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

Stable isotopes of N provide a new approach to the study of algal production in the ocean, yet know!edge of the isctopc fractionation (c) in various oceanic regime5 is i d ~k i ~~y .tiere w e report large and rapid changes in isotope composit~on (6'") of 2 coastal diatoms and 2 clones (open and coastal) of a coccolithophore grown in the simultaneous presence of nitrate, ammonium and urea under varylng conditions of N availability (i.e.N-sufficiency and N-starvation followed by N-resupply) and hence different physiological states.Du.ring N-sufficiency, the SI5N of particulate organic N (PON) was well reproduced, uslng a model derived from Rayleigh distillation theory, with constant E similar to that for growth on each individual N source.However, following N-resupply, the variations in S15NpoU could be well explained only in the case of the open ocean Emiliania huxleyi, with E 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.