Extracellular Polysaccharide-producing Marine Bacteria Inhibit Cd²⁺ Absorption In Lettuce And Basic Research On The Preparation Of Its Complex Microbial Inoculants

With the rapid development of China’s agriculture,industry,and social economy,the problem of heavy metal cadmium（Cd）contamination in arable land is becoming more and more serious,which not only adversely affects soil microorganisms and agricultural production,but also enters the human body through the food chain,posing a serious threat to human health and safety.Compared with traditional chemical and physical remediation methods,microbial remediation is a remediation technology that is suitable for medium and light heavy metal pollution without secondary pollution,which can effectively reduce the migration and biological effectiveness of heavy metals in soil and block the uptake of heavy metals by plants.Previous studies have found that extracellular polysaccharides produced by microorganisms can act as passivating agents to reduce the effectiveness of heavy metals in soil and reduce the transfer of heavy metals to plants.However,there are few studies on the application of extracellular polysaccharide-producing marine bacteria in soil remediation of heavy metals in China.The research on the remediation of soil heavy metal pollution by extracellular polysaccharide-producing marine bacteria can provide a useful reference for the development of safe,economical,and environmentally friendly remediation techniques for heavy metal pollution.Meanwhile,the plant-promoting bacteria can improve the soil environment through their promoting characteristics,optimize the fermentation conditions of inter-root promoting bacteria to further enhance their promoting characteristics,compound with the marine bacteria for remediation of heavy metals,and develop a composite microbial agent to provide technical support and development strategies for the remediation of heavy metals in soil and the sustainable development of agriculture.In this study,a strain of Pseudoalteromonas agarivorans Hao 2018,a marine bacteriophage capable of producing extracellular polysaccharides,was selected to study the adsorption of Cd²⁺and blocking the uptake of Cd²⁺by lettuce through soil filtrate experiments and pot experiments,and to optimize the fermentation conditions for Hao2018 and the maize inter-rhizosphere endophytes with growth-promoting properties obtained from the pre-laboratory screening,and to conduct a basic study for the preparation of multifunctional composite bacterial agents.The main findings were as follows:（1）Strain Hao 2018 was able to produce extracellular polysaccharides in the soil filtrate and was able to reduce the Cd²⁺content in the soil filtrate（24%-37%）.（2）Application of strain Hao 2018 was able to mitigate heavy metal Cd damage to lettuce,increase lettuce biomass,reduce Cd content in lettuce roots and edible tissues（23%-26%and 12%-40%）,and reduce the effective Cd content in Cd-contaminated soil（21%-25%）.Inoculation with Hao 2018 increased soil pH and soil enzyme activities（alkaline phosphatase and urease and dehydrogenase）,nitrogen content（NH₄⁺-N and NO₃^--N）and lettuce quality（Vc and soluble protein content）,as well as the relative values of bacteria that promote plant growth and have the ability to fix metals（Streptomyces spp.and Sphingomonas spp.）in relative abundance.（3）We identified two maize rhizosphere growth-promoting bacteria,named Bacillus aerophilus H23 and Bacillus licheniformis H7,which were previously screened in our laboratory.The growth-promoting characteristics of the above two strains were identified.The IAA production ability of strain H23 and the phosphorus solubilizing ability of strain H7 were better than other growth-promoting characteristics,so the fermentation conditions of the two strains were optimized based on the above two characteristics.The optimum conditions for IAA fermentation of strain H23 were as follows:pH 8.3,inoculum volume 11.6%,shaker speed 170 rpm,and the highest IAA production of 49.341 mg L^-1.The optimum process condition for phosphorus solubilization of strain H7 was a cultivation temperature of 31.3℃.The culture time was 7.4 days,the shaking speed was 153 rpm,and the soluble phosphorus content was315.482μg L^-1.At the same time,the fermentation conditions of strain Hao 2018 were optimized based on the EPS production,and the optimal fermentation condition was35.5 h.The yield of EPS was 2.679 g L^-1with 9%inoculum and 181 rpm of the shaker.The carrier characteristics of sodium humate were explored.The pH of sodium humate was 9.19,and the adsorption capacity of sodium humate to strains H23,H7,and Hao2018 was 0.37 L kg^-1,0.35 L kg^-1,and 0.34 L kg^-1,respectively.The results showed that sodium humate had good carrier properties.