Nitrogen isotopic fractionation associated with growth on dinitrogen gas and nitrate by cyanobacteria

This study investigated the bulk nitrogen isotopic composition and fractionation of marine and freshwater cyanobacteria growing at the expense of atmospheric dinitrogen gas (N-2) or nitrate (NO3-). In agreement with previous studies, cyanobacteria using N-2 had delta N-15 values ranging from -2 parts per thousand to -1 parts per thousand and fractionated by 2-3 parts per thousand against atmospheric N-2. When grown on NO3-, the analyzed cyanobacteria had a remarkably large range in delta N-15, from -14 parts per thousand to 1 parts per thousand, and fractionated by 4-19 parts per thousand relative to the supplied NO3-. Interestingly, several of the NO3--utilizing cyanobacteria expressed a nitrogen isotopic signal similar to the one expected for diazotrophic species. Extrapolation to the natural environment indicates that delta N-15 signals considered to be associated with dinitrogen fixation could also be generated by NO3- utilization, depending on the isotopic composition of the nitrogen source, the degree of fractionation, and the type of cyanobacterium involved.