Utilization of different forms of nitrogen by heterotrophic bacteria under varying organic carbon concentrations: From isolates to communities.

摘要

It is important to understand fate of nutrients like nitrogen, in streams given that anthropogenic activity, such as agriculture, have increased inputs of biologically reactive nitrogen to the environment leading to deterioration of stream health and eutrophication. Bacteria play a crucial role in the driving essential biogeochemical transformations. The purpose of this research was to improve our understanding of organic and inorganic nitrogen utilization by bacterial isolates and complex bacterial communities. Role of bacterial diversity in resource utilization is mostly neglected in biogeochemical models. Identification of bacteria based on molecular methods, like 16S rDNA sequencing, yield a wealth of information related to prokaryotic diversity and its importance in driving essential biogeochemical cycles. In this research utilization of organic and inorganic forms of nitrogen by stream heterotrophic bacterial isolates were examined. Our results reveal differences in bacterial resource utilization not as a function of the different taxa involved but of the enrichments the isolates were obtained from, as immediate environment dictate bacterial response to different nutrients and exerts a selection pressure. Carbon availability also influences nitrogen dynamics. To examine the impact of carbon on bacterial uptake of organic and inorganic nitrogen, bacterial abundance and community composition were examined in controlled, laboratory microcosms. There was a strong influence of carbon availability on bacterial nitrogen utilization, with preferential uptake of organic forms under low carbon concentrations. Carbon and nitrogen treatments likely drove changes in bacterial community composition that, in turn, affected rates of nitrogen utilization under various carbon concentrations. Metabolic functions, such as particular biogeochemical reactions are catalyzed by microbial extracellular enzymes, which are likely linked to the constituting taxa in a given microbial community. This study tracked temporal patterns of bacterial community structure and potential microbial enzyme activities in response to seasonal changes in stream organic matter pool. Differences in enzyme activities were highly correlated to nutrient (carbon and nitrogen) availability while differences in bacterial community structure were mostly driven by environmental conditions. However, the absence of link between bacterial community structure and potential enzyme activity is indicative of functional redundancy within microbial communities.