# Presence of Cyanotoxins in a Mexican Subtropical Monometric Crater Lake
## 4 Discussion  
While occurrences of cyanobacteria related with hepatoxins and neurotoxins in freshwater have been recognized as a problem in many countries, mainly in temperate regions [39], a gap of information still exists regarding phytoplankton toxins in tropical zones [40]. The case of Mexico is not the exception, in spite of being a country with economically important freshwater systems. To our knowledge, MCs have been recorded in a few aquatic systems of Mexico City and the states of Mexico [41; 42; 43; 44], Sinaloa [45] and Hidalgo [43]. However, microcystin presence has been quantified based on the ELISA technique and, therefore, there is no data available on the MC congeners present [46]. For some of these aquatic systems, matrix-assisted laser desorption/ionization time-of-flight mass spectrometry analysis was performed and showed the presence of MC-LR, MC-LR, MC-RR, MC-H4 YR, MC-YR, MC-WR and MC-HtyR congeners [43]. However, no quantitative values were given. To our knowledge, this is the first quantitative report of an MC congener profile for a Mexican lake. Neurotoxins, particularly PSTs, have been recorded only in Lago Catemaco, Veracruz [19]. In this work, we provided information regarding MCs and PSTs of the Santa Maria del Oro lake, Nayarit, with at least seven MC analogs and an absence of PSTs. The use of LC-MS/MS allowed us to confirm the presence of seven MC congeners (MC-WR, MC-LR, MC-LA, MC-HiIR, MC-LF, MC-YR and MC LY). Previous studies using ELISA were unable to provide information on the presence MC congeners. Although we only considered morphological characteristics to identify the cyanobacteria _L. robusta_ and _M. aeruginosa_ as the dominant species, these results agree with recent studies based on polyphasic analysis, which confirmed the presence of both species during the algal blooms in SMO lake [21; 22]. It is plausible that, in our study, both species were correctly identified and that both species were present. However, _L. robusta_ does not produce toxins, as revealed by the primer pairs DQmcyF/DQmcyR for microcystin/nodularin; sxtA F/sxtA-R for savitoxin; and cyrf/cyrfR for cylindrospermopsin [47]. The cyanobacteria _M. aeruginosa_ is the only MC producer detected in the lake to date, so it is plausible to conclude that the MC profile found in this study corresponds to this species. Therefore, this could be the first report of the MC profile for a Mexican population of _M. aeruginosa_. _Microcystis smithii_, another MC producer, has been recorded in the lake [21; 22]. However, we did not detect this species in our samples. Studies, including polyphasic analysis of the cyanobacterial community, are needed to confirm the presence of this species in the lake. Although the number of MC structural variants in blooms is high, generally, just a few analogs contribute to the bulk share of the total of microcystin concentrations [48]. In our samples, MC-WR, MC-LR and MC-LA represented 94.5% of the bulk, which also agrees with other studies. For instance, a study of MC profiles in freshwater bloom samples from a tropical lake (Guatemala) demonstrated the dominance of MC-LR, MC-RR and MC-YR, accounting for 98% of the toxin burden in _M. aeruginosa_ samples [49]. In other studies, high relative concentrations of MC-LR, MC-RR and MC-YR have also been reported from _Microcystis_ samples [50; 51]. Interestingly, two of the most potent MC congeners (LD50 of 50 \(\mu\)g kg\({}^{-1}\) b,w, i.p. mouse [2]), MC-LR and MC-LA, represented more than 54% of the total MC congeners. Therefore, if these analogs increase in the future, their potential effects may result in ecosystem disruption and higher effects on animals and humans. The genus _Microcystis_ in the urban lakes of Mexico City is present when the temperature ranges between 20 \({}^{\circ}\)C and 25 \({}^{\circ}\)C, with a nitrate:phosphate proportion below 1 and a pH \(\geq\) 10, similar to data referred to from other geographic regions [52; 53].