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 (δ¹⁵N_DB) 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 δ¹⁵N_DB is thought to be protected from diagenetic alteration and bacterial degradation. Despite the fact that the δ¹⁵N_DB 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 δ¹⁵N_DB, and to what extent N-isotope fractionation during frustule-bound N synthesis varies among species and with environmental conditions. Only few δ¹⁵N_DB 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, δ¹⁵N_DB 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 δ¹⁵N_DB and bulk biomass δ¹⁵N values in samples that are near mono-specific. By contrast, in more mixed-species samples, δ¹⁵N_DB 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.