Batch culture and species effects modulate the decoupling between diatom frustule-bound and biomass nitrogen isotope signatures

The stable nitrogen (N) isotope composition of organic matter encapsulated in diatom silica frustules (δ ¹⁵ N _DB ) from sedimentary records has been used as a proxy for reconstructing N consumption dynamics in the ocean over geologic time scales. This proxy relies on the assumption that δ ¹⁵ N _DB tracks biomass δ ¹⁵ N, without being affected by internal N-isotope fractionation. However, recent ground-truthing efforts have shown that δ ¹⁵ N _DB can diverge from biomass δ ¹⁵ N values, though the extent and mechanisms behind this decoupling remain unclear.

In this study, we cultured two freshwater and two marine diatom species in batch cultures to test whether δ ¹⁵ N _DB (1) is subject to species-dependent internal ¹⁵ N fractionation, and (2) reflects the δ ¹⁵ N of source nitrate to the same extent as biomass δ ¹⁵ N values, assessing asynchronous integration of the N isotope signal. We monitored the N isotope systematics during diatom growth by measuring δ ¹⁵ N values in nitrate, bulk biomass and frustule-bound organic N throughout batch culture progression. We found that δ ¹⁵ N _DB did not follow typical Rayleigh fractionation dynamics and remained relatively stable, while biomass δ ¹⁵ N increased predictably with progressive fractional nitrate consumption. The observed divergence could not be explained by de-synchronized integration of source-nitrate δ ¹⁵ N values into biomass versus frustule-bound organic N (i.e. delayed incorporation into frustule-bound material), as newly formed frustules clearly recorded the δ ¹⁵ N of the ¹⁵ N-labeled nitrate added during growth. This confirmed that δ ¹⁵ N _DB values capture the isotopic signature of newly assimilated nitrate rather than N derived from internal or legacy pools. We hypothesize that shifts in growth conditions during batch culture progression altered the interaction between carbon and nitrogen metabolism, leading to physiologically driven variation in internal ¹⁵ N fractionation and corresponding offsets between δ ¹⁵ N _DB and biomass δ ¹⁵ N. Such sensitivity to internal isotope fractionation during biosynthesis has important implications for the interpretation of sedimentary δ ¹⁵ N _DB records.