The use of marine algae in agriculture is one of the oldest practices since many coastal peoples have always harvested algae to use as fertilizer; livestock feed or even as food for their own consumption. The first traces of the use of seaweed by humans date back to 2700 JC. The Chinese and Japanese used them for human and animal consumption. It is also cited that the British and the Scots had used algae as organic soil amendments since the 16th century and a little later in France (Brittany) where this amendment took the name of seaweed.
The search for biopesticides and biofertilizers to replace pesticides and chemical fertilizers is a significant trend in the evolution of agriculture. Thus, in the interest of sustainable, less expensive and environmentally friendly agriculture, the use of seaweed extracts could have its place in this type of reasoning. Indeed, research carried out in this direction demonstrates that the contributions of marine algae extracts, at the recommended doses and periods, promote increased plant growth, good resistance to diseases and better quality of the products harvested.
These new “products from the seabed” are not yet real substitutes for conventional phytosanitary products but are nevertheless good complements to limit their use. Products based on marine algae extracts can therefore be part of a type of agriculture which combines both conventional chemical control and biological control while respecting as much as possible the natural flora and fauna as well as human health. They therefore represent an important asset for the evolution towards a « sustainable agriculture ».
Initially used whole, in the form of an organic amendment, algae are currently used more and more in the form of liquid extracts. The first foliar sprays of algae extracts on plants took place around the 1950s, a time when the concept of plant nutrition was still based on the principle that the roots were the organs of absorption of mineral elements from the soil and the leaves those of carbon assimilation. Although foliar nutrition was already used at that time to correct trace element deficiencies, it only developed as part of general plant fertilization around the 1960s, promoting the sale of algae extracts. Since that time, numerous trials have been undertaken to demonstrate the effectiveness of these extracts. However, many articles were written with more commercial than scientific interest in mind, and should therefore be viewed with caution.
Various phyto-active (or bio-stimulant) effects of these marine algae extracts have, however, been demonstrated despite sometimes irregular results. Among the beneficial effects reported by the majority of studies carried out in this direction we find: improvement in the germination rate of seeds, increased yields, increased resistance to cold and other abiotic stresses, resistance to certain diseases, the intensification of the absorption of mineral elements from the soil or even the improvement of the shelf life of fruits.
At present, the mechanisms of action of these extracts are not satisfactorily known. Whatever their origin, or their method of preparation, these extracts are very complex and contain numerous mineral elements and organic constituents. Today, it is agreed that seaweed extracts contain five types of particularly interesting components:
Amino acids (Betaines): molecules of great importance for plant nutrition. Betaines, derived from amino acids, are osmo-regulatory molecules which have a beneficial effect in plants and are essential for their adaptation to thermal, water and saline stress.
Polysaccharides and Oligosaccharides: marine algae also contain reserve saccharides. These molecules have significant effects on the plant but also on the rhizosphere around the roots. Thus, Alginates have a great power of retention of water and mineral elements at the roots in the soil. Mannitol and alginates have chelating properties towards trace elements. Laminarins have an important role in stimulating the natural immune system of plants while Fucoidans have significant anti-viral properties.
Phytohormones: several phytohormones have been identified in marine algae extracts. Thus, we find the presence of auxins and cytokynins (growth hormones responsible for cell division (cytokinins) and cell elongation (auxins)) in most extracts. The presence of gibberellins has also been noted in fresh products, but their activity drops drastically to negligible levels following extraction and packaging methods.
Anti-oxidants (Vitamins, Polyphenols, Fucoxanthins and Chlorophyll): heat-sensitive molecules which have great antioxidant activity by neutralizing the free radicals produced by plants during times of stress (attack by a pathogen, water stress, high salinity, high heat, intense cold… etc.).
Trace elements and other mineral elements: analysis of the composition of fresh algae shows that they contain between 50 and 60 mineral elements essential to plants but in low doses.
Although the mode of action of marine algae extracts is not entirely elucidated, the effects observed following the application of these products come mainly from phytohormones, anti-oxidants and polysaccharides. Phytohormones present in small quantities (mainly auxins and cytokinins) act on the growth and development of organs (cell division and elongation). Antioxidants act as osmo-regulators against various biotic and abiotic stresses while polysaccharides are involved in stimulating the natural defense reactions of plants (contribution of elicitors: molecules responsible for the induction of resistance mechanisms of the plant in relation to a pathogen). The presence of mannitol and alginic acid would also contribute to the retention, absorption and translocation of mineral elements due to their important complexing (chelating) property. As for the mineral elements present in algae extracts, they only contribute an insignificant proportion to the needs of the treated plant, given the low applied dose of this type of product.
Around ten species of marine algae are used across different regions of the world for the manufacture of biofertilizers. The main species of agronomic interest are:
Ascophyllum nodosum: brown algae in the shape of «knotted rope» which can reach more than 1.50 m in length. It is common along the rocky coasts of the North Atlantic. It is the main species of algae collected for the production of liquid extracts used in the biofertilizer industry.
Fucus vesiculosus: brown algae which has vesicles filled with gas to allow it good buoyancy. It is also common along the rocky coasts of the North Atlantic and the Pacific. It is also one of the main species of algae collected for its agronomic interest.
Laminaria digitata: Brown perennial algae in the shape of digitate thalli. It measures 1 to 1.5m in length and can reach 4m. It is answered in the North Atlantic, South Atlantic, North Sea and English Channel. It is also one of the main species of algae collected for the production of liquid extracts used in the biofertilizer industry.
Sargassum muticom (Sargassum): Brown algae in the form of fronds can reach several meters long and attached to the bottom by a so-called “coriaceous” thallus. Some Sargassum sometimes live exclusively floating in the form of thalli. It is an invasive species which is found in the majority of seas and oceans.
Ecklonia maxima (Kelp): Brown algae also called sea bamboo, it is a species which is very common on the African coasts of the South Atlantic (Angola and South Africa). It is also one of the main species of algae collected for the production of liquid extracts used in the biofertilizer industry.
Ulva lactuca: bright green algae, with lobed leaves with serrated edges that resemble a lettuce leaf. It is characterized by an elastic, fine and very flexible texture, allowing it not to tear at the slightest touch. This algae is of growing interest in the production of organic fertilizers because of its high nitrogen and phosphorus content.