Final answer:
Eubacteria can act as both consumers and producers in ecosystems, depending on their mode of nutrition. They can perform photosynthesis or chemosynthesis to create food for themselves and others in various environments.
Step-by-step explanation:
Eubacteria, or true bacteria, can play different roles in ecosystems when it comes to nutrition. They can act as consumers, heterotrophs that depend on other organisms for food, or as producers, autotrophs that produce food for themselves and other organisms. Some bacteria can perform photosynthesis, similar to plants, and create their own food source. Others can use chemosynthesis, obtaining energy from sources other than the sun. This allows them to be producers even in ecosystems without sunlight, like certain marine environments.