From Ocean Symbionts to Crop Allies: A Critical Appraisal of Seaweed-associated Microbiomes as Resources for Sustainable Agriculture

Abstract

Seaweeds sustain dense and metabolically distinctive microbial communities whose members produce morphogens, phytohormone analogues, siderophores, vitamins, polysaccharide-degrading enzymes and antimicrobial metabolites. These properties have prompted the proposition that the seaweed-associated microbiome constitutes an underexploited reservoir of biostimulant and biocontrol agents for terrestrial crop production. This review critically examines whether the available evidence supports that proposition, and identifies where the argument currently rests on mechanistic plausibility rather than demonstrated agronomic performance. Literature was identified through structured searching of open scholarly databases and indexes, supplemented by backward and forward citation searching, with critical appraisal focused on design adequacy, control structure, ecological realism and the correspondence between laboratory phenotypes and field outcomes. Three findings emerge. First, the mechanistic literature on seaweed–bacteria interactions is conceptually mature but taxonomically narrow, resting disproportionately on a small number of model systems, principally the green macroalgal genus Ulva, and on host functions such as thallus morphogenesis that have no direct terrestrial equivalent. Second, the applied literature consists predominantly of isolate-level in vitro screening and short-term pot experiments; replicated multi-season field trials using defined seaweed-derived inocula are essentially absent, and the most frequently cited agronomic evidence concerns seaweed extracts rather than seaweed-associated organisms, which conflates two distinct interventions. Third, the transition from a marine epibiotic niche to the rhizosphere imposes osmotic, nutritional and competitive filters that are rarely measured, so reported in vitro traits cannot be assumed to be expressed in soil. The most defensible near-term contribution of this resource lies in discovery of chemically novel metabolites and defence elicitors rather than in whole-organism inoculants. Progress will require host-range and colonisation studies under realistic soil conditions, standardised reporting of strain provenance, and field evaluation designed to detect modest effect sizes.