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Metabolism of metals in microalgae and adaptation to metal excess are of significant environmental importance. We report a three-step mechanism that the green microalga Chlorella sorokiniana activates during the acquisition of and adaptation to manganese (Mn), which is both an essential trace metal and a pollutant of waters. In the early stage, Mn2+ was mainly bound to membrane phospholipids and phosphates in released mucilage. The outer cell wall was reorganized and lipids were accumulated, with a relative increase in lipid saturation. Intracellular redox settings were rapidly altered in the presence of Mn excess, with increased production of reactive oxygen species that resulted in lipid peroxidation and a decrease in the concentration of thiols. In the later stage, Mn2+ was chelated by polyphosphates and accumulated in the cells. The structure of the inner cell wall was modified and the redox milieu established a new balance. Polyphosphates serve as a transient Mn2+ storage ligand, as proposed previously. In the final stage, Mn was stored in multivalent Mn clusters that resemble the structure of the tetramanganese–calcium core of the oxygen-evolving complex. The present findings elucidate the bioinorganic chemistry and metabolism of Mn in microalgae, and may shed new light on water-splitting Mn clusters.

Nickel is utilized by microalgae as a co-factor of urease. On the other hand, this transition metal represents an important pollutant of aquatic ecosystems. The effects of Ni(II) excess on microalgae and the mechanisms of adaptation are poorly understood. Redox processes represent an important component of the mechanisms of interaction of microalgae with transition metals. Pertinent to this, we analyzed redox changes in Chlorella sorokiniana culture that are induced by high levels of Ni(II). The intracellular level of reactive oxygen species (ROS) showed a rapid two-phase increase that took place prior to Ni accumulation in the cell. This was accompanied by oxidation of thiols and drastic deglutathyonilation of proteins. PAM fluorimetry showed that Ni excess induced an increase in the efficiency of photosystem II and promoted electron flow in chloroplasts, which is most likely responsible for ROS rise. In addition, a rising trend in the chlorophyll level was observed. On the other hand, the level of lipid peroxidation and activities of key antioxidative enzymes were not increased, which implies that oxidative stress is not an important player in Ni adaptation/toxicity. After prolonged exposure the efficiency of photosystem II drops, nickel is accumulated in the cells, and new redox balance is established. Our results imply that redox signalling is involved in Ni-induced metabolic activation and that key changes take place in photosynthetic machinery.

METHODS The impact of a set of concentrations of Mn2+ and Ni2+ on growth of C. sorokiniana culture in 3N-BBM+V medium in the early stationary phase was evaluated by changes in optical density at 750 nm and biomass during 7 days treatment. Mucilage release was analyzed using SEM microscopy. Redox settings were analyzed by oxidation-sensitive fluorescent probe and assays for thiols. RESULTS Ni was more toxic than Mn and affected culture growth at lower concentrations. Microalgal cells started releasing mucilage polymers within 1 h of exposure to 1 mM Mn2+, whereas no mucilage was observed even at 24 h of treatment with equimolar Ni2+. The peak of reactive oxygen species production was reached faster for Ni2+ than Mn2+. Mn-induced drops in the concentration of reduced thiols showed a recovery after 1 h and 24 h. Ni2+-induced drop was irreversible. The observed differences between the impact of Mn and Ni may be related to different redox and coordinative properties and to higher capacities of microalgae to sequester Mn in relation to higher quotas than Ni that are required for normal functio