Abstract: At least one elongated photobioreactor, at a small angle relative to horizontal and mixed substantially or entirely by large bubble flow, is used for contained cell culture, e.g., microalgae cultivation. Elongated, flexible, transparent, polymeric photobioreactor tubes, in near-grade and near-horizontal (e.g. sloped 1 degree to 3 degrees) orientation, and use of low-pressure air mixing, allow very inexpensive construction and operation. Multiple elongated tubes may be used for an independent operation of the multiple photobioreactor tubes for the same or different cells, e.g., microalgae and different applications. Low-pressure air is delivered near the low end of the bioreactor at less than 10 psig and without sparging, to produce large air bubbles that travel from the low end to the high end of the bioreactor, for turbulent mixing and gas exchange.