Global warming enhances the dominance of poorly palatable PUFA-deprived bloom-forming cyanobacteria. Chytrid fungal parasites increase herbivory and dietary access to polyunsaturated fatty acids (PUFA) across the phytoplankton-zooplankton interface. Little is known however about the role chytrids may play in compensating for the decrease of algae-derived PUFA under global warming scenarios. We tested experimentally the combined effects of water temperature increase and the presence of chytrids with Daphnia magna as the consumer and the cyanobacterium Planktothrix rubescens as the main diet. We hypothesised that the diet including chytrids would enhance Daphnia fitness due to increased PUFA transfer irrespective of water temperature. Chytrid-infected diet significantly increased Daphnia survival, somatic growth, and reproduction, irrespective of water temperature. The PUFA content of Daphnia feeding on the chytrid-infected diet was unaffected by heat at the onset of the first successful reproduction. Carbon stable isotopes of fatty acids highlighted preferential n-3 PUFA upgrading by chytrids and an ~3x higher endogenous n-3 PUFA conversion compared with n-6 PUFA by Daphnia, irrespective of water temperature. Diet including chytrids enhanced the retention of eicosapentaenoic acid (EPA; 20:5n-3) and arachidonic acid (ARA; 20:4n-6) in Daphnia. The heat did not decrease EPA and even increased ARA retention by enhanced endogenous bioconversion in Daphnia when feeding on the chytrid-infected diet. We conclude that chytrids support Daphnia fitness at higher water temperatures via increased n-3 and n-6 PUFA retention and preferential n-3 PUFA bioconversion. Thus, they help function pelagic ecosystems with PUFA availability at the phytoplankton-zooplankton interface in a warmer climate.
