The removal of excess nitrogen is a crucial step in the ecological restoration of eutrophic lakes. The microbial-mediated dissimilatory nitrate reduction processes in lakes represents an important mechanism for nitrogen removal. However, lake ecosystems are facing an increasingly complex and severe risk of invasive submerged macrophytes and microplastic pollution. The differences between invasive and native submersed macrophytes in terms of root secretion of organic matter and nitrogen assimilation efficiency can alter the dissimilatory nitrate reduction processes in sediments. It is evident that microplastics can directly alter the physicochemical properties of sediments and indirectly affect the rate of dissimilatory nitrate reduction processes and the interrelationships of the system by influencing the health of submerged macrophytes. However, it is not clear whether the diversity of invasive submerged macrophytes affects sediment dissimilatory nitrate reduction processes in the context of microplastic contamination.

Supervised by Professor XING Wei, GAO Xueyuan, a doctoral candidate from Wuhan Botanical Garden, conducted a cross-experiment on the addition of polystyrene microplastics (PS-MPs) and the diversity of invasive submerged macrophytes to analyze its effect on the physicochemical properties of the sediment, the structure of the microbial community, and the potential rates of the various dissimilatory nitrate reduction processes.

The results showed that denitrification processes were more sensitive to invasive submerged macrophyte diversity than dissimilatory nitrate reduction to ammonium (DNRA) and Anammox. However, the addition of PS-MPs appeared to interfere with the effect of invasive submerged macrophyte diversity on the dissimilatory nitrate reduction processes.

The addition of PS-MPs significantly altered the physicochemical environment of the sediment, including a notable reduction in sediment pH, elevated nitrate and ammonia concentrations, and other notable changes. Furthermore, the addition of PS-MPs induced changes in the sediment microbial community. As the concentration of PS-MPs increased, the number of unique genera of the microbial community increased. Endemic genera of the microbial community increased, alpha diversity rose, and resulted in a more stable microbial community structure. The physicochemical alterations to the sediment environment induced by PS-MPs created a more conducive setting for denitrification to occur, thereby further enhancing the competitive advantage of this process over DNRA. In addition, the addition of PS-MPs inhibited the Anammox.

Overall, the response of dissimilatory nitrate reduction processes to invasive submerged macrophyte diversity exhibited process specificity. Nevertheless, the incorporation of PS-MPs resulted in a non-synergistic effect that weakened the expression of the model of invasive submerged macrophyte diversity's influence on dissimilatory nitrate reduction processes.

The results of this study expand the understanding of invasive submerged macrophytes in lake ecosystems and the impacts of microplastic accumulation on functional microbial communities and nitrogen cycling systems in sediments.

The research results were published in the Journal of Hazardous Materials under the title "Does invasive submerged macrophyte diversity affect dissimilatory nitrate reduction processes in sediments with varying microplastics? ” It was funded by the National Natural Science Foundation of China.