Settling selection of Chlamydomonas reinhardtii for samarium uptake.

Samarium (Sm) is a rare-earth element recently included in the list of critical elements due to its vital role in emerging new technologies. With an increasing demand for Sm, microbial bioremediation may provide a cost-effective and a more ecologically responsible alternative to remove and recover Sm. We capitalized on a previously selected Chlamydomonas reinhardtii strain tolerant to Sm (1.33 × 10-4 M) and acidic pH and carried out settling selection to increase the Sm uptake performance. We observed a rapid response to selection in terms of cellular phenotype. Cellular size decreased and circularity increased in a stepwise manner with every cycle of selection. After four cycles of selection, the derived CSm4 strain was significantly smaller and was capable of sequestrating 41% more Sm per cell (1.7 × 10-05 ± 1.7 × 10-06 ng) and twice as much Sm in terms of wet biomass (4.0 ± 0.4 mg Sm · g-1) compared to the ancestral candidate strain. The majority (~70%) of the Sm was bioaccumulated intracellularly, near acidocalcisomes or autophagic vacuoles as per TEM-EDX microanalyses. However, Sm analyses suggest a stronger response toward bioabsorption resulting from settling selection. Despite working with Sm and pH-tolerant strains, we observed an effect on fitness and photosynthesis inhibition when the strains were grown with Sm. Our results clearly show that phenotypic selection, such as settling selection, can significantly enhance Sm uptake. Laboratory selection of microalgae for rare-earth metal bioaccumulation and sorption can be a promising biotechnological approach.

Microbial biominers: Sequential bioleaching and biouptake of metals from electronic scraps.

Electronic scraps (e-scraps) represent an attractive raw material to mine demanded metals, as well as rare earth elements (REEs). A sequential microbial-mediated process developed in two steps was examined to recover multiple elements. First, we made use of an acidophilic bacteria consortium, mainly composed of Acidiphilium multivorum and Leptospidillum ferriphilum, isolated from acid mine drainages. The consortium was inoculated in a dissolution of e-scraps powder and cultured for 15 days. Forty-five elements were analyzed in the liquid phase over time, including silver, gold, and 15 REEs. The bioleaching efficiencies of the consortium were >99% for Cu, Co, Al, and Zn, 53% for Cd, and around 10% for Cr and Li on Day 7. The second step consisted of a microalgae-mediated uptake from e-scraps leachate. The strains used were two acidophilic extremotolerant microalgae, Euglena sp. (EugVP) and Chlamydomonas sp. (ChlSG) strains, isolated from the same extreme environment. Up to 7.3, 4.1, 1.3, and 0.7 µg by wet biomass (WB) of Zn, Al, Cu, and Mn, respectively, were uptaken by ChlSG biomass in 12 days, presenting higher efficiency than EugVP. Concerning REEs, ChlSG biouptake 14.9, 20.3, 13.7, 8.3 ng of Gd, Pr, Ce, La per WB. Meanwhile, EugVP captured 1.1, 1.5, 1.4, and 7.5, respectively. This paper shows the potential of a microbial sequential process to revalorize e-scraps and recover metals and REEs, harnessing extremotolerant microorganisms.

Fluctuation analysis to select for Samarium bio-uptaking microalgae clones the repurposing of a classical evolution experiment.

Rare Earth Elements (REE) increasing demand prompts the research of biotechnological approaches to exploit secondary resources. We made use of the adapted Fluctuation analyses experiment to obtain Chlamydomonas reinhardtii ChlA strains resistant to Samarium (Sm) as the reference REE. The starting hypothesis was that adaptation to metal-containing media leads to an enhanced metal uptake. ChlA was able to adapt to 1.33·10-4 Sm M and pH~3 by pre-existing genetic variability, allowing the evolutionary rescue of 13 of the 99 populations studied. The rescuing resistant genotypes presented a mutation rate of 8.65·10-7 resistant cells per division. The resulting resistant population contradicted the expected fitness cost associated with the adaptation to Sm, selection resulted in larger and faster-growing resistant cells. Among the three isolated strains studied for Sm uptake, only one presented uplifted performance compared to the control population (46.64 µg Sm g-¹ of wet biomass and 3.26·10-7 ng Sm per cell, mainly bioaccumulated within the cells). The selection of microalgae strains with improved tolerance to REEs by this methodology could be a promising solution for REES sequestration. However, increased tolerance can be independent or have negative effects on uptake performance and cellular features studied are not directly correlated with the metal uptake. SUMMARY SENTENCE: Repurposing a classic laboratory evolution experiment to select for microalgae Samarium adapted strains for metals recovery and biotechnology approaches. DATA AVAILABILITY STATEMENT: All data generated or analyzed during this study are included in this published article (and its raw files).