Dynasym

Phytoplankton and associated heterotrophic bacteria communities form a symbiosis. In this relationship phytoplankton besides of habitat provide the major organic carbon and nutrient source to the bacteria while the bacteria provide micronutrients, vitamins and remineralised macronutrients to the phytoplankton. The form of symbiosis is not fixed and can shift from mutualism over commensalism to parasitism. The form of symbiosis likely depends on the partners’ absolute and relative densities and their community composition that are regulated by the environment (i.e. nutrient regime/ temperature). In nutrient rich and pulsed conditions, the relationship between phytoplankton and heterotrophic bacteria is temporary mutualistic during the build-up of a bloom followed by commensalism and parasitism during stationary phase when nutrients are limited. Here the phytoplankton is dominated by larger diatoms. Absolute densities of partners are high with a low bacteria biomass in relation to phytoplankton. In nutrient poor non-pulsed conditions, the plankton is dominated by small picoplankton. The association appears stably mutualistic with low absolute densities of both partners and a high share of bacteria biomass in relation to phytoplankton. In this project we aim to experimentally study the role of density dependence and community composition for the form of symbiosis between phytoplankton and heterotrophic bacteria and to understand their consequences for pelagic microbial community dynamics and functioning. In Objective 1 we aim to disentangle the role of phytoplankton and bacteria absolute and relative densities for the form of symbiosis. In Objective 2 we aim to understand how the form of symbiosis, that was investigated in Objective 1, and potentially the partner’s community compositions determine the dynamics and functioning of the entire plankton community. We aim to further understand how predicted environmental changes, mediated through density changes, shift the form of symbiosis with predictable functional consequences. The goal of Objective 3 is to identify and understand the stabilising factors maintaining mutualism in picoplankton symbiosis in oligotrophic conditions. To address these objectives, we have developed an experimental model system with three phytoplankton species (a diatom, a coccolithophore and a picoplankton species) and their associated bacteria.