Microbenthos - includes organisms smaller than 0.1 mm. These include bacteria, infusoria, algae, mites, etc.

From micro... and benthos.

A set of benthic (benthic) organisms less than 0.1 mm in size. The most massive representatives of microbenthos are bacteria, algae, protozoa (especially infusoria), small nematodes, mites, lower crustaceans, embryos of various hydrobionts. A distinction is made between eumicrobenthos (including organisms with a maximum size of 0.1 mm as an adult) and pseudomicrobenthos (consisting of organisms up to 0.1 mm in size at early stages of development).