The diatom Haslea ostrearia was first studied by Gaillon in the year 1820 because of the greening phenomenon of oysters in western France. This microalga has the capacity to produce and excrete a blue pigment, called marennine, that has antioxidant, anti-bacterial and anti-viral properties, with possible industrial applications related to aquaculture and cosmetics. However, it is difficult to produce biomass in large concentrations in photobioreactors (i.e, usually 1 kg m−3 dry weight or higher) due to stirring sensitivity, and also because diatoms have special requirements, such as a supply of silica. This work presents a design for a new, completely artificial, seawater medium named NX (i.e, Nghiem Xuan) for optimizing cultivation of the diatom H. ostrearia in photobioreactors. NX takes account of the carbon and phosphorus sources in either organic or inorganic form, and the composition of the main elements (C, H, O, N, P, S, Si). Optimization of the calcium, magnesium and iron concentrations was found to be essential in order to achieve the highest productivities. The resulting NX medium was validated in an airlift photobioreactor. This led to productivities of 4.9 × 108 cell L−1 (ca. 500 mg L−1 of biomass) and 15.7 mg L−1 of marennine, higher than has previously been reported in the literature.