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Thermoacidophilic Cyanidiales maintain a competitive edge in inhabiting extreme environments enriched with metals. Here, species of Cyanidioschyzon merolae (Cm), Cyanidium caldarium (Cc), and Galdieria partita (Gp) were exploited to remove hexavalent chromium [Cr(VI)]. Cm and Gp could remove 168.1 and 93.7 mg g−1 of Cr(VI) at pH 2.0 and 7.0, respectively, wherein 89% and 62% of sorbed Cr on Cm and Gp occurred as trivalent chromium [Cr(III)]. Apart from surface-sorbed Cr(VI), the in vitro Cr(III) bound with polysaccharide and in vivo chromium(III) hydroxide [Cr(OH) 3 ] attested to the reduction capability of Cyanidiales. The distribution of Cr species varied as a function of sorbed Cr amount, yet a relatively consistent proportion of Cr(OH) 3 , irrespective of Cr sorption capacity, was found only on Cm and Cc at pH 2.0. In conjunction with TXM (transmission X-ray microscopy) images that showed less impaired cell integrity and possible intracellular Cr distribution on Cm and Cc at pH 2.0, the in vivo Cr(OH) 3 might be the key to promoting the Cr sorption capacity (≥ 152 mg g−1). Cyanidiales are promising candidates for the green and sustainable remediation of Cr(VI) due to their great removal capacity, the spontaneous reduction under oxic conditions, and in vivo accumulation. [Display omitted] • C. merolae removed 168 mg g−1 of Cr(VI) at pH 2.0% and 89% was reduced to Cr(III). • G. partita removed 94 mg g−1 of Cr(VI) at pH 7.0% and 62% was reduced to Cr(III). • Cr retained as surface-sorbed Cr(VI) and in vitro as well as in vivo Cr(III). • Cr(III) species included Cr(III)-polysaccharide complexes and Cr(OH) 3 precipitates. • Cr(OH) 3 promoted Cr sorption capacity of Cyanidiales. [ABSTRACT FROM AUTHOR]

• Cyanidium caldarium (Cc) showed superior sorption capacity (298 mg g−1) for Pb(II). • Modulation of Pb tolerance by Cyanidiales was classified into four mechanisms. • Formation of organic complexes promoted Pb(II) ion sorption capacity of Cyanidiales. • Cyanidiales are innovative green materials for the environmental remediation. Thermoacidophilic Cyanidiales are capable to survive in extreme environments (20–56 °C; pH 0.5–5.0) with concentrated metals, allowing them to serve as promising green materials applied in metal remediation. It is the first attempt to determine the capacity and related mechanisms for Pb(II) ions sorption on three Cyanidiales genera: Galdieria maximum (Gm), Cyanidioschyzon merolae (Cm), and Cyanidium caldarium (Cc) in relation to changes in Pb speciation obtained from Pb L III -edge X-ray absorption, organic functional groups, and protein secondary structures derived from synchrotron-based Fourier-transform infrared spectroscopy. Three-dimensional images of Cyanidiales were collected using Transmission X-ray microscopy. Lead tolerance on Cyanidiales was modulated according to four mechanisms: the defense line provide by polysaccharide, the inorganic Pb-PO 4 precipitation, the organic Pb complexation concomitant with the transport to cell vacuoles, and the specific thiol-Pb chelation involved in disruption of protein secondary structures. Despite Cyanidiales generally performed all mechanisms against Pb toxicity, individual defense responses were highlighted by specific Cyanidiales species. The redistribution of Pb-polysaccharide species and inorganic Pb-PO 4 precipitates toward organic complexation promoted Pb(II) ions sorption capacity of Cyanidiales, accounting for the almost eight-time-greater sorbed Pb (298.4 mg g−1) on Cc in comparison to Gm. The knowledge provided here could improve the application of the Cyanidiales in environmental remediation as an innovative green technology. [ABSTRACT FROM AUTHOR]