A method of operating a closed photobioreactor for cultivation of phototrophic microorganisms. The photobioreactor comprises a culture liquid and is partially or completely surrounded by water of a water body. A density difference between the culture liquid and the surrounding water is provided so that the position of the photobioreactor in the water body is controlled. A closed photobioreactor for cultivation of phototrophic microorganisms. The photobioreactor is adapted to comprise a culture liquid and to be partially or completely surrounded by water of a water body. The photobioreactor comprises means for determining the density difference between the culture liquid and the surrounding water.
