The Marine-Origin Exopolysaccharide-Producing Bacteria Micrococcus Antarcticus HZ Inhibits Pb Uptake in Pakchoi (Brassica chinensis L.) and Affects Rhizosphere Microbial Communities

Abstract

Exopolysaccharides (EPSs) produced by microorganisms play an important role in biotolerance and reducing heavy metal (HM) contamination by limiting the migration of HMs into plants. However, research on the application of EPS-producing marine bacteria for soil heavy metal remediation remains limited, particularly regarding their mechanisms of HM immobilization in soil and impact on plant growth. In this study, the EPS-producing marine bacterium Micrococcus antarcticus HZ was investigated for its ability to immobilize Pb and produce EPSs in soil filtrate. The effects on the growth quality and biomass of pakchoi (Brassica chinensis L.), as well as bacterial communities in inter-root soil contaminated with Pb, were also investigated. The results indicated that HZ could reduce the Pb concentration in the soil filtrate, achieving a removal rate of 43.25-63.5%. The EPS content and pH levels increased in the presence of Pb. Pot experiments showed that adding HZ significantly increased the biomass of pakchoi (9.45-14.69%), vitamin C (Vc) (9.69-12.92%), and soluble protein content (22.58-49.7%). HZ reduced the Pb content in the roots (17.52-47.48%) and leaves (edible tissues) (43.82-52.83%) of pakchoi. HZ increased soil enzyme activities (alkaline phosphatase, dehydrogenase, and urease), and the contents of ammonium nitrogen and nitrate nitrogen. Additionally, HZ also increased the relative abundance of beneficial bacteria (e.g., Proteobacteria, Cyanobacteria, and Chlorobacteria) in the inter-root soil, which have prophylactic and heavy-metal fixation functions. In summary, HZ reduces effective Pb content in edible tissues, roots, and inter-root soil by regulating inter-root soil microbial community structure, increasing soil pH, nitrogen content, and soil enzyme activity, and altering dominant phylum abundance.

Keywords: Pb-immobilizing; bacterial community; bioremediation; exopolysaccharides (EPSs); heavy-metal-contaminated soil; marine bacteria.

Figure 1

The cell number (indicated by OD600) (A), EPS (B), Pb (C), content clearance rate of Pb (D), and PH (E) in the culture solution at different Pb concentrations inoculated with HZ. Each bar represents the mean ± SD (n = 4). The same letter indicates that there were no significant differences between all treatments (p > 0.05).

Figure 2

Effects of HZ on Pb content (C) of DTPA in edible tissue (A), roots (B), and rhizosphere soil of pakchoi. Each bar represents the mean ± SD (n = 4). The same letter indicates that there were no significant differences between all treatments (p > 0.05).

Figure 3

Effects of HZ on edible tissue biomass (A), Vc content (B), soluble protein content (C), and nitrite content (D) of pakchoi. Each bar represents the mean ± SD (n = 4). The same letter indicates that there were no significant differences between all treatments (p > 0.05).

Figure 4

Effects of HZ on NH4+-N content (A), NO₃-N content (B), pH (C), alkaline phosphatase content (D), dehydrogenase (E), and urease content (F) in rhizosphere soil of pakchoi. Each bar represents the mean ± SD (n = 4). The same letter indicates that there were no significant differences between all treatments (p > 0.05).

Figure 5

Analysis of principal coordinates (PCoA) of bacterial communities in soil samples under different treatments.

Figure 6

Relative abundance of bacterial communities in soil under different treatments.

Figure 7

Heat maps of the top 30 genera of rhizosphere soil under different Pb pollution.

Figure 8

The influence of HZ on the relative abundance of some phyla in rhizosphere soil bacteria (A–D), and the influence of HZ on the relative abundance of some rhizosphere soil bacteria genera (E–I). Each bar represents the mean ± SD (n = 4). The same letter indicates no significant differences between the groups (p > 0.05).