Acetylene reduction assays for nitrogen fixation in freshwaters: a note of caution.

Lake water samples were observed to transform [14-C]ethylene into water-soluble compounds that were undetectable by conventional acetylene reduction assay procedures. Methane oxidizing bacteria, which are known to be common in freshwaters, appeared to be responsible for this activity. As much as 28 percent of added ethylene has been observed to be transformed and this figure is probably an underestimate. It is suggested that acetylene reduction assays may not be accurately applied to samples containing methane oxidizing bacteria.