Organic fertilizer regulates multispecies biofilm formation and structure to enhance Cd removal efficiency

Cadmium (Cd) contamination of soil poses a risk to both environmental and agricultural safety. Although microbial biofilms have the potential to immobilize heavy metals, there is a lack of strategies to enhance their functionality under Cd-stress.

Investigating the effects of organic fertilizer application on the formation of multispecies biofilms in paddy soils and the cadmium adsorption capacity of biofilms under fertilization management.

A field experiment was conducted in Zhuzhou paddy soils, comparing control and organic fertilizer treatments. Multispecies biofilms cultured from rhizosphere soil were assessed under Cd stress through minimum inhibitory concentration determination, 16S rRNA sequencing, and confocal laser scanning microscopy to evaluate their resistance, biomass, community composition, and architecture. Cd accumulation and distribution were analyzed via atomic absorption spectroscopy, while biofilm compositional responses were characterized by quantifying extracellular polymeric substances and employing Fourier-transform infrared spectroscopy.

This study demonstrates that organic fertilizer application alters the composition of biofilm bacterial communities, enriching Novosphingobium, Pirellula, and Ellin6067, promotes multispecies biofilm formation, and significantly enhances Cd resistance. Organic fertilizer application significantly increased biofilm biomass by 0.49-fold compared with that of the control. With the increase in Cd concentration, the biofilm biomass significantly decreased, but low Cd concentrations (0.2 mmol/L) promoted biofilm development. Notably, organic fertilizer-amended biofilms demonstrated 88% Cd removal efficiency, markedly exceeding that of the controls. Fourier-transform infrared (FTIR) spectroscopy revealed that various functional groups, including C=O, -NH, C-N, and -COO^-, along with those indicative of polysaccharides (C-O, C-C, C-O-H, and C-O-C) and the eDNA biomarker PO₂^-, were associated with Cd exposure, suggesting their might serve as potential metal binding sites. Enhanced secretion of extracellular polysaccharides was identified as the primary mechanism of Cd stress resistance in these biofilms.

These findings elucidate the co-regulatory effects of organic fertilization and Cd in promoting biofilm assembly and function for effective Cd adsorption, offering critical insights for optimizing microbial remediation in Cd-contaminated soil.