Abstract: The present invention relates to the cultivation of photosynthetic microorganisms, and more particularly, to a low-cost flexible photobioreactor (1), as well as systems and methods using the same for optimizing the growth of microalgal species. The photobioreactor of the invention is in the shape of a flexible transparent elongated body (6) adapted to be positioned horizontally in a body of water, and comprises i) an elongated gas dispensing system (7) for providing nutrients by means of bubbling a gas mixture to a liquid suspension and for performing a vertical low stress mixing of said liquid suspension, ii) an elongated filling/draining system (11) for controlling the volume of said liquid suspension in the photobioreactor, and iii) at least one gas evacuation opening (13).