The present invention concerns methods and devices for cultivating coenocytic algae and for producing cellulose, in particular microfibrillated cellulose (MFC). In one embodiment, the algae are grown on a solid substrate immersed in liquid culture medium. The solid substrate preferably comprises a carbonite mineral. The invention also concerns a method for producing spores of said coenocytic algae and a device comprising two separate containers, a first container for growing said algae and a second container for growing larger algae and inducing the generation of spores.