tetraselmis (TmmRU) Chlorophyta strains, in different media supplemented with DU. ChlGS is an extremophile isolated from an acid U mine tailings pond, tolerant up to 25 mg U L −1 and other metals, and artificially selected for U uptake 33 . The isotopic ratio n( 235 U)/n( 238 U) with a value of 0.007375 ± 0.000013 found in the mine water (see Supplementary Fig.…”

“…The U concentration for TmmRU was obtained from the dose-response curve and represented the higher dose before the IC 50 values for growth inhibition. In ChlGS strain, the U concentration used was selected based upon previous U uptake experiments 33 . At different times along the cells’ growth curve (3, 12 and 24 days), four independent cultures were centrifuged at 4000 rpm for 15 minutes, and the two phases obtained (supernatant and pellet) of each replicate were frozen.…”

The `greed ́ and `nationalism ́ in vaccination against Covid-19 lead to the Tragedy of the Commons, Nash equilibrium and Giffen's goods, giving SARS-CoV-2 more opportunities to escape mutants.

“…[51][52][53] , others more recently have also proved them right. For example, in microorganism populations spontaneously and recurringly arise new mutant strains, resistant to substances of late synthesis even before industry used them 54,55,64,[56][57][58][59][60][61][62][63] or of military use never seen in nature before 65 or radioisotopes of nuclear industry [66][67][68][69] .…”

The effects of ionizing radiation on the structure and antioxidative and metal-binding capacity of the cell wall of microalga Chlorella sorokiniana

“…Seasonal algal blooms in the spent nuclear fuel storage at Sellafield, UK, is a phenomenon that probably best illustrates the potential of microalgae to adapt to radiation and to thrive in such ecosystems (Foster et al, 2020;MeGraw et al, 2018). It is worth mentioning that these properties demonstrate that microalgae are capable pioneer species in the colonization of highly inhospitable environments (Baselga-Cervera et al, 2018;Rivasseau et al, 2016). Clearly, the effects of ionizing radiation on microalgae and the mechanisms of their adaptation are of fundamental environmental interest, as highlighted recently by the disastrous contamination of water in the Fukushima-Daiichi nuclear power plant accident (Fukuda et al, 2014).…”

“…Conversely, microalgae appear to show resilience to radiation stress; for example they are very efficient in the remediation of freshwaters that are contaminated with radioactive metals, such as strontium, uranium, and caesium (Fukuda et al, 2014;Kalin et al, 2005;Vanhoudt et al, 2018). Furthermore, some microalgal species are known to colonize spent nuclear fuel storage pools and uranium tailings pounds showing high levels of radiation and heavy metals pollution (Baselga-Cervera et al, 2018;MeGraw et al, 2018;Rivasseau et al, 2016). Seasonal algal blooms in the spent nuclear fuel storage at Sellafield, UK, is a phenomenon that probably best illustrates the potential of microalgae to adapt to radiation and to thrive in such ecosystems (Foster et al, 2020;MeGraw et al, 2018).…”