ABSTRACT: Permeable sediments are highly bioactive compartments in coral reefs. The associated dense microbial communities sustain fast degradation of organic matter, thereby playing a key role in nutrient recycling within the reef. Besides nutrient recycling, new nutrients (i.e. nitrogen) are acquired by dinitrogen (N₂) fixing microbial communities, but knowledge about the influence of sand mineralogy and key environmental factors on this process is scarce. Therefore, this study quantified seasonal N₂ fixation (via acetylene reduction) along with gross photosynthesis (via O₂ fluxes) by adjacent carbonate and silicate sands in a Northern Red Sea coral reef. Findings revealed significantly higher N₂ fixation in carbonate than in silicate sands (2.88 and 1.52 nmol C₂H₄ cm^-2 h^-1, respectively) and a more pronounced seasonal response in the former, likely caused by its higher permeability, grain size and microbial abundance. Ambient light and organic matter availability were the main controlling environmental factors for sand-associated N₂ fixation. Carbonate and silicate sands showed similar gross photosynthesis rates (270 and 233 nmol O₂ cm^-2 h^-1) that positively (carbonate sands) or negatively (silicate sands) correlated with N₂ fixation, likely due to different diazotrophic communities. Seasonal appearance of microbial mats on carbonate sands increased N₂ fixation and gross photosynthesis by up to one order of magnitude. On an annual average, carbonate and silicate sands obtain ~8% and microbial mat communities obtain ~13% of their photo-metabolic N demand via N₂ fixation.