Desiccation affects bacterial community structure and function in temperate stream sediments

Headwaters in extended temperate regions of the world, including Central Europe, are going to be exposed to more frequent and longer lasting periods of drought, which can be attributed to global climate change and enhanced human withdrawal of water. The effects of desiccation on bacterial community composition and on microbial extracellular enzyme activities, a key process in the carbon flow of lotic environments, were investigated in the Breitenbach, a typical temperate stream located in Central Europe. Sandy sediment, a most important habitat of temperate streams, was sampled and exposed at 20 degrees C in the laboratory to different types of desiccation (fast and slow) over 8 weeks. Bacterial community composition was determined via CARD-FISH (catalyzed reporter deposition fluorescence in-situ hybridization) and extracellular enzyme activities by means of fluorogenic model substrates. The structure of the bacterial community changed during the desiccation process. It developed from a typical temperate streambed community towards a composition similar to that of terrestrial soils. Proportions of Betaproteobacteria and Bacteroidetes decreased, whereas the occurrence of Actinobacteria and Alphaproteobacteria increased. Extracellular enzyme multifunctionality (an important component of ecosystem functionality) was reduced during desiccation, particularly with fast desiccation. Aminopeptidases were most affected with their activity reduced to 10 % of the initial value, whereas activities of beta-glucosidases were reduced less markedly. After 8 weeks of desiccation, even the most affected enzymes remained remarkably active. This ensures that upon rewetting the complete ecosystem functions delivered by extracellular enzymes can start recovering without delay.