Surface Southern Ocean community growouts to evaluate the diatom bound N isotope proxy

Tracking variations in the surface ocean supply and demand of nitrate, a key marine nutrient, can help constrain the contribution of biological production in driving past climate shifts. The nitrogen isotopic composition (as δ15N) of organic matter in marine sediments is a proxy for surface ocean nitrate supply and demand over time but it may be subject to alteration during sinking and burial. The isotopic composition of nitrogen contained in the shells, or frustules, of diatoms (δ15NDB) is protected and is therefore a potentially more robust tracer of nitrate use in the past. Here we show that δ15NDB in Southern Ocean community cultures does not depend on species composition. We found the εDB (= biomass δ15N - δ15NDB) of the community growouts was -4.8 ± 0.8‰, more than 10‰ different from previous monospecific growouts, but statistically indistinguishable from previous Southern Ocean and North Pacific surface ocean observations. The two community growouts, seeded with populations from 66° and 61°S, had distinct community compositions but indistinguishable εDB, suggesting species composition does not primarily set δ15NDB values in the Southern Ocean. Our results demonstrate that under nitrate-replete conditions, δ15NDB values of frustules sinking from the surface ocean robustly track surface ocean nitrate δ15N values and therefore nitrate supply and demand.