Figure 11.7Microorganisms can form layered deposits called microbial mats. The top part of the mat, which is exposed to the Sun, contains photosynthetic autotrophic microorganisms, as revealed by the green color. Farther down in the mat, but still within the aerobic zone, are nonphotosynthetic autotrophs, as revealed by the purple color. Deeper in the mat, the color turns gray, revealing the anaerobic zone where heterotrophs live.