Nitrogen isotope fractionation during nitrate, ammonium and urea uptake by marine diatoms and coccolithophores under various conditions of N availability

Abstract

Stable isotopes of N provide a new approach to the study of algal production in the ocean, yet knowledge of the isotope fractionation (ε) in various oceanic regimes is lacking. Here we report large and rapid changes in isotope composition (δ15N) of 2 coastal diatoms and 2 clones (open and coastal) of a coccolithophore grown in the simultaneous presence of nitrate, ammonium and urea under varying conditions of N availability (i.e. N-sufficiency and N-starvation followed by N-resupply) and hence different physiological states. During N-sufficiency, the δ15N of particulate organic N (PON) was well reproduced, using a model derived from Rayleigh distillation theory, with constant ε similar to that for growth on each individual N source. However, following N-resupply, the variations in δ15N(PON) could be well explained only in the case of the open ocean Emiliania huxleyi, with ε similar to N-sufficient conditions. It was concluded that the mechanism of isotope fractionation changed rapidly with N availability for the 3 coastal clones. However, in the case of E. huxleyi isolated from the Subarctic Pacific Ocean, no evidence of a change in mechanism was found, suggesting that perhaps open ocean species can quickly recover from N-depleted conditions.