The invention provides a stirring apparatus for stirring microorganisms such as, an alga in a culturing medium. The stirring apparatus includes one or more supporting structures. The stirring apparatus further includes a plurality of paddle units operatively connected to the one or more supporting structures. The plurality of paddle units are submerged in the culturing medium holding the microorganisms. Further, the plurality of paddle units are configured to rotate for stirring the microorganisms in the culturing medium. In response to the rotation of the plurality of paddle units, the stirring apparatus propels in the culturing medium.