The secret to extreme manganese tolerance in microalgal cells
Manganese is an essential nutrient, but at high concentrations it becomes a widespread pollutant of waters affected by mining, ore processing and electrolytic manganese production. Removing it is difficult, particularly from acidic waters. Some microorganisms can survive in these extreme environments, but understanding how they do so is essential before they can be exploited for environmental or biotechnological applications. Our study focused on Chlamydomonas acidophila PM01, a green microalga originally isolated from an acid mine drainage pond. Remarkably, PM01 survived manganese concentrations of up to 50 mM, corresponding to 2.75 g/L, which is the highest concentration reported for any microalga to date. Rather than accumulating large amounts of manganese inside the cell, the microalga appears to survive mainly by keeping the metal out of the cytoplasm or confining it to safer cellular compartments.
To uncover this survival strategy, we combined three complementary synchrotron techniques at Elettra Synchrotron Trieste. Low-energy X-ray Fluorescence Microscopy (XRF) and Scanning Transmission X-ray Microscopy (STXM) at the TwinMic beamline were used to map the intracellular distribution of key elements while simultaneously revealing the major structural features of the cells. The physicochemical speciation of manganese was determined by X-ray Absorption Near-Edge Structure (XANES) spectroscopy at the IAEA Multi-Technique X-ray Spectrometry Experimental Station of the XRF beamline of Elettra. The cellular biochemical response to manganese exposure was characterized by synchrotron-radiation Fourier-transform infrared (SR-FTIR) spectromicroscopy at the SISSI-Bio beamline.
Figure 1: Characteristic LE-µXFM and STXM absorption micrographs of C. acidophila PM01 cell in spatial resolution of 400 nms that were untreated (control) or treated with 20 mM MnCl2 for 24 h or 72 h.
The elemental maps showed that manganese is not evenly distributed inside exposed cells (Figure 1). Instead, it is found to accumulate within the oxygen-rich regions around the nucleus and near the cell periphery, consistent with sequestration in vacuoles. These compartments can isolate potentially harmful substances from the rest of the cell. XANES further revealed that manganese remains in the Mn2+ oxidation state after entering the biomass (Figure 2a). It is predominantly surrounded by oxygen-containing ligands in an approximately octahedral arrangement. Complementary measurements indicated that phosphates play an important role in binding manganese in vacuoles and at the cell wall.
Figure 2: (a) XANES spectra of Mn in C. acidophila PM01 biomass and (b) SR-FTIR analysis of biomolecular profiles (fingerprint regions provided).
The SR-FTIR measurements added the metabolic dimension of this study (Figure 2b) disclosing changes in carbohydrate- and phosphate-related signals, together with a reduction in lipid reserves and signs of mild oxidative damage. Combined with microscopy and photosynthesis measurements, these findings indicate that keeping manganese out is energetically demanding. The cells respond by reallocating their metabolic reserves, enlarging their vacuolar system and slightly improving photosynthetic performance.
Together, the findings support a new model of metal tolerance in acidophilic eukaryotic microalgae: “survival through low intracellular retention”. PM01 binds part of the manganese at the cell surface, transfers some into vacuoles and may ultimately remove it through vacuolar excretion. In simple terms, this microalga tolerates extreme metal concentrations largely by keeping the metal away from its sensitive cellular machinery.
This strategy has important practical implications. PM01 is unlikely to be ideal candidate for manganese bioremediation because it accumulates relatively little metal. However, its exceptional resistance suggests that it could be cultivated in manganese-contaminated acidic waters to produce biomass or biofuels without consuming freshwater. Looking forward, this study also opens a new research direction: determining whether similar “keep-it-out” strategies are employed by other acidophilic microalgae, against other metals, and under real mine-water conditions.
This research was conducted by the following research team:
Isidora Santrac1, Milena Dimitrijevic1, Milan Žižic1,2, Marina Stanic1, Valentina Curic1, Valentina Bonanni2, Alessandra Gianoncelli2, Giuliana Aquilanti2, Chiaramaria Stani2, Giovanni Birarda2, Wilfred Hagen3, Uroš Javornik4, Bernd Zechmann5, Jon K. Pittman6, Ivan Spasojevic1, Jelena Danilovic Lukovica1
1 University of Belgrade - Institute of Multidisciplinary Research, National Institute of the Republic of Serbia, Belgrade, Serbia
2 Elettra Sincrotrone Trieste S.C.p.A., Basovizza, Trieste, Italy
3 Department of Biotechnology, Delft University of Technology, Delft, the Netherlands
4 Slovenian NMR Center, National Institute of Chemistry, Hajdrihova, Ljubljana, Slovenia
5 Center for Microscopy and Imaging, Baylor University, Waco, TX, USA
6 Department of Earth and Environmental Sciences, School of Natural Sciences, The University of Manchester, Manchester, UK
Contact persons email:
Local contact emails: (SISSI-BIO), (XRF), (TwinMic)
Reference
I. Santrac, M. Dimitrijevic, M. Žižic et al.; “The extremophile Chlamydomonas acidophila PM01 survives high manganese contamination by restricting non-vacuolar intracellular content”; Bioresour. Technol., art. no. 135510 (2026); DOI: 10.1016/j.biortech.2026.135510.
