Synthetic and biological surfactant effects on freshwater biofilm community composition and metabolic activity.

Surfactants are used to control microbial biofilms in industrial and medical settings. Their known toxicity on aquatic biota, and their longevity in the environment, has encouraged research on biodegradable alternatives such as rhamnolipids. While previous research has investigated the effects of biological surfactants on single species biofilms, there remains a lack of information regarding the effects of synthetic and biological surfactants in freshwater ecosystems. We conducted a mesocosm experiment to test how the surfactant sodium dodecyl sulfate (SDS) and the biological surfactant rhamnolipid altered community composition and metabolic activity of freshwater biofilms. Biofilms were cultured in the flumes using lake water from Lake Lunz in Austria, under high (300 ppm) and low (150 ppm) concentrations of either surfactant over a four-week period. Our results show that both surfactants significantly affected microbial diversity. Up to 36% of microbial operational taxonomic units were lost after surfactant exposure. Rhamnolipid exposure also increased the production of the extracellular enzymes, leucine aminopeptidase, and glucosidase, while SDS exposure reduced leucine aminopeptidase and glucosidase. This study demonstrates that exposure of freshwater biofilms to chemical and biological surfactants caused a reduction of microbial diversity and changes in biofilm metabolism, exemplified by shifts in extracellular enzyme activities. KEY POINTS: • Microbial biofilm diversity decreased significantly after surfactant exposure. • Exposure to either surfactant altered extracellular enzyme activity. • Overall metabolic activity was not altered, suggesting functional redundancy.

Effects of intermittent flow on biofilms are driven by stream characteristics rather than history of intermittency.

Intermittent streams are found all over the world, however most studies focus on intermittency in hot, arid climates. As flow intermittency is expected to increase with climate change, it is important to understand how stream biofilms in temperate regions respond to these changing conditions. In this study, 20 different streams from around Austria were sampled under flowing and non-flowing conditions to evaluate the effect of intermittency on temperate stream biofilms. These streams encompassed two distinct stream types: fine-grained with high agricultural land use and coarse sediments from relatively pristine areas. Half of these streams were historically intermittent and half historically perennial. Samples were taken from all streams during the spring and fall, when the intermittent streams were flowing and dry, respectively. Subsets of the sediments were subjected to controlled drying to evaluate the effects of history of intermittency on the biofilms. Samples were analyzed for respiration, extracellular enzyme activities, and extracellular polysaccharides in the wet and dry sediments from the field, as well as the lab-dried sediments. This study found that lab-dried perennial sediments showed similar responses to the intermittent sediments, indicating that history of intermittency does not affect biofilm response to drought. This study also found that the effects of grain size, seasonal growth, and nutrient levels have a larger impact on the biofilms than moisture content and history of intermittency.