Protists

Protists are single-celled eukaryotes that occupy the microbial loop as bacterivores: they graze the bacteria that grow on dissolved organic matter, packaging that otherwise-inaccessible production into a particle size that larger grazers can eat. Without them, the carbon and nitrogen that bacteria pull out of DOM would be a dead end. They reproduce fast (hours to a day) and are held in check largely by density-dependent viral lysis.

The two protist microzooplankton in the model span the size range of the loop:

• Heterotrophic Nanoflagellate (HNF) — the smallest grazers (2–20 µm), the first link in the loop, consuming bacteria at 30–50% of bacterial production.
• Ciliate Community — larger bacterivores (10–300 µm) with high clearance rates, reproducing by binary fission.

These are protists specifically — microscopic animals such as rotifers are metazoans and are catalogued with the invertebrate grazers under Consumers.