Host-Parasite-Herbicide Interactions in Lake Phytoplankton: an Eco-Evolutionary Approach

This project was my PhD thesis which I conducted at the water research institut EAWAG (ETH Z├╝rich) under the supervision of Dr. Bas Ibelings and Dr. Piet Spaak. In this project I studied the influence of anthropogenic pollution on host-parasite interactions. In nature, organisms experience simultaneous biotic and abiotic stressors. Parasites are ubiquitous and they represent an important natural stress factor that influences host populations. There is increasing evidence from a variety of host-parasite systems that parasitism acting in concert with other stressors has more detrimental effects on host organisms. I studied this concept with the aquatic host-parasite system Asterionella formosa (freshwater diatom) and Zygorhyzidium planktonicum (chytrid) and a photosynthesis inhibiting herbicide as anthropogenic stressor. The main result of this project was that herbicide exposure modifies infection dynamics and impact of disease on host populations through the complex interplay between stress, host and parasite growth dynamics and host population phenotype (Van den Wyngaert et al. 2013). More detailed experiments and agent based model simulations revealed that interference of the herbicide with host finding mechanisms (chemotaxis) was one of the key factors explaining the difference in parasite spread upon herbicide exposure (Van den Wyngaert et al. 2014). In terms of implications for disease dynamics the results of this project show that herbicide exposure decreases spread of disease through its negative effect on infectivity, except under very high host densities.

Additionally, I looked at the genetic diversity and population structure of the diatom host Asterionella formosa with high resolution microsatellite markers. Intraspecific diversity (genetic polymorphism) represents the evolutionary and adaptive potential of each species to a changing world, whether these changes are due to natural factors like coevolving parasites or human caused environmental pressures. Results reveal a complex pattern of population structure with evidence for both local and cosmopolitan genotypes, suggesting the presence of cryptic species in Asterionella formosa (Van den Wyngaert et al. 2015 Molecular Ecology). These results have not only implications for future work on Asterionella-chytrid interactions but also for the general use of A. formosa in biogeographical and ecological studies.

This PhD gave me the opportunity to broaden my knowledge on host-parasite interactions, multiple stressors and introduced me to population genetics and individual based modeling.

Acquired skills:

  • isolation and culturing techniques of phytoplankton and their parasites
  • experimental design and statistical analysis (SPSS and R)
  • population genetic molecular tools (microsatellites).
  • individual based modeling

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