Control of bacterial biofilm communities by protozoans under semi-natural conditions
Final Report Abstract
The major part of the bacterial biomass in most shallow freshwater systems is found in biofilms, surface-attached communities of bacteria, protists and small metazoa. These communities stand at the basis of biogeochemical cycles as well as the food web, especially in turbid systems where autotrophic production is limited. However, little is known about the regulation of these communities, which is in part due to their difficult accessibility. Within this project, we developed new approaches to analyze protozoa-bacteria interactions in situ by molecular, microscopical (light and confocal laser-scanning) and new experimental approaches. Focusing on food web interactions, we based our hypotheses on the general assumption that bacteria can be consumed by protozoa (heterotrophic protists, e.g. heterotrophic flagellates (HF) and ciliates) in the biofilm, although, contrary to planktonic systems, a certain grazing resistance through embedment into an extracellular matrix (EPS) was expected. In a river bypass system ensuring near-natural background parameters and highly diverse bacterial and protist communities, we were able to show that certain protozoa (ciliates) strongly facilitate biofilm microcolony formation and overall biomass and interpret this as a grazer defense mechanism. However, we did not record such a reaction to the presence of HF. Nevertheless, bacterial richness and diversity were affected by protozoa in all stages of biofilm formation, surprisingly even in the first two hours of substrate colonization and irrespective of morphological changes of the biofilm. In a similar river-bypass approach, we observed certain relevant gliding/surface-feeding (benthivorous) and sessile/plankton-feeding (planktivorous) protozoan species and their effects on biofilms via longterm video-microscopy. Here, we could show that different species of benthivorous protozoa of the same functional group can display very different grazing behaviors in situ. Moreover, planktivorous protozoa were shown to have indirect positive effects on bacterial biofilm biomass and the abundance of associated flagellates, supposedly by “sloppy feeding” and other mechanisms recycling organic material into their immediate surroundings. Together, we could for the first time take a differentiated look at the complex protozoa-bacteria interactions in near-natural biofilms, covering aspects from biomass to diversity to autecological parameters such as in situ feeding rates of individual protozoa. During the project, we integrated questions regarding the effects of nutrient stoichiometry (i.e., the ratio of C:N:P in the growth medium) on diverse bacterial biofilms and subsequent effects for protozoa-bacteria interactions. Again, we aimed to analyze near-natural communities, this time in laboratory systems with a bacterial community and protozoa isolated from a large river. We showed that the bacterial biofilm community is homeostatically regulated and maintains a relatively stable C:N:P stoichiometry of approximately 106:16.5:1.2, irrespective of the nutrient composition in the growth medium. Nevertheless, bacterial biomass and biofilm morphology are affected by the nutrient stoichiometry, with the strongest reactions to low N fractions. First results also show that the abundance of the biofilm associated hoover feeder Chilodonella sp. is positively influenced by higher biofilm biomass, the biofilm therefore seems to offer no grazing protection through enhanced growth. We expect different effects for other protozoan feeding types in subsequent experiments. The research within this project gave new and differentiated insights into protozoa-bacteria interactions in near-natural biofilm communities. Our results also provide methodological advances for further investigations, making it possible to overcome the limitations of approaches with a limited number of bacterial strains and/or protozoan species in laboratory systems.
Publications
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(2011) Control of heterotrophic biofilm communities - The importance of grazing and dispersal. Dissertation thesis, University of Cologne
Wey, JK
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(2011) From the laboratory into the field: Testing defense mechanisms of bacterial biofilms against protozoan grazing. Dissertation thesis, University of Cologne
Erken M
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(2012) Quantification of individual flagellate-bacteria interactions within semi-natural biofilms. Protist 163:632-642
Erken M, Farrenschon N, Speckmann S, Arndt H, Weitere M
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(2012) Seasonal and successional influences on bacterial community composition exceed that of protozoan grazing in river biofilms. Appl. Environ. Microbiol. 78(6):2013-2024
Wey JK, Jürgens K, Weitere M
