Current Strengths and Limitations of Toxicokinetic-Toxicodynamic (TKTD) Models for Freshwater Microalgae

Toxicokinetic-toxicodynamic (TKTD) models are increasingly used to enhance environmental risk assessment by linking laboratory toxicity data with dynamic exposure scenarios. For freshwater microalgae, which are crucial primary producers, TKTD approaches offer a framework to predict population growth inhibition and recovery under fluctuating chemical stress. A review of 12 studies (2012-2023) applying TKTD models to freshwater planktonic microalgae was conducted to compare their structural assumptions, end points, and validation methods. These models mainly focus on green algae and herbicides that inhibit photosystem II, with limited exploration of other taxonomic groups or modes of action. The complexity of models varies significantly, from simple structures assuming rapid equilibrium between external and internal chemical concentrations to mechanistic models that explicitly incorporate TK components and damage progression. While simple models support regulatory use, omitting TK processes can reduce their effectiveness for chemicals with delayed effects or complex uptake behaviors. Expanding taxonomic coverage and enhancing external validation under realistic exposure conditions is crucial to increasing the ecological relevance of algal TKTD modeling.