Method for culturing photosynthetic microorganisms, in particular algae, using a photosynthetic reactor comprising: - at least one photosynthetic reaction pipe in which the liquid culture medium circulates and which is provided with at least one substantially horizontal reaction section that is at least partially made of a material that is transparent to light radiation, said reaction pipe having a bottom end placed in the bottom part of the reactor and a top end placed in the top part of the reactor above the bottom end; - at least one return pipe which ensures the fluidic link between the bottom end and the top end of the reaction pipe; - at least one means for circulating the liquid culture medium; - at least one gas injection means placed in the reaction section or upstream of said reaction section relative to the direction of circulation of the gas, said gas injection means making it possible to inject gas into the reactor; - at least one liquid injection means making it possible to inject liquid into the reactor; and - at least one discharge means arranged in the top part of the reactor and allowing discharge of the gas injected into the reactor; said method comprising the following steps: - injecting a liquid culture medium into the reactor according to a controlled flow rate with the liquid injection means; - injecting a gas into the reactor according to a controlled flow rate with the gas injection means; - circulating the liquid culture medium with the circulating means; - controlling the circulating means and the gas injection means so as to establish, in the reaction section, a horizontal gas/liquid two-phase flow regime of the stratified flow or pocket or bubble flow type, where the gas injected by the injection means rises back up to the discharge means by circulating in the reaction pipe, in a direction of circulation which goes from the bottom end to the top end of the reaction pipe, such that the gas injected and the liquid culture medium establish said horizontal gas/liquid two-phase flow regime in the substantially horizontal reaction section; where the circulating of the liquid culture medium is not carried out by an air-lift or gas-lift device, such that the gas/liquid transfer does not take place inside a bubble column, but along a substantially horizontal reaction section in which the flow follows said horizontal gas/liquid two-phase flow regime. - at least one photosynthetic reaction pipe in which the liquid culture medium circulates and which is provided with at least one substantially horizontal reaction section that is at least partially made of a material that is transparent to light radiation, said reaction pipe having a bottom end placed in the bottom part of the reactor and a top end placed in the top part of the reactor above the bottom end; - at least one return pipe which ensures the fluidic link between the bottom end and the top end of the reaction pipe; - at least one means for circulating the liquid culture medium; - at least one gas injection means placed in the reaction section or upstream of said reaction section relative to the direction of circulation of the gas, said gas injection means making it possible to inject gas into the reactor; - at least one liquid injection means making it possible to inject liquid into the reactor; and - at least one discharge means arranged in the top part of the reactor and allowing discharge of the gas injected into the reactor; - injecting a liquid culture medium into the reactor according to a controlled flow rate with the liquid injection means; - injecting a gas into the reactor according to a controlled flow rate with the gas injection means; - circulating the liquid culture medium with the circulating means; - controlling the circulating means and the gas injection means so as to establish, in the reaction section, a horizontal gas/liquid two-phase flow regime of the stratified flow or pocket or bubble flow type, where the gas injected by the injection means rises back up to the discharge means by circulating in the reaction pipe, in a direction of circulation which goes from the bottom end to the top end of the reaction pipe, such that the gas injected and the liquid culture medium establish said horizontal gas/liquid two-phase flow regime in the substantially horizontal reaction section;