Microbiological Processes of Submicrometer Plastics Affecting Submerged Plant Growth in a Chronic Exposure Microcosm

Laboratory microcosms were designed to investigate the chronic influence of sedimental submicrometer plastics (sMPs) on the growth of an aquatic submerged plant, Vallisneria denseserrulata, and the involved microbiological processes. A dose–response experiment (0–1000 μg/g) showed that the growth of V. denseserrulata was not affected by 8-week exposure to sedimental sMPs (100 and 1000 nm) until the dose reached 1000 μg/g (i.e., 0.1% w/w) in wet sediment. The observed-effect dosage of 0.1% w/w sMPs significantly suppressed the plant height and biomass of V. denseserrulata by 19.19–22.26% and 10.81–15.80%, respectively. Changes in rhizosphere microbial community structure were detected under the observed-effect dosage, with relatively higher community diversity and weaker conflicting interaction. Sedimental sMPs decreased the stochasticity ratio of rhizosphere microbial community assembly by 11.0–27.7% compared with the control group. Furthermore, functional analysis showed that the ecological processes related to the carbon and nitrogen cycle were suppressed at the observed-effect dosage, among which phototrophy, nitrogen fixation, and nitrate reduction were significantly decreased by 68.30%, 23.56%, and 17.59%, respectively. Our results revealed the responses of an aquatic plant to sedimental sMPs exposures, and the adverse impacts of sMPs were associated with the ecological assembly processes of the rhizosphere microbial community.