Abonyi, A. | Search bioRxiv

bioRxiv2022

Chytrids-conveyed long-chain polyunsaturated fatty acids to Daphnia alleviate the detrimental effect of heat when combined with limiting dietary organic matter quantity and nutritional quality

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.

ecology↗

bioRxiv2022

Dispersal provides trophic-level dependent insurance against a heatwave in freshwater ecosystems

Climate change-related heatwaves are major recent threats to biodiversity and ecosystem functioning. However, our current understanding of the mechanisms governing community resilience (resistance and recovery) to extreme temperature events is still rudimentary. The spatial insurance hypothesis postulates that diverse regional species pools can buffer ecosystem functioning against local disturbances through immigration of better adapted taxa. However, experimental evidence for such predictions from multi-trophic communities and pulse-type disturbances, like heatwaves, are largely missing. We performed an experimental mesocosm study with alpine lake plankton to test whether a dispersal event from natural lakes prior to a simulated heatwave could increase resistance and recovery of local communities. As the buffering effect of dispersal may differ among trophic groups, we independently manipulated dispersal of organisms from lower (microorganisms) and higher (zooplankton) trophic levels. The experimental heatwave suppressed total community biomass by having a strong negative effect on zooplankton biomass, probably due to a heat-induced increase in metabolic costs that in turn caused mortality. Heating thus resulted in weaker top-down control and a subsequent shift to bottom-heavy food webs. While zooplankton dispersal did not alleviate the negative heatwave effects on zooplankton biomass, dispersal of microorganism enhanced biomass recovery at the level of phytoplankton, thereby providing evidence for spatial insurance. The different response of trophic groups may be related to the timing of dispersal, which happened under strongly monopolized resource conditions by zooplankton, creating limited opportunity for competitors to establish. At the same time, the heatwave released phytoplankton from grazing pressure and increased nutrient recycling, which may have facilitated the establishment of new phytoplankton taxa. Our findings clearly show that even a short heatwave can strongly alter energy flow in aquatic ecosystems. Although dispersal can enhance community resilience, the strength of its buffering effects depends on the trophic level.

ecology↗