How are natural and synthetic astaxanthin made?

Natural astaxanthin is made by living organisms, such as the green microalga Haematococcus pluvialis, which builds it up when grown under stress, the red yeast Phaffia rhodozyma (also called Xanthophyllomyces dendrorhous) and some bacteria. Synthetic astaxanthin is built from smaller chemical building blocks, classically by a reaction called the Wittig reaction. The basic molecule is the same, C40H52O4. What differs is how it is made, the mix of molecular shapes (algae make the (3S,3'S) form, yeast the (3R,3'R) form, and synthesis a 1:2:1 mixture of three forms), and whether the astaxanthin is free or bound to fatty acids.

In short:

• From algae: Haematococcus is grown green, then stressed to turn red, harvested, dried, broken open and extracted
• From yeast and bacteria: grown in culture; Phaffia yeast makes the (3R,3'R) form, not bound to fatty acids
• Synthetic: joined together from C15 and C10 building blocks; gives a mixture of three shapes

This article compares only how each type is made and how the molecules differ. For what astaxanthin is and where it comes from in nature, see What is astaxanthin, and where does it come from?

What do "natural" and "synthetic" astaxanthin mean?

The same molecule, made in different ways

Astaxanthin is a red carotenoid with the formula C40H52O4 and a molecular weight of about 596.8, according to PubChem, the chemistry database of the US National Library of Medicine. "Natural" here means made by a living organism and then extracted. "Synthetic" means built from smaller molecules by chemical reactions.

Where each type comes from

The 2014 review by Ambati and colleagues states that commercial astaxanthin comes mainly from Phaffia yeast, Haematococcus algae and chemical synthesis.

How is astaxanthin made from Haematococcus algae?

Haematococcus pluvialis builds up astaxanthin only when it is under stress, so production runs in two stages.

Two stages: green, then red

A 2016 review by Shah and colleagues at Peking University describes a first "green stage", in which the alga is grown under favourable conditions such as low light and plenty of nitrogen until the cells are dense. In the second "red stage", the cells are put under stress, such as high light and a lack of nitrogen. The 2021 review by Stachowiak and Szulc at Poznań University of Life Sciences in Poland lists further triggers, including a lack of phosphorus and high salt, and notes that the cells then form thick-walled resting cells (cysts) that hold the pigment.

Open ponds or closed photobioreactors

The alga is grown in open raceway ponds or in closed photobioreactors, such as tubular, bubble column and airlift designs (Shah and colleagues). Stachowiak and Szulc note that open ponds risk contamination with other algae, bacteria and fungi, which is why high-capacity photobioreactors are used.

Research in Thailand

Researchers at Chulalongkorn University in Bangkok have studied this process. A 2007 study grew green Haematococcus cells in an airlift bioreactor and found that it gave better growth than a bubble column under the same conditions, with a harvest possible every four days in semi-continuous culture. A 2015 study then induced the red stage in flat-panel airlift photobioreactors of 17 to 90 litres. Indoors, astaxanthin reached 5.34% of the cell weight; outdoors, results were slightly lower because strong sunlight inhibited the cells, and the best result, 4.47%, came from a reactor covered with one layer of shading net.

From red cells to extract

Stachowiak and Szulc explain that supercritical CO2 needs no solvent removal afterwards, because the CO2 returns to a gas at normal temperature and pressure.

How is astaxanthin made from yeast and bacteria?

Phaffia yeast

Stachowiak and Szulc describe Xanthophyllomyces dendrorhous (Phaffia rhodozyma) as a red yeast that makes unesterified astaxanthin, mainly in the (3R,3'R) form, kept in droplets of fat inside the cell. Wild strains contain only 0.01 to 0.03% astaxanthin in their dry matter, so strains making 5 to 10 times more, obtained for example by mutagenesis, are used for industrial production. A 2007 study from Anhalt University of Applied Sciences in Germany confirmed that the yeast makes astaxanthin mainly in the R,R form, while a green microalga (Scenedesmus) made mainly the S,S form.

Bacteria

Ambati and colleagues list the bacterium Paracoccus carotinifaciens at 2.2% astaxanthin by dry weight. Stachowiak and Szulc add several other bacteria that make it, including Brevibacterium and Agrobacterium aurantiacum.

How is synthetic astaxanthin made?

The Wittig reaction

Stachowiak and Szulc describe the oldest route as a Wittig reaction joining two C15 building blocks (phosphonium salts) to a symmetrical C10 central building block (a dialdehyde), giving the 40-carbon astaxanthin molecule. Shah and colleagues describe the same reaction between an asta-C15-triarylphosphonium salt and a C10-dialdehyde.

Other routes

The Stachowiak review lists other strategies: adding hydroxyl groups to canthaxanthin (a related carotenoid), a C10 + C10 + C10 synthesis, and converting lutein extracted from marigold flowers into astaxanthin.

What is synthetic astaxanthin used for?

Stachowiak and Szulc report that synthetic preparations make up over 95% of the feed market. In May 1995 the US FDA permitted synthetic astaxanthin as a pigment in animal and fish feeds, and in the European Union it is registered as a feed additive for salmon, trout, shellfish and ornamental fish. It gives farmed fish and shrimp their pink to red colour, as explained in Which foods contain astaxanthin, and how much?

How do the molecules differ chemically?

Three shapes of the same molecule

Astaxanthin has two carbon atoms, at positions 3 and 3', that each carry a hydroxyl group, and each can point one of two ways. Stachowiak and Szulc explain that this gives three configurational isomers: (3R,3'R) and (3S,3'S), which are mirror images of each other, and the meso form (3R,3'S). All three occur in nature, in different shares depending on the source.

The algae and yeast rows are from the table in Stachowiak and Szulc (2021). The synthetic row shows the 1:2:1 ratio of (3R,3'R), (3R,3'S) and (3S,3'S) that both Stachowiak and Szulc and Shah and colleagues report, written as shares.

Free or bound to fatty acids

Each end ring of astaxanthin has a hydroxyl group that can join to a fatty acid. One fatty acid makes a monoester and two make a diester. Shah and colleagues report that mature red Haematococcus cells contain about 70% monoesters, 25% diesters and only 5% free astaxanthin, usually esters of palmitic, oleic or linoleic acid.

Straight and bent forms (trans and cis)

Besides the three shapes above, the long chain in the middle of the molecule can be straight (all-trans) or bent at one double bond (cis forms such as 13-cis). A 2015 study from Ocean University of China found that when shrimp were heated, the all-trans form decreased and the 13-cis form increased, so cooking can also change the form.

How can scientists tell which source astaxanthin came from?

Because each source has its own mix of shapes, the mix works like a fingerprint.

The FDA method for salmon

In a 1997 paper, scientists at the US FDA described a liquid chromatography method that separates the shapes of astaxanthin. The shape pattern in wild Atlantic and wild Pacific salmon was similar, but significantly different from that in farmed salmon fed synthetic astaxanthin. The authors note the method can also identify astaxanthin from Phaffia yeast and Haematococcus algae.

A check with fatty acids

A 2009 study from the United States compared a simpler fatty acid test with the FDA's astaxanthin shape method in 30 salmon samples and found 100% agreement between the two in telling farmed from wild fish.

Is there a safety limit for astaxanthin?

Astaxanthin is not a nutrient with a Thai Reference Daily Intake (Thai RDI), so there is no recommended daily amount in Thailand.

EFSA's ADI

The European Food Safety Authority (EFSA) explains that its feed additives panel set an acceptable daily intake (ADI) of 0.2 mg per kg of body weight in 2019, during its assessment of a feed additive form of astaxanthin (astaxanthin dimethyldisuccinate). In 2020 its nutrition panel applied the same ADI when assessing astaxanthin from Haematococcus algae, counting astaxanthin from fish, shellfish and supplements together.

When should I talk to a doctor or pharmacist?

If you are pregnant or breastfeeding, take a regular medicine, or have a fish or shellfish allergy, talk to a doctor or pharmacist before taking dietary supplements.

Dietary supplements are not intended to prevent, treat or cure any disease.