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

In order to provide biological systematics from which to interpret nitrogen (N) and oxygen (O) isotope ratios of nitrate (N-15/N-14, O-18/O-16, 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, (15)epsilon and (18)epsilon, during nitrate consumption by cultures of prokaryotic strains and by additional eukaryotic phytoplankton strains (where epsilon is the ratio of reaction rate constants of the light vs. heavy isotopologues, (light)k and (heavy)k; epsilon = (light)k/(heavy)k - 1 x 1000, expressed in per mil). The observed (15)epsilon ranged from 5 parts per thousand to 8 parts per thousand among eukaryotes, whereas it did not exceed 5 parts per thousand for three cyanobacterial strains, and was as low as 0.4 parts per thousand for a heterotrophic alpha-protoeobacterium. Eukaryotic phytoplankton fractionated the N and O isotopes of nitrate to the same extent (i.e., (18)epsilon similar to (15)epsilon). The (18)epsilon:(15)epsilon among the cyanobacteria was also similar to 1, whereas the heterotrophic alpha-proteobacterial strain, which showed the lowest (15)epsilon, between 0.4 parts per thousand and 1 parts per thousand, had a distinct (18)epsilon:(15)epsilon, of similar to 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 (18)epsilon:(15)epsilon of similar to 1. We hypothesize that the higher in the (18)epsilon:(15)epsilon of the alpha-proteobacterium may result from isotope discrimination by nitrate transport, which is evident only at low amplitude of epsilon. These observations warrant investigating whether heterotrophic bacterial assimilation of nitrate decreases the community isotope effects at the surface ocean. (C) 2009 Elsevier Ltd. All rights reserved.