Lake water chemistry and population of origin interact to shape fecundity and growth in Daphnia ambigua

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

Calcium is a highly demanded metal, and its transport across the intestine of Daphnia magna remains a significant unresolved question. Due to technical constraints, the visualization of the kinetic process of Ca passage through D. magna has been challenging. Here, we developed the second near-infrared Ca sensor (NIR-II Ca) and conducted real-time in vivo 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.

Lake water chemistry and local adaptation shape NaCl toxicity in Daphnia ambigua

The increasing application of road deicing agents (e.g., NaCl) has caused widespread salinization of freshwater environments. Chronic exposure to toxic NaCl levels can impact freshwater biota at genome to ecosystem scales, yet the degree of harm caused by road salt pollution is likely to vary among habitats and populations. The background ion chemistry of freshwater environments may strongly impact NaCl toxicity, with greater harm occurring in ion-poor, soft water conditions. In addition, populations exposed to salinization may evolve increased NaCl tolerance. Notably, if organisms are adapted to the water chemistry of their natal environment, toxicity responses may also vary among populations in a given test medium. We examined the potential for this evolutionary and environmental context to interact in shaping NaCl toxicity with a pair of laboratory reciprocal transplant toxicity experiments, using natural populations of the water flea Daphnia ambigua collected from three lakes that vary in ion availability and composition. We observed a strong effect of the lake water environment on NaCl toxicity in both trials. NaCl caused a much greater decline in reproduction and r in lake water from a low-ion/calcium-poor environment (20 μS/cm specific conductance; 1.7 mg/L Ca2+) compared with water from both a Ca2+-rich lake (55 μS/cm; 7.2 mg/L Ca2+) and an ion-rich coastal lake (420 μS/cm; 3.4 mg/L Ca2+). Daphnia from this coastal lake were most robust to the effects of NaCl on reproduction and r. A significant interaction between the population and lake water environment shaped survival in both trials, suggesting that local adaptation to the test waters used may have contributed to toxicity responses. Our findings that the lake water environment, adaptation to that environment, and adaptation to a contaminant of interest may shape toxicity demonstrate the importance of considering environmental and biological complexity in mitigating pollution impacts.