Predator-prey relationships between mixotrophic protists influence summertime plankton in the Baltic Sea

Illustrative photo by
Illustrative photo by

A doctoral thesis defended at TUT explains how predator-prey relationships between mixotrophic protists influence summertime plankton in the Baltic Sea.

Recently Karin Ojamäe defended her doctoral thesis “The Ecology and Photobiology of Mixotrophic Alveolates in the Baltic Sea” at the Marine Systems Institute at Tallinn University of Technology. The doctoral thesis defended at TUT investigated the impact of environmental conditions and the predator-prey relationships between mixotrophic protists on the plankton community and biogeochemical cycles in the Baltic Sea.

Mixotrophic protists (unicellular eukaryotes) are characterised by flexible nutritional strategy, i.e. the ability to photosynthesize like plants and absorb energy by consuming prey. Over the last few decades it has been found that such a flexible way of life is very common among protists. However, what remains unclear is the contribution of mixotrophic
protists to the net microplankton community production, the predator-prey relationships (who eats whom and how much), the balance between two different nutritional strategies and what happens when the environmental conditions do not favour one or the other or both nutritional strategies.

During the period of seasonal water column stratification and nutrient deficiency in the upper mixed layer, a significant proportion of the biomass of the microplankton community in the Baltic Sea is formed by the mixotrophic species. The objective of the defended PhD thesis was to understand better the ecophysiology of mixotrophs, i.e to investigate how different environmental factors and predator-prey relationships influence the population dynamics of common summertime species.

In the framework of the doctoral thesis laboratory experiments were conducted to study everyday life of dinoflagellate: predator-prey interactions, feeding behaviour, photosynthetic capacity in varying environmental conditions supported by kleptochloroplasts (which the predators use for photosynthesis), capacity for inorganic nutrient uptake in cold and dark environmental conditions and the subsequent improvement of the population photophysiology after dark nutrient uptake.

The supervisor of the doctoral thesis, Lead Researcher at Marine Systems Institute, Inga Lips said, “The doctoral thesis described the impact of microscopic unicellular organisms on the prey population and it was found that toxic predators can have direct (consuming prey) as well as indirect (prey lysis caused by toxic mucus secreted by predator cells) impact on the prey population.” In addition, the laboratory experiments conducted confirmed the hypothesis that in the Gulf of Finland in summer the populations of mixotrophic dinoflagellate, Heterocapsa triquetra, migrate vertically from surface layers to the deeper layers (to the depth of 15-35 meters) to acquire nutrients and thereafter assimilate the acquired nutrients in the euphotic layer in order to improve the photosynthetic efficiency of cells.

“The results of the doctoral thesis contribute to a more explicit understanding of the success of the mixotrophic species in marine ecosystems (formation of biomass maxima in various vertical depths) in optimal and nonoptimal environmental conditions”, the Lead Researcher Inga Lips added.

The supervisor of the doctoral thesis was Inga Lips, Lead Researcher at the Marine Systems Institute at Tallinn University of Technology
Opponents: Janne-Markus Rintala from University of Helsinki and Kersti Kangro from Tartu Observatory

The doctoral thesis has been published in the digital collection of TUT library:

Original post published by Tallinn University of Technology.