Spirulina or Chlorella: What Is the Difference

Spirulina and chlorella are sold as interchangeable “green superfoods,” although one of them is a bacterium and the other a true alga. The editorial team examined how they differ in structure, composition and digestibility, which of the marketing promises are backed by studies, and what to watch out for from a safety standpoint.

Two different organisms under one word, “algae”

Spirulina and chlorella are often called “green superfoods” and sold side by side as if they were close relatives. From a biological standpoint this is not so. Spirulina is a cyanobacterium of the genusArthrospira, that is, a prokaryote whose cells have no defined nucleus. Chlorella is a single-celled green alga, a true eukaryotic organism with a nucleus and chloroplasts, evolutionarily closer to plants.

Spirulina forms long spiral-shaped filaments that were traditionally harvested from alkaline lakes in Mexico and Africa. Chlorella has the appearance of tiny spherical cells and is cultivated mainly in ponds and bioreactors, primarily in Asia. Both organisms are capable of photosynthesis but use a different set of pigments.

This difference is not purely academic. The structure of the cell determines how the product is digested, which nutrients predominate in it and how it must be processed before sale. That is why in the description of chlorella you will find the phrase “with broken cell wall,” while for spirulina it is not needed.

One should also distinguish spirulina from other cyanobacteria, for example AFA (Aphanizomenon flos-aquae), which is harvested from natural bodies of water. Safety questions for them are framed differently, and we will return to this topic later.

Cell wall and digestibility

The cell wall of spirulina is comparatively thin and built mainly of murein and polysaccharides characteristic of bacteria. Human digestive enzymes break it down without particular difficulty, so the proteins and pigments of spirulina are absorbed fairly well even after simple drying.

With chlorella the situation is different: its cell wall is dense and multilayered. Without special processing, a significant part of the cell contents passes through the intestine undigested. That is why manufacturers use mechanical breaking of the wall, for example grinding or pressure treatment. The quality of this processing directly affects the value of the product.

For the buyer this means a simple rule: with chlorella you should look for a note about a broken cell wall, while with spirulina you can disregard this parameter. The absence of such a note on chlorella is a reason to doubt how many useful substances will actually be absorbed.

The dense wall of chlorella can bind certain substances in the intestine, so it is sometimes marketed as a means of “detoxifying” from heavy metals. The editorial team points out: there is no convincing clinical evidence of such an effect in humans, and these promises should be treated critically.

Comparison of composition

In protein content both products are among the richest food sources: dry spirulina is usually more than half its mass, chlorella a comparable amount, depending on the strain and growing conditions. At the same time, a daily serving of these supplements is measured in a few grams, so as a protein source for an athlete they cannot replace ordinary food or protein powder.

Spirulina stands out for its content of phycocyanin — a blue pigment-protein with which its antioxidant properties are associated. It also contains gamma-linolenic acid, beta-carotene and relatively a lot of iron. Chlorella is richer in chlorophyll and also contains lutein and other carotenoids.

The most widespread myth concerns vitamin B12. Studies by Watanabe and colleagues (2014) showed that spirulina contains mainly pseudo-vitamin B12 — a form inactive for humans. In some samples of chlorella true cobalamin was found, but its content varies greatly (Bito et al., 2020). So vegans should not count on either of these algae as a reliable source of B12.

Figures for the mineral composition of both algae differ considerably across sources, because they depend on the growing environment. That is why the editorial team does not provide “exact” content tables: the real composition of a specific product should be checked against the manufacturer’s certificate of analysis.

What is known about effects on health and sport

The best-studied effect of spirulina is its influence on blood lipids. A meta-analysis by Serban and colleagues (2016) showed that taking spirulina was associated with reductions in total cholesterol, LDL and triglycerides and an increase in HDL. At the same time, the studies were small and heterogeneous, so this is an adjunct rather than a therapeutic agent.

In the sports context a small study by Kalafati and colleagues (2010) is often cited, in which spirulina prolonged time to fatigue during prolonged running and increased fat oxidation. However, the sample was small, and since then there have been no convincing replications in large studies.

For chlorella the evidence base is even more modest. The review by Merchant and Andre (2001) describes small clinical works in fibromyalgia, hypertension and ulcerative colitis with some positive signals, but the quality of these studies does not allow firm conclusions. A newer review by Bito and colleagues (2020) also stresses the need for high-quality trials.

• Spirulina:the most data on the lipid profile and antioxidant markers.
• Chlorella:some data on immune and metabolic markers, mostly from small studies.
• Both:no proven effect on strength, muscle mass or athletic performance.

Safety and quality of raw material

The United States Pharmacopeia assessed the safety of spirulina and assigned it to class A, that is, supplements without substantial safety concerns provided proper quality (Marles et al., 2011). The main risk is associated not with spirulina itself but with possible contamination of the raw material.

Cyanobacteria from natural bodies of water may contain microcystins — toxins that damage the liver. A study by Roy-Lachapelle and colleagues (2017) found cyanotoxins in some marketed algae supplements, with the most problems relating to products with AFA. Control of heavy metals is also important, since algae accumulate them from water.

Both algae contain phenylalanine, so they are contraindicated for people with phenylketonuria. Because of possible effects on the immune system, people with autoimmune diseases and those taking immunosuppressants should consult a doctor. Chlorella contains vitamin K, which is potentially important for patients taking warfarin.

At the start of intake, gastrointestinal complaints occur: bloating, a change in stool color, nausea. They usually pass if you start with a smaller serving. Pregnant women and children should take these supplements only after consulting a doctor.

Editorial conclusions

Spirulina and chlorella are two different organisms: a cyanobacterium and a green alga. They differ in cell structure, digestibility, pigments and nutrient profile. Chlorella requires breaking of the cell wall, spirulina does not.

Neither of them is a reliable source of vitamin B12 and neither can replace complete protein in an athlete’s diet. The best-documented effect is spirulina’s moderate influence on blood lipids; for chlorella the data are so far limited.

The main criterion for choice is the quality of the raw material: the presence of certificates for heavy metals and cyanotoxins matters more than marketing promises.

If you are choosing which of the algae to add to your diet, we advise reading “Spirulina vs Chlorella: what to choose and for whom,” as well as our materials on vitamin B12 for vegans and on sources of plant protein for athletes.

References

1. Belay A. The potential application of Spirulina (Arthrospira) as a nutritional and therapeutic supplement in health management. J Am Nutraceutical Assoc. 2002;5(2):27–48.
2. Watanabe F, Yabuta Y, Bito T, Teng F. Vitamin B12-containing plant food sources for vegetarians. Nutrients. 2014;6(5):1861–1873.
3. Bito T, Okumura E, Fujishima M, Watanabe F. Potential of Chlorella as a dietary supplement to promote human health. Nutrients. 2020;12(9):2524.
4. Marles RJ, Barrett ML, Barnes J, et al. United States Pharmacopeia safety evaluation of Spirulina. Crit Rev Food Sci Nutr. 2011;51(7):593–604.
5. Roy-Lachapelle A, Solliec M, Bouchard MF, Sauvé S. Detection of cyanotoxins in algae dietary supplements. Toxins. 2017;9(3):76.
6. Serban MC, Sahebkar A, Dragan S, et al. A systematic review and meta-analysis of the impact of Spirulina supplementation on plasma lipid concentrations. Clin Nutr. 2016;35(4):842–851.
7. Kalafati M, Jamurtas AZ, Nikolaidis MG, et al. Ergogenic and antioxidant effects of spirulina supplementation in humans. Med Sci Sports Exerc. 2010;42(1):142–151.
8. Merchant RE, Andre CA. A review of recent clinical trials of the nutritional supplement Chlorella pyrenoidosa in the treatment of fibromyalgia, hypertension, and ulcerative colitis. Altern Ther Health Med. 2001;7(3):79–91.