The invention relates to a gas-introduction unit for introducing a process gas into a liquid located in a reactor without producing bubbles. The gas-introduction unit at least comprises: - a first gas receiving chamber and a second gas receiving chamber at a distance therefrom for receiving a process gas, both gas receiving chambers being connected together via at least two flatly designed gas-conducting diffusion membranes made of mutually spaced hollow fibers which are at least partly fixed relative to one another; - a receiving area for a gas feed on at least one of the gas receiving chambers; and - a receiving area for a shaft on at least one of the gas receiving chambers; wherein the gas-introduction unit can be supplied with process gas via the gas feed receiving area in order to introduce gas into the liquid in the reactor, the gas-introduction unit can be rotated via the receiving area for the shaft, and a convection flow can be formed within the reactor via the rotational movement of the gas-introduction unit in the liquid. The invention additionally relates to a method for introducing gas into a process liquid, to a gas-liquid reactor having a gas-introduction unit according to the invention, and to the use of a gas-introduction unit according to the invention for supplying biological cultures with process gases.