Answer:
Bacteria
Explanation:
Bacterial are prokarotes, that inhabit virtually all habitats..Although some may be pathogenic, ,most bacteria are beneficial. They live in mutual relationship with their living hosts.where they help  most in catabolic  reactions especially in digestion of  food and  in nutrient cycle.
They lack membrane bound organelles,they have cell wall  made up of peptidoglycan, their nucleus is not bounded with double membrane like Eukaryotes.They  vary  in shapes, most are spherical in shape, with the shape determined by the cell wall and the cytoskeleton  in the cystosol.
