How Heavy Metals Affect Protozoa and Microfauna

Copper kills single-celled life at concentrations fish tolerate. That cuts both ways: it cures protozoan parasites but also strips a tank of its beneficial microfauna.

At the smallest end of aquarium life are the single-celled organisms: protozoa, ciliates and other microfauna. Heavy metals, copper above all, act powerfully on this microscopic world, and the effect has two faces. The same toxicity that makes copper a cure for protozoan parasites also makes it destructive to the beneficial microfauna of a mature tank.

What protozoa are

Protozoa are single-celled eukaryotes, either free-living or parasitic, that feed on organic matter. Free-living protozoa are common and often abundant in fresh, brackish and salt water, where they include the most important bacterivores and help solubilize nutrients within microbial biomass, stimulating microbial growth. In an aquarium these microbes are part of the living film and the microbial food web that supports a stable system.

The therapeutic angle: killing parasites

Many of the worst fish parasites are themselves protists. Ich, or white spot, is a ciliate protist; marine white spot (Cryptocaryon) is similar; and velvet is caused by a parasitic dinoflagellate, Amyloodinium. Copper is toxic to these organisms and is a standard treatment: freshwater ich is treated with copper sulfate, and for marine fish the recommended dose against Cryptocaryon and Amyloodinium is 0.15 to 0.20 mg/L of free copper, which targets their vulnerable free-swimming stages while the fish tolerate the dose.

The collateral angle: losing beneficial microfauna

The catch is that copper does not distinguish a parasitic protist from a beneficial one. Because the targeted ich and velvet organisms are single-celled protists, a copper dose lethal to them is also lethal to the free-living protozoa and ciliates that share the same cellular vulnerability. A copper-treated tank therefore tends to lose much of its microfauna, including the protozoa and infusoria that fry and filter-feeders graze on, and its hard-won biological maturity is set back.

Practical takeaways

• Use copper deliberately and only where it belongs: a fish-only treatment system, dosed and measured carefully.
• Expect a copper-treated tank to lose much of its microfauna; it is a cost of the treatment, not a malfunction.
• Keep delicate fry-rearing or culture systems, where living microfauna is the point, entirely free of copper and other metals.
• Remember the whole gradient: fish tolerate copper, invertebrates mostly do not, and single-celled life is hit at the lowest concentrations of all.