However, more detailed studies on a wider spectrum of monocotyledonous and dicotyledonous Proteinogenic amino acids are relatively harmless to many groups of aquatic organisms, and their exogenous addition can have beneficial effects on algal growth. Among other things, positive effects of amino acid addition have been observed on the increase in biomass of freshwater algal cultures of C. vulgaris . On the other hand, surface-active QASs containing long alkyl substituents are characterized by high toxicity to aquatic organisms, including microalgae . This is due to the same mechanism of interaction with cell membranes, which is responsible for the antibacterial activity of these compounds , as well as adverse interactions with organelles, including chloroplasts . Given the important role of algae as an essential source of food and oxygen in aquatic biocoenoses, assessing toxicity of new chemicals to this group of organisms is important in the course of the environmental risk analysis. According to the studies we performed on C. 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 ). Compounds with a short butyl or 2-(hydroxyethyl) substituent were "practically nontoxic" (El-Harbawi, 2014) and exhibited ErC50 values exceeding 100 mg dm -3 . Thus, they were less toxic than quinine hydrochloride (10-100 mg dm -3 ). Interestingly, a decrease in the toxicity against C. vulgaris (ErC50 >1000 mg dm -3 ) to a level where the chemical can be considered "relatively harmless" (El-Harbawi, 2014)