Iron limitation responses of the brown alga Saccharina japonica var. religiosa revealed by transcriptome analysis

Abstract Iron (Fe) deficiency is one of the factors that limit primary production in marine ecosystems, leading to seaweed bed depletion. However, few studies have examined Fe limitation responses in macroalgae, particularly using molecular biological approaches. In this study, Saccharina japonica var. religiosa cultivated under Fe deficiency exhibited reduced growth, Fe content, and chlorophyll a content. To investigate transcriptional responses to Fe limitation, we optimised a method for total RNA extraction from polysaccharide-rich macroalgae. Transcriptome analysis revealed 593 upregulated and 161 downregulated genes under Fe limitation. Functional analysis of these genes highlighted several responses to Fe limitation, including mobilisation and utilisation of stored Fe, induction of oxidative stress, activation of antioxidant reactions and signal transduction, carbohydrate and nitrogen metabolism adjustment. Notably, genes for the vanadium-dependent non-haem peroxidases, iodoperoxidase and bromoperoxidase, were expressed at higher levels under Fe limitation, indicating their key roles as antioxidant enzymes. Genes encoding mannuronan C-5 epimerase also showed higher expression, suggesting modification of alginate, a major cell wall polysaccharide. This study is the first to analyse macroalgal gene expression under Fe limitation and provides foundational insights for understanding Fe status in macroalgae.