Effect of Drying-Rewetting cycles on the metal adsorption and tolerance of natural biofilms

Response of bacterial and fungal communities in natural biofilms to bioavailable heavy metals in a mining-affected river

Biofilms, known as "microbial skin" in rivers, respond to rapid and sensitive environmental changes. However, the ecological response mechanisms of bacterial and fungal communities in river biofilms toward heavy metal pollution (HMP) remains poorly understood. This study focused on the key driving factors of bacterial and fungal community diversity and composition and their ecological response mechanisms within periphytic biofilms of Asia's largest Pb-Zn mining area. The diversity, dominant bacterial taxa, and bacteria structure in biofilms were influenced by biologically available heavy metal (HM) fractions, with Ni-F3 (17.96 %) and Pb-F4 (16.27 %) as the main factors affecting the bacterial community structure. Fungal community structure and α-diversity were more susceptible to physicochemical parameters (pH and nutrient elements). Partial least squares path modeling revealed that environmental factors influencing bacterial and fungal communities in biofilms were ranked as water quality > metal fractions > total metals. Dispersal limitation was the most critical ecological process in bacterial (56.9 %) and fungal (73.4 %) assembly. The proportion of heterogeneous selection by bacteria (39.5 %) was higher than that of fungus (26.2 %), which increased with increasing HMP. Bacterial communities had a higher migration rate (0.48) and ecological drift proportion (3.6 %), making them more prone to escape environmental stress. Fungal communities exhibited more keystone species, larger niche width (23.24 ± 13.04 vs. 9.72 ± 5.48), higher organization level, and a more stable co-occurrence network than bacterial communities, which enabled them to better adapt to high environmental pollution levels. These findings expanded the understanding of the spatiotemporal dynamics of microbial communities within biofilms in HM-polluted watersheds and provided new insights into the ecological responses of microbial communities to HMP.

Transport and transformations of cadmium in water-biofilm-sediment phases as affected by hydrodynamic conditions

Hydrodynamic conditions play a crucial role in governing the fate, transport, and risks of metal elements. However, the contribution of hydrodynamic conditions to the fate and transport of heavy metals among water, sediment, and biofilm phases is poorly understood. In our study, we conducted experiments in controlled hydrodynamic conditions using a total of 6 two-phase and 9 three-phase mesocosms consisting of water, biofilm, and sediment. We also measured Cd (cadmium) specification in different phases to assess how hydrodynamic forces control Cd bioavailability. We found that turbulent flow destroyed the surface morphology of the biofilm and significantly decreased the content of extracellular polymeric substances (p < 0.05). This led to a decrease in the biofilm's adsorption capacity for Cd, with the maximum adsorption capacity (0.124 mg/g) being one-tenth of that under static conditions (1.256 mg/g). The Cd chemical forms in the biofilm and sediment were significantly different, with the highest amount of Cd in the biofilm being acid-exchangeable, accounting for up to 95.1% of the total Cd content. Cd was more easily released in the biofilm due to its weak binding state, while Cd in the sediment existed in more stable chemical forms. Hydrodynamic conditions altered the migration behavior and distribution characteristics of Cd in the system by changing the adsorption capacity of the biofilm and sediment for Cd. Cd mobility increased in laminar flow but decreased in turbulent flow. These results enhance our understanding of the underlying mechanisms that control the mobility and bioavailability of metals in aquatic environments with varying hydrodynamic conditions.