Protozoa
There are both autotrophic and heterotrophic types of protozoa.
Unlike bacteria and fungi, which absorb dissolved organic compounds
from their environment, heterotrophic protozoa (such as the amoebas and Paramecium) consume other organisms
such as algae, bacteria, or other protists.
Together with other microorganisms, protozoa make up the biofilm coating sediments and hard surfaces on riverbeds,
though some protozoa are free-swimming. Certain protozoa are parasites and cause diseases such as giardia (beaver fever).