Multidimensional Cardiac Phenotyping of Daphnia magna Enables One-Hour Effect-Based Screening of Aquatic Toxicity

Abstract Operational water quality management relies on physicochemical monitoring and targeted chemical analysis, but routine surveillance covers only a prioritized subset of high-risk contaminants and does not capture biological activity from unmonitored chemicals, transformation products, or complex mixtures. Whole-organism bioassays provide complementary effect information, yet the conventional Daphnia magna (D. magna) acute immobilization end point requires 48 h. Here, we introduce D-HEART (Daphnia-based Heartbeat Evaluation and Analysis for Rapid Toxicity Screening), a microfluidic imaging and analysis framework that converts high-speed cardiac recordings into 116 descriptors spanning morphology, pumping, wall motion kinematics, and synchrony. Within the tested response space, D-HEART assigns cardiac-response classes using a six-contaminant reference library, while its Heartbeat Toxicity Index (HTI) estimates 48 h immobilization from the predicted class and phenotype. Transcriptomic profiles provided molecular context for differentiated 1 h cardiac states. Post-training Cd(II) tests across environmental water matrices tracked immobilization with greater uncertainty than in laboratory water, and two untreated industrial wastewaters showed dose-responsive HTI estimates for mixed exposures from unseen contaminant classes. By linking early physiological states to transcriptional responses and later apical toxicity, D-HEART provides a rapid, cross-time scale effect-screening layer for operational water-quality evaluation, reducing the detection window by approximately 98% relative to the conventional 48 h immobilization end point.