Which cellular organisms primarily subsist on organic matter?

Fungi are primarily known for their role as decomposers in ecosystems, subsisting on organic matter through a process called saprotrophic nutrition. They secrete enzymes into their environment to break down complex organic materials, such as dead plants and animal matter, into simpler compounds that they can absorb. This method of obtaining nutrients is vital for nutrient cycling in ecosystems, as fungi recycle organic materials back into the soil, making nutrients available for other organisms.

In contrast, bacteria are also involved in the decomposition of organic matter but can have diverse nutritional modes, including autotrophy. Viruses, on the other hand, are not classified as cellular organisms and do not consume organic matter, as they require a host cell to replicate and propagate. Protozoa may consume organic matter, but they primarily feed on bacteria and smaller organic particles rather than being direct decomposers of larger organic materials like fungi. Therefore, fungi's unique adaptations for decomposing and utilizing organic matter clearly establish them as the cellular organisms that primarily subsist on it.