Impact of the Flow Velocity and Sedimentation on Microbial Biofilms in the Stream Ilm (Thuringia/Germany)

Streams are linear definite one dimensional structures and several theoretical concepts aiming the stream ecology are existing. The River Continuum Concept (RCC) for instance describes streams as a continuous series of physical gradients and associated biotic adjustments indicating that the stream organisms interact with the longitudinal changing environment (Vannote et al., 1980). The products of streams are highly appropriate to be transported downstream than reaching the river bottom (Schönborn, 2003). Most of the organic matter available in streams is of allochthonious origin, as leaves from the riparian zones (99%). After entering the stream, allochtonious material undergoes three phases of processing: leaching, microbial colonization (conditioning) and fragmentation by physical forces and invertebrate feeding (Fisher and Likens, 1973). The amount of terrestric organic carbon transported or transformed by stream and river ecosystems world wide is about 2 Pg per year whereas the metabolic capacity in these fluvial systems may result from microbial attachements as biofilms (Battin et al., 2008)

Bacterial community composition and extracellular enzyme activity in temperate streambed sediment during drying and rewetting

Droughts are among the most important disturbance events for stream ecosystems; they not only affect stream hydrology but also the stream biota. Although desiccation of streams is common in Mediterranean regions, phases of dryness in headwaters have been observed more often and for longer periods in extended temperate regions, including Central Europe, reflecting global climate change and enhanced water withdrawal. The effects of desiccation and rewetting on the bacterial community composition and extracellular enzyme activity, a key process in the carbon flow of streams and rivers, were investigated in a typical Central European stream, the Breitenbach (Hesse, Germany). Wet streambed sediment is an important habitat in streams. It was sampled and exposed in the laboratory to different drying scenarios (fast, intermediate, slow) for 13 weeks, followed by rewetting of the sediment from the fast drying scenario via a sediment core perfusion technique for 2 weeks. Bacterial community structure was analyzed using CARD-FISH and TGGE, and extracellular enzyme activity was assessed using fluorogenic model substrates. During desiccation the bacterial community composition shifted toward composition in soil, exhibiting increasing proportions of Actinobacteria and Alphaproteobacteria and decreasing proportions of Bacteroidetes and Betaproteobacteria. Simultaneously the activities of extracellular enzymes decreased, most pronounced with aminopeptidases and less pronounced with enzymes involved in the degradation of polymeric carbohydrates. After rewetting, the general ecosystem functioning, with respect to extracellular enzyme activity, recovered after 10 to 14 days. However, the bacterial community composition had not yet achieved its original composition as in unaffected sediments within this time. Thus, whether the bacterial community eventually recovers completely after these events remains unknown. Perhaps this community undergoes permanent changes, especially after harsh desiccation, followed by loss of the specialized functions of specific groups of bacteria

Bacterial community composition and extracellular enzyme activity in temperate streambed sediment during drying and rewetting.

Droughts are among the most important disturbance events for stream ecosystems; they not only affect stream hydrology but also the stream biota. Although desiccation of streams is common in Mediterranean regions, phases of dryness in headwaters have been observed more often and for longer periods in extended temperate regions, including Central Europe, reflecting global climate change and enhanced water withdrawal. The effects of desiccation and rewetting on the bacterial community composition and extracellular enzyme activity, a key process in the carbon flow of streams and rivers, were investigated in a typical Central European stream, the Breitenbach (Hesse, Germany). Wet streambed sediment is an important habitat in streams. It was sampled and exposed in the laboratory to different drying scenarios (fast, intermediate, slow) for 13 weeks, followed by rewetting of the sediment from the fast drying scenario via a sediment core perfusion technique for 2 weeks. Bacterial community structure was analyzed using CARD-FISH and TGGE, and extracellular enzyme activity was assessed using fluorogenic model substrates. During desiccation the bacterial community composition shifted toward composition in soil, exhibiting increasing proportions of Actinobacteria and Alphaproteobacteria and decreasing proportions of Bacteroidetes and Betaproteobacteria. Simultaneously the activities of extracellular enzymes decreased, most pronounced with aminopeptidases and less pronounced with enzymes involved in the degradation of polymeric carbohydrates. After rewetting, the general ecosystem functioning, with respect to extracellular enzyme activity, recovered after 10 to 14 days. However, the bacterial community composition had not yet achieved its original composition as in unaffected sediments within this time. Thus, whether the bacterial community eventually recovers completely after these events remains unknown. Perhaps this community undergoes permanent changes, especially after harsh desiccation, followed by loss of the specialized functions of specific groups of bacteria

PCA biplot of extracellular enzyme activities and bacterial abundance in rewetted Breitenbach sediment after desiccation.

<p>Axes 1 and 2 explain 92% and 5% of the variance in enzyme activity, respectively. For further explanations cf. Fig. 7.</p

Abundances of prokaryotes in Breitenbach streambed sediments experimentally rewetted after 13 weeks of desiccation.

<p>The abundance of prokaryotes was determined after SYBR Green staining, whereas the abundances of different taxonomic groups were determined via CARD-FISH. Means with SD are given (n = 4). The asterisks indicate significant differences between dry sediment from day 0 used for rewetting and the treatment samples (ANOVA, * = P<0.05, ** = P<0.01).</p

Activities of extracellular enzymes in desiccating Breitenbach sediment.

<p>The activities were determined in wet sediments (0) and after 2, 4, 8 and 13 weeks of artificial desiccation, using 3 different scenarios: fast, intermediate and slow desiccation. (A) alpha-glucosidase, (B) beta-glucosidase, (C) beta-xylosidase, (D) phosphatase, (E) aminopeptidase. Mean values with SD are given (n = 4). The asterisks indicate significant differences between the treatments (ANOVA, P<0.05).</p

Comparison of bacterial community composition in rewetted Breitenbach sediment after artificial desiccation via correspondence analysis.

<p>The analysis is based on TGGE band patterns prepared with 16S rRNA gene fragments. Axes 1 and 2 explain 23 and 16% of the variance in bacterial community composition, respectively. The filled black boxes symbolize sediments perfused with unaffected Breitenbach stream water containing the natural microbial community (S), and the open red diamonds indicate sediments perfused with filtered and boiled stream water containing no microorganisms (F). The numbers indicate the days of rewetting, and the small letters denote the replicates.</p

Water loss over time during the sediment desiccation process.

<p>Desiccation occurred at 20°C in polypropylene containers without cover (fast), with gaze (intermediate) or with plastic foil cover (slow desiccation scenario). The final moisture content of the sediment was set to 0%. The arrows indicate the sampling times (after 2, 4 and 8 weeks of desiccation).</p

Sediment chemical characteristics.

<p>C:N ratio, concentrations of nitrogen, nitrate, and ammonium in streambed sediments during different desiccation scenarios (fast, intermediate and slow). Means with SD are given (n = 4–5). The asterisks indicate significant differences between the treatments (ANOVA, * = P<0.05).</p