N and O isotope effects during nitrate assimilation by unicellular prokaryotic and eukaryotic plankton cultures

摘要

In order to provide biological systematics from which to interpret nitrogen (N) and oxygen (O) isotope ratios of nitrate (15N/14N, 18O/16O, respectively) in the environment, we previously investigated the isotopic fractionation of nitrate during its assimilation by mono-cultures of eukaryotic algae (Granger et al., 2004). In this study, we extended our analysis to investigate nitrate assimilation by strains of prokaryotic plankton. We measured the N and O isotope effects, 15e and 18e, during nitrate consumption by cultures of prokaryotic strains and by additional eukaryotic phytoplankton strains (where e is the ratio of reaction rate constants of the light vs. heavy isotopologues, lightk and heavyk; e = lightk/ heavyk 1 1000, expressed in per mil). The observed 15e ranged from 5& to 8& among eukaryotes, whereas it did not exceed 5& for three cyanobacterial strains, and was as low as 0.4& for a heterotrophic a-protoeobacterium. Eukaryotic phytoplankton fractionated the N and O isotopes of nitrate to the same extent (i.e., 18e 15e). The 18e:15e among the cyanobacteria was also 1, whereas the heterotrophic a-proteobacterial strain, which showed the lowest 15e, between 0.4& and 1&, had a distinct 18e:15e of 2, unlike any plankton strain observed previously. Equivalent N vs. O isotope discrimination is thought to occur during internal nitrate reduction by nitrate reductase, such that the cellular efflux of the fractionated nitrate into the medium drives the typically observed 18e:15e of 1. We hypothesize that the higher in the 18e:15e of the a-proteobacterium may result from isotope discrimination by nitrate transport, which is evident only at low amplitude of e. These observations warrant investigating whether heterotrophic bacterial assimilation of nitrate decreases the community isotope effects at the surface ocean.