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 comprises 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 high concentrations of sulphites and heavy metals that are prohibitive for most other organisms. Little is known about the physiological processes underlying the long-term adaptation of these extremophiles to such hostile environments. Here, we investigated the long-term adaptive responses of a red microalga Cyanidioschyzon merolae, a representative of Cyanidiales, to extremely high nickel concentrations. By the comprehensive physiological, microscopic and elemental analyses we dissected the key physiological processes underlying the long-term adaptation of this model extremophile to high Ni exposure. These include: (i) prevention of significant Ni accumulation inside the cells; (ii) activation of the photoprotective response of non-photochemical quenching; (iii) significant changes of the chloroplast ultrastructure associated with the formation of prolamellar bodies and plastoglobuli together with loosening of the thylakoid membranes; (iv) activation of ROS amelioration machinery; and (v) maintaining the efficient respiratory chain functionality. The dynamically regulated processes identified in this study are discussed in the context of the mechanisms driving the remarkable adaptability of C. merolae to extremely high Ni levels exceeding by several orders of magnitude those found in the natural environment of the microalga. The processes identified in this study provide a solid basis for the future investigation of the specific molecular components and pathways involved in the adaptation of Cyanidiales to the extremely high Ni concentrations.

The red microalga Cyanidioschyzon merolae inhabits extreme environments with high temperatures (40–56 °C), high acidity (pH 0.05–4), and high concentrations of heavy metals that are lethal to most forms of life. However, information is scarce on the precise adaptation mechanisms of this extremophile to such hostile conditions. Gaining such knowledge is important for understanding the evolution of microorganisms in the early stages of life on Earth characterized by such extreme environments. Through an analysis of the re-programming of the global transcriptome upon the long-term (up to 15 days) exposure of C. merolae to extremely high concentrations of nickel (1 and 3 mM), the key adaptive metabolic pathways and associated molecular components were identified. Our work shows that the long-term Ni exposure of C. merolae leads to the lagged metabolic switch demonstrated via the transcriptional upregulation of the metabolic pathways critical for cell survival. DNA replication, cell cycle, and protein quality control processes were upregulated, while downregulation occurred with energetically costly processes, including the assembly of the photosynthetic apparatus and lipid biosynthesis. This study paves the way for future multi-omic studies of the molecular mechanisms of abiotic stress adaptation in phototrophs, as well as the future development of rational approaches to the bioremediation of contaminated aquatic environments.

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 remodeling 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.

Microalgal-based technology can cost-efficiently recover nitrogen from wastewaters into algal biomass, while high salinity and metal contents in mining waters (MWs) can cause challenges. This study focused on nitrogen recovery from chloride-rich MW. Firstly, the effect of NaCl concentrations 62 and 150 mM on the cell growth, photosynthetic performance and photoprotective mechanisms of the green microalga Selenastrum was studied. The highest cell growth was observed for the control culture, whereas the cell growth inhibition was proportional to the increase in NaCl concentration. The decrease in cell viability was significant only at 150 mM NaCl. With increasing salinity, the photochemical efficiency of photosystem II decreased by 5–38% and the proportion of energy dissipated as heat and fluorescence increased by 1–24% compared to the control. However, not all the differences between the detected values were significant, which indicates the efficient adaptation of Selenastrum sp. to high salinity. Selenastrum sp. was further cultivated in actual MW (62 mM Cl-), which led to dynamic microalgal growth (biomass productivity of 0.033 g L-1 d-1) and high nitrogen uptake (60% removal of total N). The level of chlorophyll a/b was over 2-fold higher, and the production of carotenoids increased by 17% in Selenastrum sp. biomass cultivated in MW compared to a synthetic medium. Thus, the algal biomass produced in MW may have higher commercial value. Overall, this research shows that bioremediation via Selenastrum sp. offers a promising sustainable route for the recovery of nitrogen from chloride-rich MW.