Effects of warming on stream biofilm organic matter use capabilities

The understanding of ecosystem responses to changing environmental conditions is becoming increasingly relevant in the context of global warming. Microbial biofilm communities in streams play a key role in organic matter cycling which might be modulated by shifts in flowing water temperature. In this study, we performed an experiment at the Candal stream (Portugal) longitudinally divided into two reaches: a control half and an experimental half where water temperature was 3 °C above that of the basal stream water. Biofilm colonization was monitored during 42 days in the two stream halves. Changes in biofilm function (extracellular enzyme activities and carbon substrate utilization profiles) as well as chlorophyll a and prokaryote densities were analyzed. The biofilm in the experimental half showed a higher capacity to decompose cellulose, hemicellulose, lignin, and peptidic compounds. Total leucine-aminopeptidase, cellobiohydrolase and β-xylosidase showed a respective 93, 66, and 61% increase in activity over the control; much higher than would be predicted by only the direct temperature physical effect. In contrast, phosphatase and lipase activity showed the lowest sensitivity to temperature. The biofilms from the experimental half also showed a distinct functional fingerprint and higher carbon usage diversity and richness, especially due to a wider use of polymers and carbohydrates. The changes in the biofilm functional capabilities might be indirectly affected by the higher prokaryote and chlorophyll density measured in the biofilm of the experimental half. The present study provides evidence that a realistic stream temperature increase by 3 °C changes the biofilm metabolism to a greater decomposition of polymeric complex compounds and peptides but lower decomposition of lipids. This might affect stream organic matter cycling and the transfer of carbon to higher trophic levels.