The impacts of cadmium exposure on epiphytic bacterial communities and water quality in mesocosmic wetlands.

Abstract

The damaging magnitudes of heavy metal pollution on freshwater ecosystems are well known, though research on the specific impacts of cadmium (Cd) on water quality and microbial communities in epiphytic biofilms is lacking. In this study, high-throughput sequencing and Scanning Electron Microscopy (SEM) were used to explore the effects of Cd on water quality and bacterial morphology, biodiversity, interactions, and ecological functions in epiphytic biofilms affixed to submerged plants that were natural and synthetic (Vallisneria natans, Potamogeton maackianus, and artificial macrophytes) in constructed wetlands. The results showed that Cd exposure significantly reduced the ability of natural and artificial plants to remove total nitrogen (TN) (5.7-50%), total phosphorus (TP) (12.5-97.1%), and chemical oxygen demand (COD) (9.45-21.8%), and affected the morphology of epiphytic bacteria. The bacterial β-diversity indices were significantly affected by Cd exposure, whereas bacterial α-diversity revealed a reverse trend. Notwithstanding Cd application induced the fluctuation (increase/decrease) in bacterial composition dynamics, but the relative abundance of Proteobacteria and Cyanobacteria spiked from 11.3-57.2% and 6.08- 94.35 %, respectively, in natural and artificial plants. Besides, Cd loading disturbed all bacterial network structures, with interactions particularly vulnerable in P. maackianus. Our study demonstrated that Cd exposure alters the bacterial diversity, food web structure, and predicted metabolic functions such as metabolism, translation, cell motility, signal transduction, membrane transport, and biodegradation of xenobiotics in epiphytic bacterial biofilms. These findings highlight the detrimental effects of Cd on epiphytic bacterial communities and nutrient removal in constructed wetlands.