How Microalgae Are Grown for Supplements

The way a microalga is grown determines what ends up in your supplement: the nutrient consistency, the contamination profile, and the cost.

Two products containing the same species can differ substantially if one was cultivated in a controlled sealed system and the other in an outdoor pond exposed to whatever the surrounding environment introduces. If you care about what you are putting in your body, understanding the cultivation basics helps you ask the right questions.

The Two Main Approaches

We covered the detailed comparison in our photobioreactor vs open pond article. The summary:

Open-pond systems grow algae in shallow outdoor raceways or circular ponds. Sunlight drives photosynthesis, paddle wheels circulate the culture. The method is inexpensive and scalable. The trade-off is environmental exposure: airborne contaminants, competing organisms, weather variation, and whatever is in the local water supply all have access to your supplement's raw material.

Closed photobioreactors grow algae in sealed transparent tubes or panels. Light enters through the walls, but nothing else gets in. Water is filtered, nutrients are controlled, temperature and pH are monitored. The method is more expensive to build and operate, but it eliminates the environmental exposure pathways that open systems introduce.

We grow our marine phytoplankton in closed photobioreactors. That is a product-specific choice we made for purity and consistency reasons, and we think it matters enough to disclose.

Heterotrophic Fermentation

Not all microalgae are grown photosynthetically. Schizochytrium, the species used for most algae-derived DHA oil, is a heterotroph. It feeds on organic carbon sources (sugars) rather than photosynthesising. It is cultivated in stainless steel fermenters, similar to those used in brewing or pharmaceutical production. This method produces high yields of DHA-rich oil in a controlled, sealed environment.

Fermentation-based cultivation has different purity characteristics from photosynthetic cultivation. There is no light requirement, no exposure to the external atmosphere, and no dependence on water clarity. The contamination risks are microbial rather than environmental. We produce our Clean Omega DHA from this type of process.

What the Growing Method Affects

Nutrient consistency: Algae grown in controlled environments produce more consistent batch-to-batch nutrient profiles than algae subject to seasonal and weather variation. If you are taking a supplement for its EPA content, knowing that each batch delivers a similar amount matters for dose reliability.

Contamination risk: We covered heavy metals in algae supplements in detail. The growing environment is the primary route by which lead, cadmium, arsenic, and mercury enter the product. Closed systems eliminate the exposure pathways. Open systems depend on post-harvest testing to detect what the environment introduced.

Species purity: Open ponds can be invaded by competing algae species, bacteria, and other microorganisms. A product labelled as a single species may contain a mixture. Sealed systems maintain species purity because the culture is physically isolated.

Cost: Closed systems cost more to build and operate, which is reflected in the retail price. We think the cost difference is justified by the purity and consistency advantages, but we acknowledge that well-managed open-pond operations with rigorous testing can produce safe products at a lower price point.

What to Ask

If the manufacturer does not tell you how their algae was grown, it is worth asking — whether you are ordering online or picking it up from a health shop. The answer is relevant to everything from contamination risk to nutrient reliability. We explain the broader quality variables in our guides to choosing phytoplankton and chlorella supplements.

This article is for informational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Consult your healthcare provider before starting any supplement.

Methodology and Disclosure

Phytality grows microalgae in closed photobioreactors and sources DHA from fermentation-based cultivation. We have a commercial interest in controlled cultivation being valued. Descriptions of cultivation methods reflect established aquaculture and fermentation science. No EFSA-authorised health claims are cited. Our assessment of cost-quality trade-offs reflects our editorial position.

Last reviewed: March 2026