Diatoms survive in dark, anoxic sediment layers for months to decades. Our investigation reveals a correlation between the dark survival potential of marine diatoms and their ability to accumulate NO 3 − intracellularly. Axenic strains of benthic and pelagic diatoms that stored 11-274 mM NO 3 − in their cells survived for 6-28 wk. After sudden shifts to dark, anoxic conditions, the benthic diatom Amphora coffeaeformis consumed 84-87% of its intracellular NO 3 Approximately 40% of marine primary production is due to diatoms, making them key players in the global carbon cycle (1). Many diatoms take up and store NO 3 − intracellularly in concentrations of up to a few 100 mM (2-4), which exceeds ambient NO 3 − concentrations by several orders of magnitude. It is well documented that diatoms use intracellular NO 3 − for assimilatory NO 3 − reduction (2); however, the intracellular NO 3 − pool also might be used for dissimilatory NO 3 − reduction. Mass sinking of pelagic diatom blooms is triggered by nutrient depletion in the surface layer of the water column (5). However, silicate depletion, not nitrate depletion, is the prevalent environmental cue for increased sinking rates (6). Therefore, the intracellular NO 3 − pool might not be depleted in diatoms that sink from the nutrient-poor surface layer to nutrient-rich deeper layers or the sediment surface. In fact, Lomstein et al. (7) found high intracellular NO 3 − pools in phytoplankton freshly deposited on the seafloor. A considerable fraction of the settled diatoms survive for months to decades as vegetative or resting cells in dark, anoxic sediment layers, in which neither photosynthesis nor aerobic respiration can occur (8-10). Benthic diatoms experience shifts to dark, anoxic conditions due to vertical migration behavior in the sediment (11) and burial by bioturbating animals (12). The occurrence of NO 3 − -storing diatoms in deep sediment layers increases the concentration of cell-bound NO 3 − , which has potential implications for nitrogen cycling (7,(13)(14)(15). However, the energy-providing metabolism that allows diatoms to survive in dark, anoxic sediments is not known, and the fate of the intracellular NO 3 − is unclear. Dissimilatory NO 3 − reduction is common in many anaerobic prokaryotes, and was recently found in several eukaryotic taxa as well. The anaerobic protozoan Loxodes spp. respires NO 3 − to NO 2 − (16), and the two fungi Fusarium oxysporum and Cylindrocarpon tonkinense respire NO 3 − to N 2 O (17, 18). Complete denitrification of NO 3 − to N 2 has been reported for NO 3 − -storing benthic foraminiferans (19,20). Dissimilatory NO 3 − reduction also can lead to NH 4 + formation in a pathway known as dissimilatory nitrate reduction to ammonium (DNRA), a process well documented in prokaryotes, such as large sulfur bacteria (21,22). The only eukaryotes known to be capable of DNRA are fungi (23,24). DNRA is involved in anaerobic energy generation via a twostep reaction sequence: NO 3 − reduction to NO 2 − is coupled to electron transport phosphor...