Biostimulants from Marine Algae

Seaweeds have been used since antiquity, either directly or in composted form, to improve soil. This continues today, with seaweeds deliberately applied in products such as biostimulants or biofertilizers. Seaweeds are a large and diverse group of plants. Some consist of just a single cell, while others can grow very long. All seaweeds are capable of photosynthesis. For use as biostimulants or biofertilizers, brown algae (Ascophyllum nodosum) are primarily used worldwide.

At least one of the following effects on the plant or rhizosphere should occur when biostimulants are applied (Yakhin et al., 2017):

• Nutrient use efficiency (availability, uptake)
• Tolerance to abiotic stress
• Quality traits (number of flowers, root length, density)
• Availability of bound nutrients in the soil or rhizosphere

Ascophyllum nodosum, also known as knotted wrack, is a hardy brown alga. Brown algae mainly inhabit the North Sea coasts of Europe and the northern Atlantic coasts (from Norway to Canada) and Greenland. This alga is adapted to extreme conditions such as strong temperature fluctuations. Its remarkable ability to survive in this environment results from a high concentration of unique bioactive substances. The plant is leathery, branched, and can reach up to one meter in length, often with characteristic, oval-shaped swellings.

The habitat of this seaweed is subject to tides. Brown algae are alternately underwater and exposed to air. They are rich in nutrients, natural growth hormones, and can fix atmospheric nitrogen, making them an excellent source of organic matter for soil improvement. Additional macro- or micronutrients may be present, such as potassium, phosphorus, and trace elements like iron and manganese. Due to high concentrations of cytokinins and gibberellins, it directly stimulates vegetative growth, flowering, and fruit set. Auxins are more abundant in Ecklonia maxima (kelp), which directly stimulate root growth. In contrast, Ascophyllum nodosum contains high levels of mannitol, betaine, vitamins, alginic acid, and amino acids.

When using brown algae as a biofertilizer, you can improve soil fertility and root growth and increase crop yields. In addition, brown algae can also act as a biostimulant by promoting vegetative plant growth, vitality, flowering, and fruit set through improved nutrient uptake. Brown algae enhance plant resistance and tolerance to abiotic stress (e.g., extreme temperatures, drought, excess moisture, frost). Their decomposition in the soil leads to better swelling and crumb formation of the soil. An increase in cation exchange capacity is also observed.