Dynasym

Symbioses, defined as close associations of different species living together are common in aquatic systems and of crucial importance for ecosystem structure and persistence. The strength and form of a symbiosis depends on the associated costs and benefits for both symbiotic partners, resulting in either commensalistic, mutualistic, competitive, parasitic, or predatory interactions. These costs and benefits for symbiotic partners can be altered by both biotic and abiotic environmental conditions, making the form of symbiosis context dependent, potentially resulting in potential shifts along environmental gradients. However, we are currently lacking a comprehensive understanding of context dependent shifts in the strength and type of symbioses and its implications for ecosystem functioning, especially in planktonic microbial food webs.

SP4 will investigate the context dependence of a freshwater ciliate-microalgae symbiosis, targeting the widespread ciliate Coleps hirtus viridis and its endosymbiont Micractinium sp.. We will study context-dependent changes in the costs, benefits and the form of this symbiosis and their consequences for the population dynamics of both symbiotic partners and other species interacting with the symbiosis in a food web context. Using laboratory experiments, we will quantify the population growth of symbiont-free and symbiont-bearing Coleps as well as free-living and endosymbiotic Micractinium along four different gradients: 1) the density of the microalgae Cryptomonas sp. (prey of Coleps, competitor of Micractinium), 2) the density of a heterotrophic ciliate feeding on the same algal prey as Coleps, 3) dissolved inorganic nutrients (altering prey quality) and 4) temperature (altering prey quality and metabolic rates of ciliates and algae). We will combine simple monoculture experiments to quantify shifts in the costs and benefits of the Coleps-Micractinium symbiosis in response to these environmental gradients with more complex community experiments allowing for feedbacks between the form of symbiosis and the performance of different species. Here, we will follow the population dynamics and potential feedbacks among the symbiont-free and the symbiont-bearing Coleps with Micractinium in competition with the pure heterotroph and the algal prey at different temperature and nutrient conditions. Experimental findings will be compared to the results of a mathematical model that will be adapted from SP2 and used to optimize experimental designs and to identify underlying mechanisms with respect to the influence of species interactions and environmental conditions on the form of symbiosis and resulting population dynamics. Furthermore, we will systematically review the extensive body of literature on ciliate-microalgae symbioses for context-dependence to support our experiments and other subprojects of DynaSym, and we will synthesize the data on ciliate-microalgae symbioses generated within different subprojects of DynaSym.