Single diploid protoplast stage in the developmental cycle of red algae (Rhodophyta). This cycle is trigenetic, meaning that there are 3 successive, cellularly different generations. A haploid thallus (the gametophyte) produces gametes, which once fertilised produce a carposporophyte. The carposporophyte forms carpospores, which are released into the environment where they develop into tetrasporophytes. After reduction division, the tetrasporophytes produce tetraspores that germinate to give a gametophyte once more.