The invention relates to a method for cultivating photoautotrophic micro-organisms, comprising the following steps: provision of a photosynthesis layer, which contains a photosynthesis medium and photoautotrophic micro-organisms suspended therein, in a closed accommodation space having a gas-diffusion-inhibiting outer boundary, wherein the photosynthesis layer has an average thickness of no more than 10 mm; exposure of the photosynthesis layer without eddying or feeding through a gas in the photosynthesis medium while limiting the diffusive gas exchange of the accommodation space by the environment, wherein the supply of micro-organisms in the photosynthesis medium with hydrogen carbonate ions is maintained by storing hydrogen carbonate ions in the photosynthesis medium and/or providing a photosynthesis substrate in the form of carbon dioxide and hydrogen carbonate from a cell-free carbon storage medium outside the photosynthesis layer; and conducting a gas formed during photosynthesis out of the accommodation space.