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

Abstract

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 (NO 3 − ). In agreement with previous studies, cyanobacteria using N 2 had Δ 15 N values ranging from ‐2‰ to ‐1‰ and fractionated by 2‐3‰ against atmospheric N 2 . When grown on NO 3 − , the analyzed cyanobacteria had a remarkably large range in Δ 15 N, from ‐14‰ to 1‰, and fractionated by 4‐19‰ relative to the supplied NO 3 − . Interestingly, several of the NO 3 − ‐utilizing cyanobacteria expressed a nitrogen isotopic signal similar to the one expected for diazotrophic species. Extrapolation to the natural environment indicates that Δ 15 N signals considered to be associated with dinitrogen fixation could also be generated by NO 3 − utilization, depending on the isotopic composition of the nitrogen source, the degree of fractionation, and the type of cyanobacterium involved.