One of the main concerns in industrialized countries is represented by per- and poly-fluoroalkyl substances (PFAS), persistent contaminants hardly to be dealt with by conventional wastewater treatment processes. Phyco-remediation was proposed as a green alternative method to treat wastewater. Synechocystis sp. PCC6803 is a unicellular photosynthetic organism candidate for bioremediation approaches based on synthetic biology, as it is able to survive in a wide range of polluted waters. In this work, we assessed the possibility of applying Synechocystis in PFAS-enriched waters, which was never reported in the previous literature. Respirometry was applied to evaluate short-term toxicity of perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS), which did not affect growth up to 0.5 and 4 mg L−1, respectively. Continuous and batch systems were used to assess the long-term effects, and no toxicity was highlighted for both compounds at quite high concentration (1 mg L−1). A partial removal was observed for PFOS and PFOA, (88% and 37%, with removal rates of about 0.15 and 0.36 mg L−1 d−1, respectively). Measurements in fractionated biomass suggested a role for Synechocystis in the sequestration of PFAS: PFOS is mainly internalized in the cell, while PFOA is somehow transformed by still unknown pathways. A preliminary bioinformatic search gave hints on transporters and enzymes possibly involved in such sequestration/transformation processes, opening the route to metabolic engineering in the perspective application of this cyanobacterium as a new phyco-remediation tool, based on synthetic biology.

The order of Cyanidiales comprise seven acido-thermophilic red microalgal species thriving in hot springs of volcanic origin characterized by extremely low pH, moderately high temperatures and the presence of elevated concentrations of sulphites and heavy metals that are prohibitive for most other organisms. Little is known about the molecular mechanisms of Cyanidiales long-term adaptation to such hostile environments, in particular to heavy metals, yet elucidation of these processes is important for understanding the evolution of the metabolic pathways underlying heavy metal detoxification for developing rational strategies for heavy metal bioremediation. Here, we investigated the long-term adaptive responses of Cyanidioschyzon merolae cells, a member of Cyanidiales, to extremely high nickel concentrations. Through complementary approaches based on physiological, microscopic and elemental analyses we dissect several molecular mechanisms underlying the long-term adaptation of this model extremophilic microalga to high Ni exposure. These include: (i) extrusion of Ni from the cells and lack of significant Ni accumulation inside the cells; (ii) maintenance of efficient photoprotective responses including non-photochemical quenching and state transitions; (iii) dynamic remodelling of the chloroplast ultrastructure such as formation of metabolically active prolamellar bodies and plastoglobuli together with loosening of the thylakoid membranes; (iv) activation of ROS amelioration metabolic pathways; and (v) preservation of the efficient respiratory chain functionality. All the dynamically regulated processes identified in this study underlie the remarkable adaptability of C. merolae to extremely high Ni levels that exceed by several orders of magnitude the levels of this heavy metal found in the natural environment of this extremophile.