Unraveling the microbial eukaryotic parasites of copepods: from individual fitness costs to seasonal dynamics
2025
Eliassen, Lasse Krøger
Parasitism profoundly impacts ecosystems yet has historically been neglected in the largest ecosystem on Earth, the marine pelagic waters. Copepods are abundant zooplankton that play important roles in global carbon cycling. Several microbial eukaryotic parasites infect copepods, but the significance of these infections is poorly understood. In my thesis, I used a combination of modelling, field, and experimental approaches to study these parasite-host systems. The model suggests that parasites can regulate copepod populations and identifies key parasite and host properties that facilitate outbreaks. Using metabarcoding, many of the known parasites of copepods were found in the Oslofjord, Norway, where they remain throughout the year despite a seasonally fluctuating copepod host community. The thesis also provides quantitative data on the fitness costs of an enigmatic parasite, the Yellow-Hyphal Parasite, and describes its biology. Finally, we find that parasites are present in a long-standing copepod culture, which holds great promise for future experimental studies, a major hurdle in this research field. The research helps bridge the gap between parasitology and pelagic ecology, and contributes to the growing consensus that parasitism is widespread and ecologically significant, in copepods and in the pelagic realm at large.
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