Detection of secondary cyanobacterial metabolites using LC-HRMS in Lake Karaoun

Harmful algal blooms events have been reported worldwide and during the last decades are occurred with increasing frequency and intensity due to the climate change and the high inputs of nutrients in freshwaters from anthropogenic activities. During blooms cyanobacteria release in water their toxic secondary metabolites, known as cyanotoxins, along with other bioactive metabolites. Due to the negative impacts of these compounds on aquatic ecosystems and public health, there is an urgent need to detect and identify known and unknown cyanobacterial metabolites in surface waters. In the frame of the present study, a method based on liquid chromatography - high resolution mass spectrometry (LC-HRMS) was developed to investigate the presence of cyanometabolites in bloom samples from Lake Karaoun, Lebanon. Data analysis was performed using Compound Discoverer software with related tools and databases in combination to the CyanoMetDB mass list for detection, identification and structural elucidation of the cyanobacterial metabolites. In the course of this study, 92 cyanometabolites were annotated including 51 cyanotoxins belonging to microcystins, 15 microginins, 10 aeruginosins, 6 cyclamides, 5 anabaenopeptins, a cyanopeptolin, the dipeptides radiosumin B and dehydroradiosumin, the planktoncyclin and a mycosporine-like amino acid. Out of them, 7 new cyanobacterial metabolites, the chlorinated MC-ClYR, [epoxyAdda5]MC-YR, MC-LI, aeruginosin 638, aeruginosin 588, microginin 755C and microginin 727 were discovered. Moreover, the presence of anthropogenic contaminants was recorded indicating the pollution of the lake and emphasizing the need for assessment of the co-occurrence of cyanotoxins, other cyanobacterial metabolites and other compounds hazardous to the environment. Overall, results prove the suitability of the proposed approach for the detection of cyanobacterial metabolites in environmental samples but also highlight the necessity of spectral libraries for these compounds, considering the absence of their reference standards.