Question

list two important roles of decomposer they play in the environment

Answer 1

1)Decomposers include microorganisms such as bacteria and fungi, which obtain nutrients by breaking down the remains of dead plants and animals. 
2) Decomposers eventually convert all the organic matter into carbon dioxide (which they respire) and nutrients. These nutrients (such as nitrogen, phosphorus, and magnesium )are then taken up by the plants and algae. This process resupplies the nutrients to the ecosystem and also converts the waste matter back to usable form. 

Answer 2

Once these deceased organisms are returned to the soil, they are used as food by bacteria and fungi by transforming the complex organic materials into simpler nutrients. The simpler products can then be used by producers to restart the cycle. These decomposers play an important role in every ecosystem.