Subcellular Localization of Manganese in Two Green Microalgae Species with Different Tolerance to Elevated Mn Concentrations

Green microalgae Lobosphaera sp. IPPAS С-2047 and Micractinium simplicissimum IPPAS С-2056 were examined for the first time for cell tolerance to elevated concentrations of manganese applied in the form of MnCl₂. Analyses of cell photosynthetic activity by chlorophyll fluorometry and the dynamic patterns of absorbance changes of cell suspensions at the peak of chlorophyll absorption revealed differential tolerance of two microalgae species to manganese. The acute toxicity assayed in 4-day treatments became apparent at manganese concentrations equal to or higher than 1 g/L for M. simplicissimum cells and at concentrations above 10 g/L for Lobosphaera sp. Transmission electron microscopy and energy-dispersive X-ray spectroscopy were used to study the subcellular distribution of manganese in microalgal cells under elevated nontoxic concentrations of manganese in the medium. The results with Lobosphaera sp. established the lack of individual manganese-containing inclusions on cell surfaces and in the cell interior; the intracellular distribution of manganese was dispersed with elevated accumulation of this element in the region of thylakoids and plastoglobules. The occurrence of manganese and phosphorus in plastoglobules was found for the first time. Apparently, these compartments become accessible for accumulation of Mn and P upon the translocation of thylakoid components during stress-induced disassembling of their structure. The cells of M. simplicissimum were able to oxidize the exoplasmic Mn²⁺ with the formation of manganese nanoparticles in the intercellular matrix as well as on the cell surface and within the cell walls. In addition, the manganese permeating into the cells was shown to compartmentalize in vacuoles and bind to the polyphosphate granules.