Nutrient and enzymatic adaptations of stream biofilms to changes in nitrogen and phosphorus supply

Abstract

We evaluated the effect of high and low nutrient nitrogen (N) and phosphorus (P) concentrations (Hn and Ln, respectively) and N:P supply (16:1 and 56:1, respectively) on the structure and activity of stream biofilms. Natural biofilms were exposed to 4 different nutrient conditions (16:1-Hn, 16:1-Ln, 56:1-Hn, 56:1-Ln) in a microcosm experiment in the laboratory over 35 d. Biofilm bacterial density and chlorophyll a (chl a) concentration decreased under lower N and P concentrations. Bacterial density was further sensitive to nutrient imbalance and decreased with decreasing P availability, while chl a concentration was not affected by P reduction. Greater P requirements and weaker ability to store large quantities of P in bacteria compared to algae may explain these differences. Biofilm responses to imbalanced N:P (56:1) were only observed under Hn conditions, and were expressed as an increase in the proportion of algal carbon with respect to bacterial carbon, and greater N and extracellular polymeric substance (EPS)-polysaccharide accumulation. Algae withstood N:P imbalance better than bacteria under Hn conditions, but this trend decreased at low water nutrient concentration (56:1-Ln condition). The cellobiohydrolase: phosphatase enzyme activity ratio was negatively correlated to the biofilm C:P molar ratio, evidencing the tight link between nutrient acquisition and storage in biofilms. Our experiment highlighted the rapid adaptation (3 to 28 d) of biofilm nutrient content and enzyme activities to changes in water nutrient availability.