Final answer:
Lichens are the dominant organism on bare rock during primary succession due to their ability to photosynthesize and contribute to soil formation in a symbiotic relationship with fungi.
Step-by-step explanation:
The dominant organism colonizing bare rock during primary succession is lichens. They are considered pioneer species since they are among the first life forms to inhabit such areas. Lichens are composed of a symbiotic relationship between a fungus and an algae or cyanobacteria, allowing them to survive in harsh conditions without soil. These organisms can photosynthesize and provide nutrients to the developing ecosystem by breaking down the rock to form the initial soil, laying the foundation for subsequent colonization by plants like mosses, grasses, shrubs, and eventually trees.