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Experimental study of the temperature rise effect on the ciliate community as part of the microbial food web in the Adriatic Sea

Ciliates represent the important component of the microbial food web, characterised by high biomass and intensive population dynamics, which enables them to adapt rapidly to ecosystem changes. Beside the changes in the community composition and trophic relationships this study also investigated the impact of temperature rise on microbial carbon fluxes in the experimental conditions. Dilution experiments were used for simultaneous estimations of protozoan grazing on picoplankton (bacteria, Prochlorococcus, Synechococcus and picoeukaryotes) under in situ and 3°C above in situ temperature. Picoplankton mortality was primarily caused by predation, especially in cold season, while viral lysis was of less intensity. The contribution of heterotrophic nanoflagellates to total grazing on picoplankton was higher than contribution of ciliates. However, direct ciliate grazing on Prochlorococcus and Synechococcus exceeded heterotrophic nanoflagellates grazing in most cases. Ciliates satisfied their carbon demand dominantly through the grazing on heterotrophic nanoflagellates and bacteria, and the role of autotrophic picoplankton as their prey increased significantly in the cold months. Differences in the pray composition were related to the ciliate community composition and the size categories of ciliate cells. Under the warm condition, the amount of picoplankton biomass transferred to protozoan predators exceeded the lysed biomass. The relative contribution of these loss factors for microbial mortality is crucial for a better understanding of the carbon fluxes in marine environments, suggesting that global warming could further increase picoplankton carbon flow toward higher trophic levels in the Adriatic Sea, which consequently affects the ecology of microbial food webs and functioning of the marine ecosystem.

ciliates, protozoan grazing, carbon flux, global warming, Adriatic Sea

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