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Seaweed Extracts: What Makes Them Work?

In many coastal regions, raw seaweed has been used for millennia to improve the productivity of farming systems. Then, in the latter half of the 20th century the use of liquid seaweed extracts on crops became widespread globally. Farmers using these extracts repeatedly reported positive results on crop yield and quality. As these results could not simply be correlated to the seaweed extract’s elemental nutrient content, it took time for a scientific explanation for these effects to catch up. As we have gained a clearer understanding of algal biochemistry and the associated modes of action, we no longer considered seaweed extracts to be fertilisers, and instead classify them as one of the key product classes in the rapidly developing sector of biostimulants.

Below I have linked the positive effects observed when using seaweed extracts to the latest scientific knowledge of the compounds present in seaweeds to further our understanding of how and when to use seaweed extract products to maximize their efficacy.

Most of the seaweed extracts used in agriculture as biostimulants are extracted from species of brown algae (Phaeophyceae); most notably the species Ascophyllum nodosum (knotted wrack). As Ascophyllum nodosum grows in the intertidal zone of the Northern Hemisphere it has been proposed that the stressful conditions of repeated exposure to desiccation, ultraviolet light, and temperature fluctuations experienced by the seaweed in this habitat, combined with a slower growth rate, lead to the high concentrations of bioactive compounds present in Ascophyllum. In comparison, tropical species, or the kelps that grow further out to sea, produce lower concentrations of these bioactive compounds and higher levels of cellulose; a compound that is neither novel or able to stimulate plant growth.

From a review of the marketing material for microalgae-based extracts, it would appear the only advantage being claimed over brown seaweed extracts is a lower sodium content, due to their cultivation in fresh water. However, microalgae cultivation is a rapidly developing area of phycology and so new insights and products are expected in the coming years.

Plant hormones or analogues

It is known that algae contain plant hormones and when seaweed extracts are applied to plants the crop exhibits similar responses to when purified/synthetic plant hormones are applied. However, it is questionable whether these responses are actually due to plant hormones in seaweed extract. This is because a) many plant hormones are known to readily break down in the presence of light, heat, and oxidizing conditions (all of which an extract will experience during its time from extraction to delivery in the field), and b) when people have analyzed extracts for known plant hormones they have found comparatively low concentrations.

Instead, the current accepted paradigm is that seaweed extracts contain plant hormone analogues. These analogues possess different chemical structures to the corresponding plant hormone, but induce the same/similar response when applied to the plant. As a result, positive growth responses similar to the action of cytokinin and/or auxin are commonly cited.

Algal polysaccharides, monosaccharides and sugar-alcohols

A range of polysaccharides are unique to algae and make up the bulk of seaweed biomass. This is illustrated in the following microscope pictures where various polysaccharides can be seen fluorescing (Mike Asquith, 2017).

The principal component of green algae cell wall is ulvans, in red seaweed cell walls it is agarans and carrageenans, and in brown seaweeds alginates and fucans are found in the cell walls and laminarin as a storage polysaccharide. These polysaccharides, along with the oligosaccharides and monomers that result from their depolymerisation during any extraction process, have been found to induce positive responses when applied to crops.

One positive response of plants to treatment with algal polysaccharides repeatedly reported in academic studies is the induction of plant defense mechanisms. As these compounds are unique to algae, plants are known to detect the presence of these polysaccharides as “non-self”. Detection of algal polysaccharides at the cell membrane has been demonstrated to activate the jasmonic acid, salicylic acid, and ethylene signalling pathways.

These hormonal pathways stimulate the accumulation of a range of chemical and physical protective compounds. The activation of plant defenses by seaweed extracts can thus be used to pre-emptively protect against abiotic and biotic stress in crops. However, it should also be noted that there is no current evidence for a specific receptor in plants for these compounds, so they most likely work by stimulating a receptor for another polysaccharide microbe-associated molecular pattern (MAMP) such as chitin. Therefore, if induction of plant defenses is your primary objective, you should consider products that act directly on key cell membrane receptors as these should prove more efficacious in the field. There may also be regulatory reasons in your region that might dissuade you from using seaweed extracts to prime against biotic stresses.

In addition to plant defense priming, there are a number of other modes of action for algal polysaccharides based on their specific biochemical properties;

Alginates (alginic acid)

Alginates are the polysaccharide that give brown seaweeds their gelatinous nature. They are present in very high levels in all brown seaweed. Alginates are extracted from brown seaweed for medical use (e.g., Gaviscon). In agriculture, the gel-forming property of alginates allows for the formation of microcolloids over the surface of plants, that are thought to act as a protective film. Alginates will also act as a feed and nutrient source for beneficial microbes. This includes antagonists that exclude pathogens from colonizing the leaf surface, and those that exude plant hormones.


The sugar alcohol mannitol is found in high concentrations in brown seaweeds. Being water soluble, many seaweed extracts contain good levels of mannitol. Mannitol is a multifunctional compound and has three main areas of activity:

  • Antioxidant. Mannitol is able to “mop up” the reactive oxygen species (ROS) that create the free radicals that damage plant tissues. ROS are produced in high levels when plants are under abiotic stresses and thus a seaweed extract containing mannitol would be a good choice in stressful growing conditions.
  • Mannitol is able to form a complex with boron atoms Boron complexing. Mannitol is able to form a complex with boron atoms, effectively chelating an important plant nutrient.
  • Signalling in fungal attack.Pathogenic fungi also use mannitol to quench ROS when attacking a plant. The interplay between mannitol and the plant’s enzymes that degrade mannitol is an important interaction in the fight between a crop plant and pathogen that determines if a disease takes hold (reference). As a result, I recommend that seaweed extracts should not be applied if a crop is already suffering from a fungal disease.


Brown seaweeds also contain significant levels of sulphonated polysaccharides known either as fucans or fucoidans. Unfortunately, there is a paucity of studies on how these function when applied to plants, however, their use and effects on animal and human is far better studied.

Vitamins and minerals

Iodine It is well known that seaweed is a good source of iodine, in the form of iodide. As is the case for fucans, the function of iodine in human/animal health is far better understood than it is for how it functions in plants. It has been proposed that it could influence the interaction with beneficial microbes or help with the biofortification of crops destined for feed/food but reports are limited.

Vitamin B12 (cobalamin)

If you study vitamin B12’s chemical structure, you will see it is a natural chelation of cobalt.

Vitamin B12 is well known for its importance in human health. However, for plants it can also prove useful. If you study its chemical structure (see image), you will see it is a natural chelation of cobalt. Cobalt is thought to be essential for the growth of a number of crop plants, and improve the growth of others. Furthermore, cobalt is essential for the enzymes involved in nitrogen fixation by bacteria. Vitamin B12 is not synthesized in plants or animals, but seaweeds contain high amounts and seaweed extracts are a good source of this nutrient.

However, vitamin B12 is highly sensitive to alkali conditions (reference), so if cobalt is of interest consider using a cold-pressed seaweed extract. Interestingly, it is thought that the vitamin B12 found in seaweed is not synthesized by the algae themselves but by the bacteria growing on their surfaces in their natural intertidal (littoral) habitat (reference).

What you won’t find in seaweed extracts

As well as all the active ingredients in seaweed, there are claims made that they are also a good source of other compounds. Quite often you will see claims made that seaweed extracts are a good source of amino acids, but if seaweeds were a good source of amino acids, we would eat them for their protein content, but this is not true. As a result seaweed extracts only contain very low levels of amino acids naturally and if you are looking for an amino acid biostimulants there are far better alternatives available.

Some seaweed extracts are formulated to contain high levels of plant macronutrients. However, seaweed extracts naturally contain very low levels of all macronutrients, with typical values of less than 1% w/v. Those seaweed extracts sold with much higher NPK values will have had extra nutrients added, or in the case of alkali extracts, there will be potassium present due to the use of potassium hydroxide required for the chemical extraction. Therefore, it is recommended that you do not choose a seaweed extract purely on its nutritional content.

In conclusion, as we gain more insights into the mechanisms by which seaweed extracts function in crops I hope we will start to see the industry move away from the “use to improve plant growth” marketing approach toward a more scientific and informed strategy. If you would like to know more about seaweed extracts, how to use them, or the difference between extraction methods please get in touch.

I first published this article in Agribusiness Global. For more of my articles on biostimulants, please click here.

Dr Russell Sharp

Eutrema Ltd

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