Duckweeds live with complex assemblages of microbes as holobionts that play an important role in duckweed growth and phytoremediation ability. In this study, the structure and diversity of duckweed-associated bacteria (DAB) among four duckweed subtypes under natural and nutrient-deficient conditions were investigated using V3-V4 16S rRNA amplicon sequencing. High throughput sequencing analysis indicated that phylum Proteobacteria was predominant in across duckweed samples. A total of 24 microbial genera were identified as a core microbiome that presented in high abundance with consistent proportions across all duckweed subtypes. The most abundant microbes belonged to the genus Rhodobacter, followed by other common DAB, including Acinetobacter, Allorhizobium-Neorhizobium-Pararhizobium-Rhizobium, and Pseudomonas. After nutrient-deficient stress, diversity of microbial communities was significantly deceased. However, the relative abundance of Allorhizobium-Neorhizobium-Pararhizobium-Rhizobium, Pelomonas, Roseateles and Novosphingobium were significantly enhanced in stressed duckweeds. Functional prediction of the metagenome data displayed the relative abundance of essential pathways involved in DAB colonization, such as bacterial motility and biofilm formation, as well as biodegradable ability, such as benzoate degradation and nitrogen metabolism, were significantly enriched under stress condition. The findings improve the understanding of the complexity of duckweed microbiomes and facilitate the establishment of a stable microbiome used for co-cultivation with duckweeds for enhancement of biomass and phytoremediation under environmental stress.

Duckweeds are commonly used as standard ecotoxicological test species and are excellent candidates for phytoremediation due to their adaptability to diverse aquatic conditions. This study investigated the response of two duckweed species, Lemna aequinoctialis and Spirodela polyrhiza, to mercury-contaminated environments, specifically focusing on their growth rates and photosynthetic performance under mercury stress. Duckweeds were treated with HgCl₂ at various concentrations (0, 0.1, 0.2, and 0.3 mg/L) in 10% Hoagland’s medium for seven days, after which growth parameters, pigment contents, and chlorophyll fluorescence levels were evaluated. The results showed that increasing mercury concentrations significantly affected growth and photosynthetic pigments in L. aequinoctialis and S. polyrhiza. Chlorophyll fluorescence analysis indicated that, under higher mercury concentrations, Fv/Fm and Y (II) decreased, while NPQ increased. The findings revealed that L. aequinoctialis was more susceptible to mercury toxicity than S. polyrhiza. Therefore, L. aequinoctialis may serve as a more sensitive species for mercury biomonitoring, whereas S. polyrhiza is more resistant and might, for this reason, be more useful for phytoremediation of mercury-contaminated soil.