Effect of availability of nitrogen compounds on community structure of aquatic bacteria in model systems

To test if the quality and concentration of dissolved nitrogen (N) species could be a selective force in shaping bacterioplankton community structure, competition for various N compounds among five heterotrophic marine bacteria (Pseudomonas strains B, B25, and AX; Bacillus strain A6; Erythrobacter strain F19) was examined. Two of the five strains (AX and B25) were capable of utilizing urea for growth. The five strains were inoculated into dilute (1/1,000 strength) ZoBell medium enriched with various N sources (free amino acids, casein, ammonium, nitrate, or urea). Regardless of the added N source, the communities were either dominated by strain B (at 50 microM N) or strain AX (at 250 microM N). Without any addition of N, strain F19 dominated. If F19 was not included in the community, strain B25 dominated. Despite these differences in community structure, consumption of the added N compounds was surprisingly similar and no advantages of urea for the urea-utilizing bacterium B25 were obvious. To examine if urea could be of selective advantage to the urea-degrading strains B25 and AX, communities with and without B25 were amended with urea N. As expected, strain B25 became dominant when present, but without this strain the non-urea-utilizing strain B outcompeted the urea-utilizing strain AX. Possibly, strain B benefited from N released during catabolism of urea by strain AX. Changes in community composition did not result in major changes in the nitrogen dynamics. The results indicate that dissolved N species can be a selective force in shaping microbial communities. Relative to nutrient generalists, nutrient specialists may either have competitive advantages or stimulate growth of other species by synergetic interactions. Results from the model communities suggest that there may be a large degree of unpredictability in the making of microbial communities, whereas major ecosystem functions such as N cycling appear relatively stable.

Effect of select nitrocompounds on ruminal fermentation; an initial look at their potential to reduce economic and environmental costs associated with ruminal methanogenesis

Methane production by ruminal microbes during the digestion of feedstuffs is an inefficient process resulting in losses of 2-12% of the gross energy consumed by ruminants. Presently, we report the effect of three inhibitors on ruminal methane production in vitro. Mixed populations of ruminal microbes collected from cannulated cows maintained on an alfalfa hay:corn diet (50:50) were incubated at 39 degrees C for 24 h under a 100% carbon dioxide gas phase in closed tubes with 72 mM added sodium formate. Cultures were supplemented with 12 mM 2-nitropropanol, nitroethane or nitroethanol (experiment 1) or with 2, 12 or 24 mM nitroethane or a combination of 12 mM nitroethane and 4 mM nitroethanol (experiment 2). Control cultures containing no added nitrocompound were incubated simultaneously with treated incubations. Methane concentrations were reduced (P<0.05) from those measured in control incubations (27.6 +/- 2.1 and 17.7 +/- 0.8 micromol/ml; mean +/- SD for experiments 1 and 2, respectively) by at least 57% and as much as 94% in the nitrocompound supplemented incubations. By comparison, the widely fed methane inhibitor, monensin, typically reduces ruminal methane production by about 33%. Concentrations of volatile fatty acids and ammonia that accumulated in the nitrocompound supplemented incubations were not markedly affected compared to those produced by control cultures despite the reductions in methane produced. Hydrogen accumulated only slightly in cultures supplemented with the nitrocompounds. These results demonstrate that 2-nitropropanol, nitroethane and nitroethanol inhibit ruminal methane production. Further research is warranted to determine the mechanisms responsible for this inhibition and to see if these inhibitors can be used in practical application to reduce economic and environmental costs associated with ruminal methanogenesis.