Sorting of bacterial communities in a productivity and dispersal gradient

Background/Question/Methods

Bacteria show both high dispersal rates as well as a capacity for strong local dynamics. Microbial communities are thus expected to be connected by dispersal while species composition is expected to reflect local environmental conditions. In a metacommunity framework, this leads to the prediction that species sorting would be a dominant structuring process of species composition of microbial communities in heterogeneous environments. A field survey on bacterioplankton communities inhabiting eutrophic shallow lakes and ponds along a North-South gradient in Europe indeed revealed a strong match of community composition with environmental gradients at different spatial scales, with no evidence for dispersal limitation. Here we used a container experiment to quantify the impact of dispersal on species sorting along a strong environmental gradient.

We set up a container experiment involving 96 containers of 180 l content that were filled with tapwater and randomly assigned to factorial combinations of low/high productivity and three dispersal levels (no dispersal; weekly exchange of 40 ml; weekly 2 l). The 16 containers in each treatment were inoculated with zooplankton, algae and bacterioplankton originating from 16 different shallow lakes, except for the high dispersal treatment that received a fully mixed inoculum at the start.  After one month, samples were taken with a tube sampler. Taxon composition of the bacterioplankton was assessed using molecular fingerprinting (DGGE), and presence and relative abundance of OTUs were used to compare microbial communities across treatments.   

Results/Conclusions

Taxon composition of microbial communities in our mesocosm were strongly impacted by the productivity gradient, suggesting an important influence of species sorting along the imposed environmental gradient. There was, however, also a significant effect of dispersal on the composition of the microbial communities. Especially the high dispersal treatment differed from the other two dispersal treatments. There are three possible explanations: (1) dispersal limitation; (2) priority effects (the high dispersal treatment was started from a fully mixed bacterioplankton sample); (3) confounding effects from a gradient in grazer density (Daphnia). Daphnia was more widespread and abundant in the high dispersal treatments. The impact of dispersal treatment on taxon composition of bacterioplankton could thus also be an indirect impact of the treatment on the abundance of a keystone grazer known to influence bacterial communities.