Invertebrates possess an innate immune system that acts non-specifically against pathogens and is regulated by the circadian clock. Using the host–parasite system, Daphnia magna and its bacterial parasite Pasteuria ramosa, we investigated day–night differences in susceptibility. In an infection experiment where hosts were exposed to spores either during the day or night, infection was slightly higher during daytime exposure and in animals treated with exogenous melatonin. Daphnia exhibited rhythmic expression of five immune genes, with low daytime expression and a pronounced, synchronized peak immediately after the transition from day to night. This timing aligns with the documented increase in daytime susceptibility, which may benefit Pasteuria as encounter rates rise when Daphnia forage in sediment during diel vertical migration. Melatonin exposure altered immune gene expression and increased susceptibility both day and night. Melatonin can act as an immune suppressor and may also influence parasite spore maturation. Disruption of circadian rhythms and melatonin signaling by anthropogenic stressors alters the infection dynamics in this freshwater keystone organism, with consequences for population stability, ecosystem functioning, and the conservation of freshwater biodiversity. Our results spotlight the mechanisms underlying infection risk in host–pathogen systems, highlighting the importance of circadian regulation for disease dynamics in freshwater ecosystems.
Schwarzenberger et al. (Mon,) studied this question.