Microbiology and Biotechnology Research: An Overview Vol. 8

Fish processing generates large volumes of under-utilised organic residues, including viscera, heads, frames, skin, scales and process waters, and their conversion into agricultural inputs has become increasingly attractive within circular bioeconomy strategies. At the same time, plant growth-promoting rhizobacteria (PGPR) are central to low-input crop management because they stimulate root development, improve nutrient capture, moderate stress ethylene, and contribute to rhizosphere resilience. Recent studies indicate that commercial fish emulsion can serve as a nutrient base for selected rhizobacteria and actinomycetes, thereby enhancing plant growth and increasing endogenous plant growth regulator levels in planta. The intersection between these two domains remains insufficiently synthesised. This review examines how fish waste and fish-waste-derived products can influence the phytostimulation potential of plant growth-promoting rhizobacteria, with particular attention to fish emulsions, silages, hydrolysates and microbially transformed liquid biofertilisers. A narrative review approach was adopted to integrate mechanistic microbiology, plant physiology, soil fertility and bioresource valorisation studies published between 1996 and April 2026. The evidence indicates that fish waste is not merely a nutrient source. Depending on processing method and dose, it can function as a rhizosphere substrate, a carrier matrix, and a source of peptides, amino acids and minerals that modify microbial competition, root exudation dynamics, nutrient mineralisation and stress physiology. The most direct evidence shows that fish emulsion can act as a food base that amplifies the plant-growth effects of selected rhizobacteria, while a broader body of work on fish hydrolysates demonstrates improved root growth, chlorophyll status, nutrient acquisition and temperature or salinity tolerance in crops. Nevertheless, rigorous studies directly measuring inoculant persistence, rhizosphere community shifts and field-scale consistency remain limited. The review argues that fish-waste-derived inputs have genuine promise for strengthening PGPR-mediated phytostimulation, but that their success depends on controlled hydrolysis, composition standardisation, compatibility with inoculant strains, and careful management of salinity, phytotoxicity, odour and shelf life. Priority research needs include formulation science, multi-omics validation, soil-specific dose optimisation and long-term agronomic trials under realistic farming conditions.