Phlorotannins and Fucans from Phaeophyceae as Phaeophyceae Agrochemical Leads: Structural Diversity, Molecular Mechanisms, and Delivery Systems

The increasing demand for sustainable agricultural practices has intensified the search for biobased alternatives to synthetic agrochemicals. In this context, marine macroalgae, particularly brown algae (Phaeophyceae), represent a largely underexplored reservoir of structurally diverse secondary metabolites with significant agrochemical potential. Among these, phlorotannins and sulfated polysaccharides such as fucoidans stand out because of their multifunctional biological activities. These compounds exhibit direct effects against phytopathogens and agricultural pests through mechanisms such as enzyme inhibition, disruption of cellular and midgut homeostasis, and interference with key metabolic pathways while also acting as elicitors of plant defense by activating systemic resistance responses. This review critically examines the structural diversity of phlorotannins and fucans from Phaeophyceae, correlating their molecular features with their reported biological activities and modes of action relevant to crop protection. Particular emphasis is placed on the chemical vulnerability of these metabolites under field conditions, where photodegradation, oxidative stress, and limited bioavailability restrict their practical application. Advances in green extraction technologies—such as ultrasound-assisted, microwave-assisted, and supercritical fluid extraction—are discussed as essential tools for preserving structural integrity and bioactivity. In parallel, state-of-the-art delivery systems, including nanoencapsulation using biopolymers such as alginate and chitosan, are highlighted as effective strategies to enhance stability, controlled release, and environmental persistence. Finally, the review addresses critical challenges related to the extraction of standardization, formulation scalability, and regulatory frameworks that currently limit the commercial adoption of marine-derived agrochemicals. By integrating metabolic diversity, green chemistry, and advanced formulation technologies, Phaeophyceae-derived metabolites have emerged as promising leads for the development of innovative, low-impact agrochemicals that are compatible with integrated best management strategies and the principles of blue chemistry.