Influence of pH on amine toxicology and implications for harmful algal bloom ecology

In marine and estuarine conditions, pH-dependent aquatic toxicity associated with Prymnesium parvum has been previously demonstrated with greater toxicity observed at higher pH. Recent research by our group extended such observations to inland waters in Texas, USA. Because prymnesins (prymnesin-1, prymnesin-2) represent the only P. parvum toxins reported in the peer-reviewed literature, we hypothesized that P. parvum toxins may behave like weak bases in surface waters due to the presence of a primary amine. Employing a widely accepted computational model (ACD/Labs), we predicted physicochemical properties to support interpretation of experimental findings, specifically focusing on the amine-containing hydrophobic portion of these molecules. Using different computational models, alternative pKa values for prymnesins have been proposed by others. Herein, additional examination of our proposed hypothesis identifies the importance of considering subtle changes in ionization state on the lipophilicity, bioavailability (e.g., log D, log Dlip−water, Bioconcentration factor), and aquatic toxicity of ionizable chemicals over an environmentally-relevant pH gradient. If prymnesins represent causative toxins released by P. parvum as previously reported or if new toxins are identified in the future, a closer evaluation of our findings further highlights the importance of considering site-specific pH during environmental assessment and management efforts for P. parvum in inland waters.