Mircobacterium foliorum strain SZ1 assisted melastoma malabathricum L. Phytoremediation of arsenic

Abstract

Arsenic contamination in soil is a serious problem as the toxic metalloid impacts both environmental and public health. Phytoremediation is an environmental friendly method that can be applied to remediate the arsenic-contaminated soil. Arsenic is a non-essential element and it is generally toxic to plants. The efficiency of plant arsenic uptake is usually low as most soils contain inorganic arsenic in the form of stable arsenate. Bacterial association with the plant can improve plant growth and arsenic uptake. Therefore, this project aimed to investigate the efficiency of a local plant Melastoma malabathricum L. incorporated with an arsenate-reducing bacterium Microbacterium foliorum strain SZ1 in arsenic phytoremediation. The effects of M. foliorum SZ1 inoculation on the soil bacterial community and the arsenic-exposed M. malabathricum L. leaf proteins expression were also studied. A two-month experiment was conducted at the greenhouse using M. malabathricum L. treated with four arsenic concentrations (0, 10, 30 and 50 ppm), inoculated or uninoculated with M. foliorum SZ1. Plant dry weight, root length and shoot length were measured as growth assessment. Arsenic contents in soil and plant tissues (roots and shoots) were quantified using inductively coupled plasma-optical emission spectrometry (ICP-OES). Illumina MiSeq 16S rRNA was applied to determine the total soil bacterial composition. Leaf proteins were separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and identified by liquid chromatography-mass spectrometry (LC-MS). At the end of the treatment, plant’s survival was only observed in the uninoculated 0 ppm treatment and all 50 ppm treatments. The 50 ppm-treated plants showed a significantly higher plant dry weight than the respective inoculated 0, 10 and 30 ppm plants by 115%, 80%, and 77%. Soil arsenic concentration of the inoculated 30 ppm treatment was 7.7% and 23.6% higher than the uninoculated 50 ppm and inoculated 50 ppm soils, respectively. The bioconcentration factor of M. malabathricum L. in 50 ppm arsenic was more than 1, suggesting the plant’s ability in arsenic phytoextraction. Establishment of M. foliorum SZ1 in soil was not observed. M. foliorum SZ1 may have increased the abundance of its order Micrococcales in the 50 ppm arsenic-contaminated soil. However, the effect of inoculation on the soil was not as prominent as the arsenic toxicity. All survived plants expressed proteins that mainly involved in cellular respiration and energy metabolism. Arsenic treatment increased the leaf protein expression by almost 3-fold. Plant defense against the toxicity was determined by the discovered ROS-scavenging enzymes such as peroxidases, glutathione-S-transferase, 2-cysteine peroxiredoxin and catalase. This project assesses the efficiency of local plant-bacteria association in remediating arsenic-contaminated soil and provides information on the soil indigenous community and plant physiology. The presented data can be used as a reference to optimize the application of bacteria-assisted phytoremediation technique in future.