The effects of N:P ratio and nitrogen form on four major freshwater bacterial taxa in biofilms

Bacteria in freshwater systems play an important role in nutrient cycling through both assimilatory and dissimilatory processes. Biotic and abiotic components of the environment affect these transformations as does the stoichiometry of the nutrients. We examined responses of four major taxa of bacteria in biofilms subjected to various N:P molar ratios using either nitrate or ammonium as a nitrogen source. Fluorescent in situ hybridization was used to enumerate the Domain bacteria as well as the α-, β-, and γ-proteobacteria, and the Cytophaga–Flavobacteria cluster. Generally, bacterial responses to the treatments were limited. However, the Cytophaga–Flavobacteria and β-proteobacteria both responded more to the ammonium additions than nitrate, whereas, the α-proteobacteria responded more to nitrate additions. The β-proteobacteria also exhibited peak relative abundance at the highest N:P ratio. Nutrient concentrations were significantly different after the incubation period, and there were distinct changes i...