Question

cyanobacteria developed near the end of the archean eon, and it thrived on ____ dissolved in water.
answer input
spelling counts but capitalization doesnt.

Explanation:

Cyanobacteria are photosynthetic bacteria. During the Archean Eon, they thrived by using carbon dioxide ($\ce{CO_2}$) dissolved in water for photosynthesis (to produce organic matter and release oxygen). So the substance they thrived on, dissolved in water, is carbon dioxide (or other key nutrients, but carbon dioxide is a primary one for their photosynthetic activity in this context).

Answer:

carbon dioxide