Some bacteria are decomposers, which fill an important role in the food web. what would happen to an ecosystem without decomposers

First, waste matter wouldn't decompose and add nutrients to the ground. then all the dead plants and animals would build up until all the nutrients had been taken from the ground because it couldn't be replenished. then the herbivores would starve followed by omnivores and carnivores. Everything would die out eventually.

Water and air would become polluted and the entire ecosystem would be unable to survive

Organism, organ system, organs, tissue, cells