Protista Single-celled eukaryotes of an enormous diversity are lumped together as the Kingdom Protista. Photosynthetic forms called phytoplankton, living in both fresh and saltwater, are probably responsible for more than half of all photosynthesis on earth. Heterotrophic protists may directly absorb dissolved substances or may ingest particulates. Viruses Viruses are agents that infect cells, yet lack many of the characteristics of life.
Viruses hijack the host cell and force it to synthesize more virus.
As such, they are responsible for many severe diseases of other organisms, including bacteria, plants, and animals.