Novel Near-Infrared-II <i>In Vivo</i> Visualization Revealed Rapid Calcium Intestine Turnover in <i>Daphnia magna</i> with Delayed Impact by Cadmium and Acidification

Calcium is a highly demanded metal, and its transport across the intestine of <i>Daphnia magna</i> remains a significant unresolved question. Due to technical constraints, the visualization of the kinetic process of Ca passage through <i>D. magna</i> has been challenging. Here, we developed the second near-infrared Ca sensor (NIR-II Ca) and conducted real-time <i>in vivo</i> imaging of Ca in daphnids with a high signal-to-noise ratio, deep tissue penetration, and minimal damage. Through the utilization of the NIR-II Ca sensor, we for the first time visualized and quantified the kinetic process of Ca passage in the intestine in real time. The results revealed that trophically available Ca passed through the intestines in 24 h, whereas waterborne Ca required only 35 min. This rapid "flushing through" mechanism established waterborne Ca as the primary source of Ca absorption. However, environmental stressors such as water acidification and cadmium significantly delayed the Ca passage and absorption. The development of NIR imaging and sensors allows for real-time dynamic visualization of contaminants/nutrients in organisms and holds great potential as a powerful tool for future studies into material kinetic processes in living animals.