Advanced description of the morphology of Cylindrospermopsis raciborskii in a shallow mediterranean lake

This study aims to identify the C. raciborskii species based on its morphology, examine its morphological variability by tracking its annual life cycle and environmental parameters fluctuations, and assess its toxicity by measuring cylindrospermopsin (CYN) in Lake Oubeira between October 2021 and September 2022. Microscopic observations revealed the existence of straight or slightly curved filaments. Filament ends exhibited various shapes: rounded (39%), intermediate (40%), pencil (16%), and needle (5%). However, only rounded filament ends were present throughout this cycle. Four morphotypes were observed: filaments with heterocytes and akinetes (< 1%), akinetes only (< 2%), heterocytes only (6%), and neither heterocytes nor akinetes (> 90%). Results showed that filaments without heterocytes or akinetes, and those with only heterocytes, positively correlated with nitrates, conductivity, turbidity, atmopspheric temperature, water temperature, and pH, but negatively with transparency and dissolved oxygen. Conversely, filaments with heterocytes and akinetes were positively correlated with precipitation (Preci) (p ≤ 0.001), showing a positive correlation with phosphates while filaments with only akinetes displayed negative correlations with WT and Trans. Only extracellular Cylindrospermopsin was detected during the study period, excluding February and April, with concentrations ranging from 80 to 750 ng/. Four CYN peaks exceeding 500 ng/L were offset from C. raciborskii biomass peaks. The observed morphological variability in filaments sampled from Lake Oubeira confirms the species as C. raciborskii. However, employing additional approaches, such as culturing, and molecular taxonomy, would be necessary to complete the study of this cyanobacteria species.