Top-down structuring of freshwater bacterial communities by mixotrophic and heterotrophic protists

Mixotrophic and heterotrophic protists hold a key position in aquatic microbial food webs. They account for the bulk of bacterivory in pelagic systems. However, the potential structuring effect of heterotrophic and especially mixotrophic protists on bacterial communities is far from clear. We conducted standardized short-term grazing experiments, to test for the overall impact on bacterial community structure and possible prey preferences of protist taxa with different phagotrophic nutritional modes. The used protist taxa covered a range from more phototrophic towards heterotrophic strategies, with obligate mixotrophic taxa employing phototrophy as the dominant strategy, towards facultative mixotrophic taxa relying more on phagotrophy, with the end of this gradient being covered by a phagoheterotroph lacking phototrophic capacity. Bacterioplankton from different lake systems was enriched and used to represent semi-natural bacterial assemblages that served as prey communities. Our study showed that similarities in protistan nutritional modes were reflected in similarities in their overall impact on bacterial communities. The impact intensity increased towards clear phagotrophic strategies, with mixotrophs in the phototrophic end of the gradient rather having a stabilizing impact on bacterial communities. Obligate mixotrophs grazed diverse prey in small amounts, while towards strict heterotrophic nutrition we observed an ingestion increase and selective depletion of bacteria with potential high growth rates. Future global change scenarios supporting the domination of obligate mixotrophic bacterivores might promote higher bacterial diversity in the illuminated zone of lakes.