Transfer of the nitrogen isotope signature from the nitrate pool to the diatom biomass and into the diatom frustule

The nitrogen (N) isotopic composition of diatom frustule-bound organic matter (δ15NDB) from sedimentary archives has been used as a promising proxy indicator to reconstruct nitrate utilization in the high-latitude oceans on different timescales. The advantage of this proxy over conventional N isotopic approaches, such as measuring δ15N values of the bulk sediment, is that δ15NDB is thought to be protected from diagenetic alteration and bacterial degradation. Despite the fact that the δ15NDB proxy has been applied in palaeoceanographic research for two decades, little is known about the propagation of the δ15N signature of assimilated nitrate into biomass δ15N and subsequently into δ15NDB, and to what extent N-isotope fractionation during frustule-bound N synthesis varies among species and with environmental conditions. Only few δ15NDB data exist for living diatoms in natural environments or laboratory cultures, and implications for paleo-environmental reconstructions appear controversial between existing studies. Here, we present novel constraints on the relationship between δ15N values of nitrate, diatom bulk biomass, and diatom frustule-bound N across samples from different natural environments and from controlled mono-specific diatom cultures. While previous ground-truthing work has focussed on marine diatom species both in culture and in the ocean, we extend our study to freshwater species and lacustrine environments. We find that, in mono-specific diatom cultures, δ15NDB values are generally relatively close to biomass δ15N values, irrespective of the variable 15N-fractionation imparted by nitrate assimilation. Similarly, analysis of diatom samples from natural environments revealed little offset between δ15NDB and bulk biomass δ15N values in samples that are near mono-specific. By contrast, in more mixed-species samples, δ15NDB values can be shifted in both directions relative to biomass δ15N values, possibly as a result of i) species-specific N isotope fractionation during frustule-bound N synthesis, and/or ii) non-uniform contribution of N to the total biomass and diatom-bound N pools between different species.