Microbial decomposers communities are mainly structured by trophic status in circumneutral and alkaline streams

Abstract

In streams, the release of nitrogen and phosphorus is reported to affect microbial communities and the ecological processes they govern. Moreover, the type of inorganic nitrogen (NO3, NO2 or NH4) may differently impact microbial communities. We aimed to identify the environmental factors that structure aquatic microbial communities and drive leaf-litter decomposition along a gradient of eutrophication. We selected 5 circumneutral (Portuguese) and 5 alkaline (French) streams differing in nutrient concentrations to monitor mass loss of alder leaves, bacterial and fungal diversity by PCR-DGGE, fungal biomass and reproduction, and bacterial biomass during 11 weeks of leaf immersion. The concentrations of inorganic nutrients in the stream water ranged from 5 - 300 µg L-1 SRP; 0.30 – 5.50 mg L-1 NO3-N; 2 - 103 µg L-1 NO2-N; and < 4 - 7,100 µg L-1 NH4-N. Species richness was maximum in moderately anthropized (eutrophic) streams but decreased in the most anthropized (hypertrophic) streams. Different species assemblages were found in subsets of streams of different trophic status. In both geographic areas, the limiting nutrient, either nitrate or phosphate, stimulated the microbial activity in streams of intermediate trophic status. In the hypertrophic streams, fungal biomass and reproduction were significantly lower and bacterial biomass dramatically decreased at the site with the highest ammonium concentration. The limiting nutrients that defined the trophic status were the main factor structuring fungal and bacterial communities whatever the geographic area. Very high ammonium concentration in the stream water most probably has negative impacts on microbial decomposer communities.