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 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.

Influence of light intensity and nutrient source on δ13C and δ15N signatures in Ulva pertusa

“…In theory, in light-saturated U. pertusa a slight increase in pH could have led to efflux of NH 3 , which in turn would favour the retention of 15 N resulting in the fractionation observed in this treatment. This positive discrimination for 15 N during assimilation of ammonium has previously been documented in studies conducted on microalgae (Wada & Hattori 1978, Waser et al 1998b) and supports evidence for short-term trends in positive discrimination for 15 N in U. (previously Enteromorpha) intestinalis (Cohen & Fong 2005).…”

“…Previous work on other photoautotrophs suggests that light availability (Wada & Hattori 1978, Heikoop et al 1998, growth rate (McKee et al 2002), nutrient availability (Waser et al 1998b, McKee et al 2002, turbulence (Trudeau & Rasmussen 2003) and nitrogen source (Waser et al 1998a) may all have some effect on fractionation of nitrogen during its incorporation in photoautotrophs. Similarly, other studies have shown that fractionation of carbon by algae may vary as a function of the physical environment as well as taxonomic difference (Wiencke & Fischer 1990, Raven et al 2002.…”

Nitrite consumption and associated isotope changes during a river flood event

“…5). This is on the low end of isotope effects reported for nitrate assimilation (Granger et al, 2004;Waser et al, 1998), but fractionation can be affected by residence times, such that the isotope effect is lower when residence times are low (Kendall, 1998). Moreover, we cannot exclude co-occurring nitrate production, which may also contribute isotopically depleted nitrate to the total pool.…”

“…As phytoplankton recovers (evidenced by increased [O 2 ]), it might well contribute to ammonium consumption. Phytoplankton assimilation of ammonium can have an isotope effect of ∼ −19 ‰ (Waser et al, 1998). If ammonium is fractionated during uptake, but also permanently supplied from remineralisation, a moderate enrichment of the pool is at least possible.…”

Seasonally varying nitrogen isotope biogeochemistry of particulate organic matter in Lake Kinneret, Israel

“…depletion, phytoplankton NO { 3 assimilation appeared to occur without isotopic fractionation. It is well known that the magnitude of the fractionation factor varies as a function of growth conditions and species such that little fractionation could be readily obtained (Montoya and McCarthy 1995;Waser et al 1998;Needoba et al 2003).…”