Stuck in the mud: suspended sediments as a key issue for survival of chrysomonad flagellates
The effect of suspended fine sediments on chrysomonad flagellates was investigated for 'Spumella-like' flagellates in laboratory studies and exemplarily for a flagellate community (with a focus on chrysomonads) originating from the oligomesotrophic Lake Mondsee, Austria, using different clay minerals and silicate beads. In the community experiment, the abundances of Spumella-like flagellates decreased significantly after introduction of suspended clays, but on the community level suspended clays did not negatively affect flagellate abundance. In order to understand this taxon-specific response, the influence of different clay characteristics, specifically of particle concentration and size, was investigated for Spumella-like flagellates in laboratory studies. We used ultra-microbacteria, i.e. typical bacterioplankton of Lake Mondsee, as food. For comparison we also investigated growth rates of the Spumella-like flagellates feeding on the large bacterium Listonella pelagia. These experiments confirmed that at bacterial abundances realized in oligotrophic and mesotrophic lakes, the flagellates are severely food limited. The presence of suspended sediments generally decreased the growth at any food concentration tested. This decrease was reflected in shifts of the growth kinetic parameters, i.e. in a positive correlation between the half-saturation constant and the threshold food concentration and a negative correlation between the maximal growth rate and the suspended sediment concentration. The clearance rates strongly decreased when small and large particles were present, but were only slightly affected for intermediate-sized particles. We assume that small particles in the size range of ingestible bacteria interfere with the feeding process and cause lower clearance rates, while intermediate-sized particles may serve as substrate for the attachment of flagellates and to subsequently optimize their clearance rates.
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