Ciliates as ecological engineers

Posted on January 5, 2013

The biofilms that occur on submerged surfaces in freshwater systems contain photosynthetic microbes and non-photosynthetic bacteria within a polymer-based matrix.  In these  biofilms, the arrangements of cells and groups of cells create microlandscapes that resemble mazes, mushrooms, regular patterns and other forms.  Because the clustering of microlandscape forms reduces the surface area : volume ratio, and therefore rates of diffusion and enzyme action, the structure of biofilms can influence ecosystem processes like primary production and respiration.  Biofilms also act as a source of food for invertebrate grazers, so it’s possible that the feeding and movement activities of these organisms have an impact on biofilm architecture, and thus ecosystem metabolism.  To explore this possibility, Dutch ecologists used a confocal laser scanning microscope to compare the three-dimensional structure of biofilms in the presence and absence of two species of ciliated protozoans (Urostyla sp. and Paramecium bursaria) that graze on bacteria.  The presence of either  species had the effect of reducing the aggregation of matrix material, but Urostyla also increased matrix volume. This showed that ciliates can have beneficial effects on photosynthesis, since matrix material, which is probably secreted by algae as a protective response, increases nutrient uptake from the water.   The presence of Urostyla also reduced the aggregation of bacteria in the lower part of the biofilm near the substrate, while the presence of Paramecium reduced the clustering of photosynthetic algae near the free surface of the biofilm.  It seems that the observed changes were caused by the movements of ciliates rather than their feeding activity, since the dispersion but not the volume of algae was affected.  These contrasting impacts of different ciliate species on biofilm structure show that ecosystem engineering (the ability of organisms to cause physical changes in their environment) occurs at the microscopic level as well as at larger scales.

Reference:  Weerman, E.J. et al.  2011.  Ciliates as engineers of phototrophic biofilms.  Freshwater Biology 56, 1358–1369.