Tolerance of regional microalgae strains from the Republic of Tatarstan to heavy metal and acid gas contaminants: implications for carbon capture applications

Abstract

Green microalgae are attractive biological agents for capturing carbon dioxide from industrial flue gas and for growth on partially treated wastewater, but this dual-source cultivation strategy exposes cultures to heavy metals and the acidic gases sulfur dioxide (SO₂) and nitrogen oxides (NO_x). This study assessed the lead (Pb), cadmium (Cd), SO₂, and NO_x tolerance of nine microalgal strains (Chlamydomonas sp., Chlorococcum sp., Edaphochlamys debaryana, Chlorella sp., Chlamydomonas moewusii, Chlorella vulgaris, and Spongiosarcinopsis sp.), isolated in 2024 from various water bodies and surfaces in the Republic of Tatarstan, Russia, and was aimed at selecting the most tolerant strain for further development as a carbon-sequestering candidate. Sampling sites were characterized by redox potential (EMF, 27-1518 mV), pH (6.4-8.0) and spectral absorption coefficient (color of water). EC₅₀ values, determined by dose-response assays, ranged from 31.1 to 238.5 mg/L for Pb and from 8.3 to 176.3 mg/L for Cd, generally exceeding literature values reported for the same or related taxa. Sensitivity to 50 ppm SO₂ and 50 ppm NO_x was low across all isolates, whereas 100 ppm NO_x and 100 ppm NO₂ produced significant growth inhibition in three and five isolates, respectively. A point-scoring procedure integrating Pb, Cd, and gas tolerance identified Chlorella vulgaris (isolate 14) as the best-performing strain overall (35 of a maximum 36 points), combining the highest Pb tolerance, a near-maximum Cd tolerance, and complete tolerance across all tested gas conditions. These findings support Chlorella vulgaris as the primary candidate among the isolates examined for further evaluation in the flue gas- and wastewater-based microalgal cultivation systems.