Axenic cultures of Posidonia coriacea and Halophila ovalis were established to investigate the effect of nitrogen addition and the optimum concentration of MS (Murashige and Skoog Physiol Plant 15:473–497, 1962) macronutrients, micronutrients and organics. Chlorophyll content and dissolved oxygen of H. ovalis was optimal at the control concentration of macronutrients (half strength MS). Below and above this concentration, growth of this species was compromised. A similar effect was observed for H. ovalis growth in response to MS micronutrient. However, changing the MS vitamins concentration had no impact on the growth of H. ovalis or P. coriacea. Leaf and root numbers of H. ovalis were highest in the treatment with the most nitrogen (50.6 mM). Root architecture of H. ovalis was affected by nitrogen source; shorter roots without root hairs were produced when NH4NO3 was present in the media. Longer roots with root hairs were produced when the cultures were grown in the absence of NH4NO3. P. coriacea growth did not exceed that of the control for any of the treatments. The response of H. ovalis to nutrient addition suggests its opportunistic nature, whereas the lack of response by P. coriacea suggests there may be some other limiting factor. This study shows that the proposed formulation of ½ strength MS can be suitable for both H. ovalis and P. coriacea.