Exploiting Galdieria sulphuraria Genetic Resources for the Biorecovery of Rare Earth Elements (REEs)

Exploration of life in extreme environments allows the discovery of intriguing organisms with extraordinary biotechnological potential. Example of extreme environments is represented by hot springs, where harsh conditions (pH < 1; T > 50°C; high concentrations of metals) are prohibitive for most living organisms, except for archaea, bacteria and a few eukaryotes like the unicellular alga Galdieria sulphuraria. In the present project, I used G. sulphuraria to explore its potentiality in recovering Rare Earth Elements (REEs). Firstly, I studied its phylogeny and evolution using the complete plastid and mitochondrial genomes. Results from the organelle phylogenies showed the subdivision of the species into 6 sublineages. The influence of selective pressure could mainly explain divergent evolution since high variability of the dS-dN values was shown when comparing each pair of sublineages. Secondly, I used G. sulphuraria, strain SAG 107.79, to perform exposition tests to the REEs cerium. Evaluation of the growth rate and cell vitality demonstrated the absence of significant effects on the microalgal physiology. Transcriptomic data highlighted the activation of pathways probably involved in metal homeostasis. Among the Biological processes, transcripts involved in Localization and Transport were subjected to a higher increase in expression following cerium treatments. I performed phylogenetic analysis using the amino acid sequences of these transcripts from the representative members of the 6 lineages, integrated with the orthologous sequences from the NCBI non-redundant database. All phylogenies identified a close relationship of these transcripts with archaea, bacteria, or fungi, thus suggesting the intervention of Horizontal Gene Transfer (HGT) events in the biorecovery potential of G. sulphuraria. The biotechnological application of G. sulphuraria was also evaluated at strain level. Two genetically distant strains, SAG 107.79 and ACUF 427, were selected to test the different performances in the bioremoval of yttrium, cerium, europium and terbium from single- and quaternary-metal solutions. From both test solutions, ACUF 427 was the better biosorbent in acidic conditions, while SAG 107.79 better bio removed metal ions from subneutral solutions.