What Is Marine Phytoplankton?

Marine phytoplankton are microscopic, single-celled organisms that photosynthesise in seawater. Among the oldest life forms on Earth, these organisms produce an estimated 50-80% of the planet's oxygen and form the base of nearly every marine food chain. If you have ever breathed in sea air and felt something faintly alive about it, these organisms are part of the reason.

What most people do not realise is that marine phytoplankton are also the original source of the omega-3 fatty acids found in fish. Fish do not manufacture EPA and DHA. They accumulate it by eating microalgae, or by eating things that ate microalgae. The fish is the middleman. That single fact is why phytoplankton has entered the supplement conversation, and why we work with it directly.

But the term covers thousands of species, and when we reviewed the ones commercially available for supplementation, the nutritional differences between them were not subtle. The species that matters most for omega-3 is Nannochloropsis gaditana, which produces EPA at concentrations high enough to be nutritionally useful. If the label on your product does not name a species, you are buying a category, not an ingredient.

What Marine Phytoplankton Actually Is

In biological terms, marine phytoplankton are microorganisms that live in the sunlit upper layers of the ocean and convert sunlight, water, and carbon dioxide into energy. They are not plants, though they photosynthesise like plants. They are not seaweed, which are large, multicellular organisms you can see with the naked eye.

And most of them are not the cyanobacteria sold as spirulina, which belong to a different biological kingdom entirely. Knowing what phytoplankton is not clears up a surprising amount of the confusion you will encounter on supplement shelves.

If you look up "marine phytoplankton" expecting a single organism, you will find thousands: diatoms, dinoflagellates, coccolithophores, and other microalgae. Most are ecologically fascinating but nutritionally irrelevant to your supplement decision.

Nannochloropsis Gaditana: The Key Supplement Species

The species we selected is Nannochloropsis gaditana, a eustigmatophyte with unusually high lipid content, particularly EPA (eicosapentaenoic acid). A product containing Nannochloropsis delivers a fundamentally different nutrient profile from one containing Isochrysis (which is higher in DHA) or Tetraselmis (which is more protein-focused). If you are comparing two products that both say "marine phytoplankton," you could be comparing an omega-3 supplement with a protein supplement without realising it.

Nutritional Profile and Evidence-Based Benefits of Marine Phytoplankton

EPA: The Standout Omega-3 Fatty Acid

Open a tub of whole-cell phytoplankton powder and what you see is a fine, dark green substance with a faintly marine smell, nothing like fish oil. That colour comes from the chlorophyll and carotenoid pigments. But the nutrient that earns Nannochloropsis its place on the shelf is EPA, a long-chain omega-3 fatty acid present at concentrations higher than in most commercially cultivated microalgae.

If you are taking phytoplankton for heart health, there is one claim we can make with regulatory backing: EPA and DHA contribute to the normal function of the heart at a combined daily intake of 250 mg. That is an EFSA-authorised health claim under Commission Regulation (EU) No 432/2012, and it is one of the few omega-3 claims that has actually survived scrutiny.

We use it because it is earned, not because it sounds good on the label.

The alternative route, converting plant-based ALA from sources like flaxseed, is inefficient: published estimates put the conversion rate at roughly 5-10%, varying by individual genetics and diet (Brenna et al., 2009). If you are spooning ground flaxseed onto your porridge each morning and assuming that covers your omega-3 needs, the arithmetic is not in your favour.

One caveat worth noting: Nannochloropsis delivers EPA, not DHA. If your goal is both from plant sources, a dedicated algae-derived DHA supplement covers the other half.

Chlorophyll, Carotenoids, and Micronutrients

Beyond EPA, whole-cell Nannochloropsis delivers chlorophyll, violaxanthin and other carotenoid pigments, and a range of amino acids. These carry over into supplements made from intact biomass but not into extracted algae oils, which strip out everything except the fatty acids. Worth knowing if you are deciding between a powder and a capsule: you are not just choosing a format, you are choosing a nutrient profile.

How Well Your Body Absorbs EPA from Phytoplankton

We should be straightforward here: how much EPA your body actually absorbs from whole-cell phytoplankton is not yet comprehensively studied in the way it has been for fish oil or purified algae oil. The EPA in Nannochloropsis is present in a phospholipid-bound form, which some research suggests may be well-absorbed.

But we do not yet have the head-to-head comparisons at equivalent doses that would let us make that claim with confidence.

If you are comparing products across formats, be cautious about assuming milligram-for-milligram equivalence. We would rather tell you the evidence is still developing than pretend the question is settled.

How Marine Phytoplankton Is Cultivated

Marine phytoplankton are grown commercially by providing them with light, water, carbon dioxide, and a controlled nutrient feed, then harvesting the biomass once the culture reaches the right density. The method used to do this varies enormously, and it is the single biggest factor in what you end up swallowing.

Open Ponds vs Closed Photobioreactors

Open ponds are the cheaper route. The algae grow in large, shallow outdoor pools exposed to sunlight, rain, dust, and whatever the local environment contributes. That means heavy metals, microplastics, competing organisms, and batch-to-batch variation that no amount of downstream processing fully corrects.

Closed photobioreactors are sealed glass or plastic tube systems where every variable is controlled: light, temperature, water purity, and nutrient delivery. The biomass that comes out is cleaner and more consistent. It also costs substantially more to produce, and that cost lands on you at the till. If you are comparing two supplements and one costs noticeably less, the cultivation method is usually the reason.

How Marine Phytoplankton Differs from Other Algae Supplements

Marine Phytoplankton vs Chlorella

Chlorella is a freshwater green alga with a very different nutritional emphasis. It is roughly 40-60% protein by dry weight and one of the richest natural sources of chlorophyll, but it provides no meaningful EPA. If you picked up chlorella expecting it to cover your omega-3 needs, it will not.

That said, chlorella earns its place for different reasons. If your goal is concentrated plant protein and chlorophyll in a single whole-food source, chlorella has a stronger case than phytoplankton. The two are genuinely complementary rather than competing.

Marine Phytoplankton vs Spirulina

Spirulina (Arthrospira platensis) is not technically an alga at all. It is a cyanobacterium, biologically closer to bacteria than to eukaryotic microalgae like Nannochloropsis. It is high in protein and phycocyanin (a blue pigment with antioxidant properties) but provides no EPA or DHA.

All three sit together on supplement shelves under "algae," which is a bit like filing salmon and seaweed under "marine food." Technically defensible, nutritionally misleading.

Marine Phytoplankton vs Algae Oil (DHA Supplements)

"Algae oil" supplements are typically made from Schizochytrium, which is DHA-dominant. Marine phytoplankton from Nannochloropsis is EPA-dominant. They are complementary, not interchangeable. If you want both EPA and DHA from plant sources, you need both types of supplement. Our guide to phytoplankton vs algae oil covers this comparison in full.

How to Choose a Marine Phytoplankton Supplement

If you are standing in a health shop or scrolling through product pages, three things are worth checking before you spend your money.

Species name. A supplement that says "marine phytoplankton" without naming the species is asking you to trust a category label. That is like buying "fish" without knowing whether it is salmon or sardine. For EPA content, look for Nannochloropsis on the label. If the species is not stated, you cannot evaluate what you are buying.

Cultivation method. Closed photobioreactor or open pond? If the product page does not say, ask. The answer affects purity and consistency. Most reputable producers will state their method clearly because it is a genuine differentiator, not a marketing flourish. Silence on this point is itself informative.

EPA per serving. Look for quantified milligrams on the nutritional information panel, not vague references to "omega-3 rich" or "nutrient dense." Your body does not respond to adjectives. It responds to milligrams. If a product does not declare its EPA content per dose, you cannot meaningfully compare it to anything else on the shelf.

What Growing Marine Phytoplankton Taught Us

Most of what we know about marine phytoplankton did not come from reading papers. It came from growing it, testing it, and making formulation decisions where the wrong call would mean a worse product on the shelf. Here is what that process surfaced.

Species selection is a nutritional decision, not a branding one. When we compared lipid profiles across commercially available phytoplankton species, Nannochloropsis gaditana consistently delivered the highest EPA concentration. Other species offered more protein or more DHA, but if your goal is plant-source EPA, the species question has a clear answer.

Closed photobioreactors cost more because contamination is not theoretical. We tested open-pond and closed-system biomass side by side during formulation. The heavy metal differences were not marginal. Sealed systems eliminate airborne contamination, agricultural runoff, and competing microorganisms. You pay more at the till, but you are paying for what is absent from the product.

The price gap between phytoplankton products usually reflects how they were grown. If you see two products with similar labels and very different prices, the cultivation method is the most likely explanation. Closed-system production requires more infrastructure, more energy, and more monitoring per batch. Cheaper products are not necessarily worse, but the question worth asking is why they cost less.

Whole-cell powder and extracted oil are fundamentally different products. Whole-cell Nannochloropsis delivers EPA alongside chlorophyll, carotenoids, and amino acids. Extracted algae oil isolates the fatty acids and discards everything else. Neither is wrong, but if you assume they are interchangeable because both say "phytoplankton" or "algae," you will misunderstand what you are taking.

Bioavailability evidence is still catching up to the product category. We can tell you the EPA is present in phospholipid-bound form, and early research on that form is encouraging. We cannot yet show you the large, controlled human trials that would let us claim superior absorption over fish oil. That honesty costs us a marketing line, but it keeps the science accurate.

Marine Phytoplankton FAQ

Is marine phytoplankton safe to take every day?

For the species used in commercial supplements (primarily Nannochloropsis), daily use at recommended doses is well-established. Check with your GP if you are pregnant, breastfeeding, or taking blood-thinning medication. EPA can have mild anticoagulant effects at high intakes (generally above 2-3g of combined EPA and DHA daily), though standard phytoplankton supplement doses are well below this threshold.

Can marine phytoplankton replace fish oil?

It can replace the EPA component of fish oil, but not the DHA. If your current fish oil provides both EPA and DHA, switching to phytoplankton alone would leave a gap. You would need a separate algae-derived DHA supplement to cover both fatty acids from plant sources.

Does marine phytoplankton taste or smell like fish?

No. Phytoplankton has a mild, slightly vegetal flavour, noticeably different from fish oil. Fish oil develops its strong taste through oxidation during processing and storage. Most people find phytoplankton far more tolerable, particularly if you have experienced the reflux that fish oil capsules sometimes cause.

How is marine phytoplankton different from algae oil?

"Algae oil" supplements are typically made from Schizochytrium, which produces DHA. Marine phytoplankton from Nannochloropsis produces EPA. They are complementary, not interchangeable. If your goal is a complete plant-based omega-3 profile, you need both.

What dose of marine phytoplankton should you take?

Dosing depends on the EPA concentration per serving of your specific product. The EFSA-authorised heart health claim applies at a combined 250 mg of EPA and DHA daily. Check the nutritional information panel for the actual EPA content per dose, then work out whether you are meeting that threshold from your chosen product alone or in combination with other sources.

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

Methodology and Disclosure

Phytality manufactures marine phytoplankton supplements from Nannochloropsis gaditana grown in closed photobioreactors. We have a direct commercial interest in this ingredient. Taxonomic descriptions reflect established biology. Nutritional claims about EPA content are drawn from published literature cited above.

The EFSA-authorised health claim for EPA and DHA is cited under Regulation EU 432/2012 with its intake condition stated. ALA conversion rates are drawn from Brenna et al. (2009). Comparisons with chlorella, spirulina, and algae oil reflect established compositional differences documented in published reviews (Barkia et al., 2019).

Bioavailability of EPA from whole-cell Nannochloropsis is an area of active research; we have noted the current limitations of the evidence. Where we describe our own cultivation method and heavy metal comparisons, this reflects our actual production and formulation process.

Last reviewed: March 2026. Next review due: March 2027.