A review of the current status of Kappaphycus alvarezii-based biostimulants in sustainable agriculture

The indiscriminate or non-strategic use of chemical fertilizers in agriculture can potentially have adverse effects on natural ecosystems due to their harmful impacts. As a cleaner, more sustainable supplementary treatment to minimize the need for synthetic nutrient inputs, seaweed-based biostimulants (phyco[bio]stimulants) are being widely studied to increase agricultural productivity for the burgeoning global population. Kappaphycus alvarezii, a fast-growing, tropical red alga widely cultivated for its hydrocolloid content, is a promising source of bioactive compounds that improve plant growth, including imparted tolerance to some levels of abiotic and biotic stressors in harsh environmental conditions. The alga also contains protein, carbohydrates, fatty acids, fibres, algal hormones, polyphenols, and various macro- and microelements. The polysaccharide κ-carrageenan, a major constituent of this particular seaweed, also has known plant growth-stimulating effects. Thus, various K. alvarezii-based biostimulants are used in agriculture, including liquid extracts and solid formulations which are applied as: drenches, foliarly and seed coatings, or seed soaking. These groups of biostimulants are reported to increase growth, yield, nutrient uptake, and biotic and abiotic resilience, depending on the type of extraction method, concentration of the applied extract, and also timing of the application within the treated plant’s growth cycle. This review summarizes various K. alvarezii-based products used in agriculture for plant biostimulation, their extraction procedures, application modes, and proposed mechanisms of action. We reviewed the scientific data supporting the use of Kappaphycus extracts as biostimulants to enhance crop productivity and quality. Additionally, the beneficial impacts of using Kappaphycus extracts on soil health and their potential as a sustainable, adaptive strategy for addressing climate variability is discussed. Several research gaps have been identified to understand the current limitations of the existing knowledge base, and a way forward has been presented to prioritize areas for further investigation.