Print PageUnderstanding the nitrogen cycle in streams is essential because anthropogenic inputs have dramatically increased nitrogen loading. Downstream transport of these nutrients can result in undesirable algal blooms and creation of “dead zones” in water bodies receiving lotic inputs. Although freshwater ecosystems harbor a substantial pool of organic nitrogen, research has primarily focused on uptake of dissolved inorganic forms of nitrogen by heterotrophic bacteria. However, organic forms of nitrogen can serve as a potential source of nitrogen for the bacterial community. In this study, we examined utilization of organic and inorganic forms of nitrogen by heterotrophic bacteria isolated from streams with varying nitrate concentrations. Samples were collected from three streams with substantially different nitrate concentrations (two in Indiana and one in Ohio). Sediments were enriched in minimal media with seven different nitrogen amendments and in a complex medium (nutrient broth). Enrichments were incubated for different lengths of time to isolate fast and slow growers. Bacteria isolated from these enrichments were grown in various sole source nitrogen compounds and their growth rates determined by measuring optical density (OD) at 600nm using a spectrophotometer.
Results/Conclusions
During the enrichments, growth with nitrate, glycine, mixtures of nitrogen compounds, and nutrient broth was heavy (> 3.4 Optical Density) whereas bacterial protein yielded lower growth (< 0.13 OD). More than 90% of bacteria isolated were Gram-negative coccobacilli. Bacteria from the nitrate enrichment showed similar growth rates in various nitrogen treatments (average rate 2.02 generations [gen]/hr). In contrast, an isolate from tryptophan showed high growth rates with nitrate (14.46 gen/hr) but low growth on organic forms, such as glycine (0.96 gen/hr) and protein (0.11 gen/hr). Tryptophan was not a preferred nitrogen source for bacteria from other nitrogen enrichments (< 0.15 OD) with the exception of bacteria from the tryptophan enrichment (>0.40 OD). An isolate from the protein enrichment was a slow grower and did not grow well in all other nitrogen treatments. Isolates from labile organic nitrogen sources, e.g. glycine, showed a preference for inorganic forms e.g. nitrate and ammonium over complex organic forms like tryptophan and protein. However, organisms isolated from recalcitrant organic nitrogen treatments may be specialized for use of these compounds. Overall, abundances of bacteria with different ranges of preferred nitrogen sources, along with concentrations of available nitrogen compounds, may influence organic nitrogen utilization in streams.
