PHOSPHORUS-ALTERED PARASITE-HOST INTERACTIONS EXAMINED AT MULTIPLE SCALES

Author

Scott William Binger, Southern Illinois University CarbondaleFollow

Date of Award

12-1-2024

Degree Name

Master of Science

Department

Zoology

First Advisor

Narr, Charlotte

Abstract

Shifts in the availability of nutrients in freshwater ecosystems can have wide-ranging impacts on parasite-host interactions. However, the mechanisms that drive these impacts are poorly understood, in part due to the challenges involved in directly observing parasites. Daphnia-microparasite systems have emerged as a common model system to examine the ecology and evolution of parasites, with a great diversity of parasitic strategies seen across parasite taxa. Daphnia are important herbivores in freshwater systems, which makes them ideal organisms to study to understand the impact of changes in freshwater primary production. Since freshwater systems are often phosphorus limited and P is linked to algal quantity and quality, my work here focuses on the effect of P availability on Daphnia-microparasite systems. Here, I chose to study the microsporidian Hamiltosporidium tvaerminnensis and the bacterium Pasteuria ramosa due to their differing infection strategies, examining H. tvaerminnensis at the host population scale, and P. ramosa at the level of the individual host. Hamiltosporidium tvaerminnensis infects its host via both vertical transmission (to hosts’ offspring) and horizontal transmission (through the ingestion of spores released by dead hosts). Previous work on individually housed Daphnia suggests that high P Daphnia diets increase H. tvaerminnensis spore loads while reducing the competitive advantage (fecundity) of uninfected Daphnia relative to infected ones. However, it is unclear how these individual level effects scale up to the population level, where effects of P availability on host density may mediate the relationship between P and this parasite’s abundance. For my first study, I chose to explore how these effects scale up to affect transmission. I manipulated phosphorus availability in mesocosms seeded with 4 vertically infected and 4 uninfected Daphnia. After sixty days, I found significant positive relationships between algal P concentration and the prevalence, spore load, and total population size of this parasite. These results are consistent with previous work suggesting that competition from uninfected individuals can be an important driver of prevalence for this parasite, and that nutrients can shift the outcome of that competition in ways that alter parasite populations. I also found a significant negative relationship between algal density and spore load. This result demonstrates one way that resource-consumer dynamics can influence how parasites move through systems. For my second study, I chose a mechanistic modeling approach at the level of the single host. The Dynamic Energy Budget framework explicitly and mechanistically models the flow of energy through an organism and has been applied to several parasite-host systems. Here, I focused on Pasteuria ramosa, which exclusively transmits horizontally. Previous work suggests that P. ramosa shows a great deal of trait plasticity, and I was interested in creating a model that could demonstrate that trait plasticity and its effect on parasite growth. My goal was to model the results of a previous study run on Daphnia in our lab, which showed acclimation to hosts fed at different P levels. I identified two potential parameters of a bacterial growth model, uptake rate and metabolic maintenance rate, that could covary and create separate strategies for the parasite—a high uptake, low efficiency strategy and a conservative, low uptake strategy. Each strategy, I predicted, would be more successful under different host diets. I then ran 30 day simulations with combinations of each parasite strategy and host diet. These simulations showed that, by varying uptake and maintenance rates, the observed trends in parasite acclimation could be modeled. These studies together demonstrate how P availability can affect host-parasite interactions at multiple scales, in parasites with multiple infection strategies. Given how widespread parasites and nutrient inputs are in natural systems, continued work is needed to detangle these complex relationships.