In the context of the reintroduction project of the European oyster Ostrea edulis (Linnaeus, 1758), a hatchery is necessary as the stocks cannot be restored naturally due to the strong decline caused by anthropogenic impacts in the past years. Optimizing the survival rate of the larvae is of great importance in this respect. The increased use of antibiotics is intended to face this challenge. However, this approach cannot be applied in the framework of ecological restoration, since it does more harm than good. Alternatives must be developed. In this work, the long chain fatty acids (LCFA) of Tetraselmis suecica exposed to different light intensities (100, 150 and 200 μmol photons m-2 s-1) were analyzed. According to va- rious references, light is one of the factors that is supposed to influence LCFA concentration. Additionally, it is hypothesized that with higher LCFA concentrations, also antibacterial activity in microalgae culture is increased. This antibacterial activity can be utilized to reduce bacterial infection of sensitive Ostrea edulis larvae grown in large-scale aquaculture. The T. suecica cultures maintained at different light intensities were each added to a 3-mixed diet with Isochrysis galbana and Chaetoceros muelleri in beakers containing 700 mL of 1 μm filtered and UV-treated seawater and 3,500 O. edulis larvae. Controls with a 2-mixed-diet addition with only I. galbana and C. muelleri and a 3-mixed-diet addition with I. galbana, C. muelleri and Rhodomonas salina served as control. The results were used to examine whether the addition of T. suecica to the holding water increases the larval survival rates. Thus, in this work, sustainable alternatives to the use of antibiotics in hatcheries were sought. Light intensity was found to have no impact on the percentage of fatty acids in their com- plete composition per T. suecica cell, but did affect the total LCFA concentration. It was found that the highest LCFA concentration per T. suecica cell was found in cultures grown at a light intensity of 150 μmol m-2 s-1. The lowest LCFA concentration per T. suecica cell was found in cultures maintained at 200 μmol m-2 s-1. Compared to the 2-mixed-diet control, the 3-mixed-diet treatments did not perform as well. For example, in the treatments with the lowest LCFA concentration in the T. suecica cells (cultivation at 200 μmol m-2 s-1), a 100 % larval mortality rate was observed. Clear results could not be obtained in this sub-trial, but a tendency for better survival of O. edulis larvae with increased long chain fatty acid content in T. suecica cultures using as feed was evident. However, besides the LCFA concentration of the food, other influencing factors regarding the survival of the sensitive O. edulis larvae should be considered and excluded in the future. Here, it is recommended to optimize the experimental set-up to get clearer results.