vulgaris, the obtained quinine-based QASs are characterized by a diverse range of acute toxicity. The estimated ErC50 values ranged from 0.1 (3Br, 3Asp, 3Ala) to more than 1000 mg dm -3 (1Asp). The determined ErC50 ranges are summarized in Fig. , while the growth inhibition values of C. vulgaris for the corresponding toxicant concentration are summarized in As in the case of antimicrobial activity, toxicity toward algae was determined by the type of substituent in the quinine-derived cation. All compounds with a dodecyl substituent (3Br, 3Asp, 3Ala) exhibited the same ErC50 range (0.1-1 mg dm -3 ), comparable to the toxicity toward C. vulgaris recorded for other QASs with a dodecyl chain, such as benzalkonium chloride (ErC50: 0.22 mg dm -3 ) or salts with an imidazolium cation (0.37 mg dm -3 ) . No differences were also noticed between the toxicity of the 3 salts with an octyl substituent (2Br, 2Asp, 2Ala, ErC50: 1-10 mg dm -3 ). It should therefore be emphasized that optimization of the quinine-derived cation structure for antimicrobial activity (especially in the case of 3Br, 3Asp and 3Ala) also leads to a significant increase in acute toxicity against C. vulgaris, which presumably results from the analogous mechanism of action of amphiphilic QASs against cells of both pathogenic microorganisms and algae . Thus, this means that despite their natural origin and enhanced biocompatibility, potential quinine-based quaternary disinfectants containing a long alkyl substituent should still be treated as a potential pollutant, and their release into the environment and concentration in wastewater should be closely monitored analogously to "conventional" QASs of synthetic origin .