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Abstract

Diatoms are a widespread group of microalgae and are estimated to contribute up to 40 % to total marine primary production. They are able to store signi cant amounts of nitrate intracellularly and survive unfavorable conditions by entering resting stages. It has been demonstrated that the benthic diatom Amphora co eaeformis is capable of consuming its intracellularly stored nitrate via dissimilatory nitrate reduction to ammo- nium (DNRA) and, thereby, survive dark and anoxic conditions. In this study, ecophysilogical labeling experiments revealed that also the pelagic diatom species Thalassiosira weiss ogii is capable of anaerobic nitrate reduction. The products released by the diatom cells under dark and anoxic conditions were mainly ammonium, low levels of nitrite, and trace amounts of nitrous oxide. This suggests that DNRA, but not denitri cation or anammox, is the pathway of anaerobic nitrate reduction used by T. weiss ogii. The rapid consumption of intercellular nitrate and decrease of cellular ATP contents further suggest that the diatoms use the energy derived by DNRA activity to enter a resting stage, which assures long-term survival until re-exposure to favorable conditions. Since ammonium as the main product of DNRA activity by T. weiss ogii is excreted by the cell, this form of xed nitrogen is potentially recycled by other microorganisms. If DNRA activity proves to be widespread in pelagic diatom species, this might especially alter our understanding of microbial nitrogen cycling in oxygen-de cient water bodies, such as open-ocean oxygen minimum zones or eutrophic coastal regions.