Although most diatoms are autotrophic, some heterotrophic or symbiotic species can be found in particular habitats. The living matter of each diatom is enclosed in a shell of silica that it secretes. These shells are marked by minute pores or depressions that allow the living organism access to its environment.iatom (dĪˈətŏmˌ, –tōmˌ) [ key ], unicellular organism of the kingdom Protista, characterized by a silica shell of often intricate and beautiful sculpturing. Most diatoms exist singly, although some join to form colonies.