Functional Diversity of Planktonic Ciliates

Ciliates are a dominant group of unicellular microorganisms (protists) that occurs in virtually all water bodies. Ciliates are key elements of aquatic food webs, acting as predators of bacteria, algae, other protists and even some metazoans. Planktonic ciliates are important food for zooplankton, and mixotrophic and functionally autotrophic species may significantly contribute to primary production in lakes and in the ocean. Therefore, the question of whether ciliate functional diversity and their ecosystem roles differ between marine and freshwater ecosystems is of fundamental importance for the global biodiversity debate and water management. To this end, this project will provide the first rigorous analysis to test if the functional diversity differs between marine and freshwater ciliates. It is based upon an established cooperation between four research laboratories located in Austria, the UK and China. This research takes an interdisciplinary approach, integrating the project partners expertise from classical and molecular taxonomy, phylogeny, ecology, and modelling, building bridges between the traditionally separated disciplines Biological Oceanography and Limnology. The project results will be published in scholarly journals and media releases and lectures for the public.

Ciliates are a dominant group of unicellular microorganisms (protists) that occurs in virtually all water bodies. Ciliates are key elements of aquatic food webs, acting as predators of bacteria, algae, other protists and even some metazoans. Planktonic ciliates are substantive food for zooplankton, and mixotrophic and functionally autotrophic species may significantly contribute to primary production in lakes and the ocean. Therefore, whether ciliate functional diversity and their ecosystem roles differ between marine and freshwater ecosystems is of fundamental importance for the global biodiversity debate and water management. An international research project led by the Research Department for Limnology, Mondsee, of the University of Innsbruck, with cooperation partners in Austria, the UK and China, addressed this issue. For the first time, the project results demonstrate that marine and freshwater ciliates are functionally different. The thermal sensitivity of growth and mortality rates, which primarily influence the ciliates' population dynamics, differed significantly in these ecosystems. The ciliates have adapted to the prevailing thermally stable environmental conditions in the sea with relatively constant growth rates. On the other hand, ciliates are better able to respond to rapidly changing temperatures often encountered in freshwater lakes. Furthermore, ciliate grazing loss rates by microcrustacean predators differed significantly in the aquatic habitats. In many lakes, ciliates are permanently or temporaily decimated by copepod and cladoceran predation. In contrast, the grazing pressure by copepods is too low in the ocean to control ciliate population sizes. Unlike lakes, ciliate population dynamics are primarily controlled by resources (i.e., food availability) in the open sea. Accordingly, the ciliate role in marine and limnic food webs differs. The new insight into physiological and structural differences must be considered in predictive models on the effects of global warming on planktonic ciliates. These differences between marine and freshwater ecosystems likely apply to other protists.