Periphytic biofilm at saline soil-water interface captured more cadmium than at non-saline ones: process, mechanism, and implication

Soil salinity inhibits rice growth and enhances cadmium (Cd) mobility, threatening rice production and human health. Periphytic biofilm (PB), widely distributed at the soil-water interface of paddy filed, exhibits strong Cd capture capability. However, PB at saline soil-water interface (SPB) and its capability for Cd capture remain unclear. Herein, SPB from coastal mudflat-reclaimed paddy filed was sampled to evaluate its Cd captured process and mechanism by advanced technologies. Results showed that the process of Cd capture by SPB was a spontaneous physical adsorption process driven mainly by extracellular polymeric substances (EPS), which contributed over 90%. Among different EPS layer of SPB, the soluble EPS (S-EPS) accounting more than 50% for Cd adsorption. The maximum Cd adsorption capability of SPB was 619.7 mg/kg, which was 69% higher than PB grown on non-saline soil (NPB). The enhanced Cd adsorption capability of SPB was mainly attributed to the higher amount of EPS, which contained more CHON and CHONS compounds. Furthermore, non-invasive micro-test technique (NMT) analysis indicated ion exchange process occurring in SPB was 3 times higher than that in NPB. Finally, microbial metagenomic sequencing analysis showed that SPB possessed higher abundances of metal-tolerant taxa (e.g., Coleofasciculus chthonoplastes ) and enhanced expression of Cd-related (e.g., cmtR ) and EPS-related genes (e.g., lptG , mleP ). These findings expand the understanding of Cd biogeochemistry in saline wetland with PB, but highlight the potential of SPB for Cd pollution remediation in paddy fields. • Saline periphytic biofilm (SPB) enriched 69.0% more Cd than nonsaline biofilm (NPB). • EPS contributed over 90% Cd enrichment, mainly adsorbed on the outermost (>50%). • The ion exchange capacity of Na and Cd was stronger in SPB than NPB. • Genes such as cmtR were more abundant in SPB than in NPB, driving Cd enrichment.