The invention relates to a photobioreactor (1) for cultivating phototrophic microorganisms (such as microalgae for example), comprising a reactor element (2) which has a plurality of rising pipes (3a) and falling pipes (3b) for a liquid culture medium (4) containing the microorganisms and which has a distributing pipe (5). The upper ends (6) of each of the rising pipes (3a) and the falling pipes (3b) are connected to the distributing pipe (5) in a liquid-permeable manner. The invention is characterized in that both the culture medium (4) as well as a gas chamber (9) above the culture medium (4) for receiving gas bubbles (10) rising out of the culture medium (4) are provided in the distributing pipe (5) in the operating state, wherein a boundary surface (11) is arranged between the culture medium (4) and the gas chamber (9) in the distributing pipe (5). By arranging the boundary surface (9) (or the boundary surface region), which is typically susceptible to dirt, in the distributing pipe (5), the aim of the invention to simplify the cleaning process of the photobioreactor or the process of keeping same clean is achieved. In this manner, the yield and service life of the photobioreactor is increased. The invention additionally relates to a method for cultivating phototrophic microorganisms.