Microbial diversity and community–environment relationships in boreal streams
The perspective that microbial communities are controlled solely by environmental factors (‘everything is everywhere, but the environment selects’) has persisted for a long time, notwithstanding that recent studies have found that both environmental factors and spatial processes are important. We examined variation in the structure of fungal and bacterial communities in boreal streams along large local environmental gradients (e.g. stream size, acidity and nutrients) at intermediate spatial extent. Owing to the intermediate spatial extent of our study area and high dispersal rates of microbes, we expected that local environmental factors would structure stream fungal and bacterial communities.
A set of 30 streams was sampled along a geographical transect from western to eastern Finland.
Leaf packs were used as sampling units in each stream. Fungal and bacterial operational taxonomic units (OTUs) were determined using pyrosequencing in the laboratory. Species accumulation curves were used to assess how well the regional species pools were sampled. Partial redundancy analysis and partial linear regression were used to determine the relative contributions of environmental and spatial variables to community structure and OTU richness, respectively.
We found that environmental control was important in structuring fungal and bacterial communities, yet much of the variation was attributable to the shared effects of environmental and spatial predictors, or remained unexplained. The composition of fungal and bacterial communities was most strongly related to water chemistry variables (i.e. pH, aluminium and total phosphorus), whereas habitat variables (i.e. riparian deciduous trees, moss cover, substratum particle size and stream width) were clearly less important. We also found that fungal richness was negatively related to water iron concentration, whereas bacterial richness showed only a weak relationship with water pH.
The composition and richness of stream microbial communities are mostly related to water chemistry variables at the spatial extent studied, emphasizing an important role for species sorting. This finding supports the traditional perspective that local environmental conditions are important drivers of variation in microbial communities. However, much of the variation in fungal and bacterial communities remained unexplained, suggesting that drivers of these communities are likely to be complex and not yet fully understood.