Shifts in carbon substrate utilization in sediment microbial communities along the Llobregat River

River sediment microorganisms have the capacity to metabolize, uptake and decompose flowing water organic materials, which strongly depend on their metabolic capabilities. The changes of microbial functional diversity in river sediments were analysed at five sites along the River Llobregat (NE Spain), using the Biolog EcoPlates incubation method. In parallel, we tested the potential application of the EcoPlates with preserved frozen samples (-80 degrees C). Although functional diversity (Shannon index) did not show significant differences, the specific carbon substrates that were used differed between sampling sites. The microbial community from the upstream site was the most active as shown by the highest respiratory activity and abundance of living bacteria, being able to metabolize more labile substrates. Downstream sediment communities showed a decrease in bacterial viability, respiration activity and EPS content, suggesting a less structured biofilm. The functional fingerprint analyses clearly distinguish the sediment microbial communities from the downstream most polluted sites, which showed a potential capacity to use more complex carbon substrates such as polymers. Freezing sediment samples is not recommended, since a significant reduction in the functional diversity and changes in the fingerprint patterns were observed together with a reduction of the number of substrates utilisation especially at two downstream sites. The Biolog EcoPlates appear to be useful for detecting changes in the microbial functional fingerprint in fresh sediments along a river pollution gradient.