Seaweed Based Hydrogels: Extraction, Gelling Characteristics, and Applications in the Agriculture Sector

Due to expanding seaweed-based industries and the growing popularity of seaweed biorefineries, there is an increasing opportunity for high-value applications of seaweed-extracted polysaccharides. One of the most fascinating applications for these biopolymers is in the field of agriculture, where they have been used as fertilizers, hydrogel/granules for slow release of urea, herbicides, insecticides, and water reservoirs to tackle drought-like situations. Polysaccharides, such as alginate, agar/agarose, ulvan, carrageenan, etc., have been used as such for a long time in multiple applications; however, their use in the development of hydrogels and subsequent utilization to deal with agricultural challenges like drought mitigation, to reduce chemicals leaching from fertilizers, or to lessen the harmful impacts of pesticides and increase crop yields, has recently gained a significant amount of interest from scientists. In this review article, we thoroughly discuss the technique generally utilized to extract polysaccharides from seaweeds, the structure, the gelation mechanism, and agricultural applications of various hydrogels, namely, alginate, agar/agarose, ulvan, and carrageenan. The impact of various bioadditives and surface modification techniques on the properties of polysaccharide-based hydrogels, such as water absorption/retention tendency, variation in fertilizers/pesticide releasing capability, etc., are discussed. Finally, the challenges along with some future possibilities are also discussed.