Protists have divergent effects on bacterial diversity along a productivity gradient

Abstract

Productivity and predation are thought to be crucial drivers of bacterial diversity. We tested how the productivity-diversity of a natural bacterial community is modified by the presence of protist predators with different feeding preferences. In the absence of predators, there was a unimodal relationship between bacterial diversity and productivity. We found that three protist species (Bodo, Spumella and Cyclidium) had widely divergent effects on bacterial diversity across the productivity gradient. Bodo and Cyclidium had little effect on the shape of the productivity-diversity gradient, while Spumella flattened the relationship. We explain these results in terms of the feeding preferences of these predators.

Figure 1.

Composition and diversity of the microcosm bacterial communities. The top row of panels shows the mean relative abundance of each bacterial taxon across the productivity gradient for (a) no predators, (b) Bodo sp., (c) Spumella sp., and (d) Cyclidium. sp. Colour intensity indicates the relative abundance of each bacterial taxon. The tRFLP fragment size (number of base pairs) associated with each taxon is shown on the y-axis. (e–h) shows the bacterial biodiversity, measured as the complement of the Simpson index (1 − λ), across the productivity gradient for each predator treatment. Broadly similar results are obtained if the Shannon–Wiener index is used (electronic supplementary material, figure S2). Curves are fitted regression lines.

Figure 2.

Relative abundance of each bacterial species in the presence of (a) Bodo sp., (b) Cyclidium sp., and (c) Spumella sp. as a function of their relative abundance in microcosms without predators. The axes are on an arcsine transformed square-root scale.