What are the usual contaminants?

Bacteria (usually present, a problem when they dominate), grazing protozoa such as ciliates and amoebae, rotifers and other zooplankton that eat cultures from within, fungi, and faster-growing algae that outcompete your strain. Each has a signature: grazing makes cultures clear from dense, bacterial takeover smells, weed algae change the texture.

Can I save a contaminated culture?

Sometimes, depending on the contaminant and the species. Grazers can sometimes be excluded by mesh or settling; some contaminants are outgrown by re-dosing nutrients and keeping conditions strongly favourable to your strain. Honestly, most contaminated small cultures are cheaper to discard and restart from your reserve — which is why reserves exist.

Does everything need to be sterile?

No. Most applied microalgae culture is 'clean', not sterile: bleach-washed vessels, filtered or boiled water, clean hands and tools, covered vessels. Full aseptic technique matters for axenic lab work, not for live-feed culture rooms.

Contamination Control in Microalgae Cultures

Contamination is not an event; it is a slow invasion that becomes visible late. Managing it is less about heroics after the fact and more about boring habits before it. This article covers the contaminant roster, how they get in, and the working hygiene that keeps a culture room alive for years.

Know your invaders

Grazers — ciliates, amoebae, rotifers. The most dramatic: a dense culture clears visibly within a day or two as something eats it. Check a drop under a microscope; movers among your cells are bad news.
Bacteria — always present at some level; a problem when culture health drops and they bloom. Signatures: film, smell, cells clumping or lysing.
Weed algae — faster-growing species riding in on water, tools or starter. They win by outgrowing your strain; a culture that changes character between transfers is being replaced.
Fungi and mold — more common in freshwater and high-organic setups; visible strands or fuzzy growth.

How they arrive

Aerosols and dust from open handling; unclean vessels; untreated water; live starters from unreliable sources; sharing tools between cultures without cleaning; and — the classic — an open culture sitting near a crashed one. Every contaminant needs a doorway; hygiene is door control.

The working hygiene set

Wash vessels with dilute bleach, rinse thoroughly, and let them dry before use.
Use filtered, boiled, or otherwise treated water for media; never top up with raw tank water.
Keep cultures covered or capped; open them briefly and deliberately.
Never share tools between cultures without cleaning; work from clean toward questionable, never the reverse.
Quarantine new starters in small volume until proven, and keep a younger reserve culture of every strain you care about.

Observation beats treatment

A weekly microscope check of every culture catches grazers weeks before the culture visibly declines. Colour, smell and settling tell you the rest. When contamination does win — and in any long-running room, it sometimes will — discard decisively, clean thoroughly, and restart from the reserve you kept. The alkaline-culture trick deserves mention: some species are cultured in conditions most contaminants hate (Arthrospira's high-pH, high-carbonate medium is the famous case — see our Arthrospira guide). Selecting forgiving chemistry is prevention too.

How to Prevent Contamination in Microalgae Cultures

Know your invaders

How they arrive

The working hygiene set

Observation beats treatment