Abstract: A closed system for producing algae moves water that contains algae through an endless reactor tube that defines a continuous pathway. At least part of the tube floats on a volume of water. The tube is transparent to light. Water partially fills the tube and there is a gas space above the water in the tube, with two phase stratified flow of water and gas in the tube, gas transfer between the gas space and the water, and algae biomass suspended in the water. Nutrients and a gas and water are supplied to the tube. Algae are harvested from the tube.