Biological N₂ fixation and nitrate/nitrite (NO₃^-/NO₂^-) assimilation are molybdenum (Mo)-dependent pathways by which inorganic nitrogen enters the biosphere and supports the production of organic carbon by photoautotrophs. In marine environments, molybdate (MoO₄^2-) availability is limited through sorption reactions, particularly in sediments rich in sulfide minerals. As a potential source of reactive oxygen species (ROS), the Mo-enzyme xanthine oxidase is also suggested to affect the viability of algal populations in enriched MoO₄^2- media. Thus, the aim of this work is to study the effect of increasing (0.5 to 200 µM) MoO₄^2- concentrations on ROS and nitrogen metabolism of the dinoflagellate Lingulodinium polyedrum maintained in cultures containing 0.1 or 0.5 g.L⁻¹ of insoluble ferrous sulfide (FeS), as an Mo-adsorbing matrix. Interestingly, the depletion of MoO₄^2- in the cultivating medium apparently resulted in oxidative stress to the microalga, as was evidenced by exacerbated activities of superoxide dismutase (SOD), catalase (CAT) and ascorbate peroxidase (APX). The lowest activities of the three antioxidant enzymes were obtained in algal cultures containing 0.1 mM MoO₄^2- (independent to the FeS content), which coincidentally is the regular Mo concentration found in oceanic and some estuarine waters. The adsorption (and retention) of Mo in sulfide residues is suggested to affect the ROS metabolism in L. polyedrum since maximum SOD and APX activities were obtained with 50 mM MoO₄^2- in cultures containing 0.1 g.L^-1 FeS, whereas this threshold was not reached in algal cultures containing 0.5 g.L^-1 FeS and MoO₄^2- concentrations up to 200 mM. The determination of lipid and protein oxidation indexes partially corroborates this hypothesis. Financial support: CNPq, FAPESP (Brazil); IFS (Sweden).