Recent studies demonstrate that marine N₂ fixation can be carried out without light. However, direct measurements of N₂ fixation in dark environments are relatively scarce. This study uses a model that represents biogeochemical cycles at a deep-ocean location in the Gulf of Aqaba (Red Sea). Different model versions are used to test assumptions about N₂ fixers. Relaxing light limitation for marine N₂ fixers improved the similarity between model results and observations of deep nitrate and oxygen.