Biological effects of selenium on Hypnum plumaeforme and the underlying mechanisms mediated by physiology and microbiome

Bryophytes provide vital ecosystem services, yet the biological effects of selenium (Se) on them remain largely unknown. This study simulated ecological scenarios with different selenite exposure levels to investigate the response patterns of growth, physiology, and symbiotic bacterial communities in Hypnum plumaeforme. Overall, its inorganic, organic, and total Se content increased with increasing selenite exposure levels. At 2 mg/L, H. plumaeforme accumulated more biomass, carotenoids and selenomethionine (SeMet), but less lipid and phosphorus (P). Enhanced methylselenocysteine (MeSeCys) biosynthesis facilitated its Se detoxification. At 4 mg/L, H. plumaeforme had more photosynthetic pigments, carbohydrates and protein, but fewer chloroplasts and less P. H₂O₂ accumulation did not result in elevated malondialdehyde (MDA) content and growth inhibition, which was attributed to the activation of superoxide dismutase (SOD), catalase (CAT) and glutathione peroxidase (GPX), as well as increased SeMet biosynthesis for Se detoxification. The enrichment of Variovorax and TM7a might support its starch accumulation and Se tolerance. At 8 mg/L, H. plumaeforme had more photosynthetic pigments and protein, but fewer chloroplasts and less P. Excessive SeMet synthesis induced Se toxicity. Although SOD, CAT and GPX were activated, H₂O₂ and MDA accumulation occurred in this bryophyte, resulting in reduced biomass and impaired nitrogen (N) uptake. The enrichment of bacterial taxa, including Alphaproteobacteria, might facilitate its antioxidant defense. This study identifies the growth-promoting, neutral, and inhibitory effects of Se on H. plumaeforme, and illustrates the underlying physiological and microbial regulatory mechanisms. These findings provide important theoretical support for bryophyte conservation and Se ecological risk assessment.