Light exposure decreases infectivity of theDaphniaparasitePasteuria ramosa

Climate change is altering light regimes in lakes, which should impact disease outbreaks, since sunlight can harm aquatic pathogens. However, some bacterial endospores are resistant to damage from light, even surviving exposure to UV-C. We examined the sensitivity ofPasteuria ramosaendospores, an aquatic parasite infectingDaphniazooplankton, to biologically relevant wavelengths of light. Laboratory exposure to increasing intensities of UV-B, UV-A, and visible light significantly decreasedP. ramosainfectivity, though there was no effect of spore exposure on parasitic castration of the host.P. ramosais more sensitive than itsDaphniahost to damage by longer wavelength UV-A and visible light; this may enableDaphniato seek an optimal light environment in the water column where both UV-B damage and parasitism are minimal. Studies of pathogen light sensitivity help us uncover factors controlling epidemics in lakes, which is especially important given that water transparency is decreasing in many lakes.