Marine microbial steroids: structural diversity, biosynthetic implications, and functional relevance

Marine microbial steroids represent a chemically diverse group of natural products arising from sterol metabolism and extensive scaffold tailoring. This critical narrative review synthesises steroids reported from marine fungi, microalgae, bacteria and actinomycetes, with emphasis on source-dependent structural diversification, stereochemical organisation, biosynthetic implications and biological relevance. Fungal metabolites are dominated by oxygenated, epidioxy, lactonised, rearranged and hybrid steroids, whereas microalgae exhibit distinctive sterol-profile diversification and bacteria and actinomycetes contribute transformation products, catabolites and nitrogen-containing derivatives. Across these groups, structural variation is concentrated at oxygenated centres, side chains, ring junctions and hybrid attachment sites. Reported activities include cytotoxic, anti-inflammatory, anti-infective, enzyme-inhibitory and tissue-repair effects, although mechanistic and in vivo validation remains limited. Future progress will depend on integrating genome mining, isotope tracing, enzymatic validation, metabolomics and matched-pair stereochemical studies to connect structure, biosynthesis and function.