Potential of Mn2+-Resistant Purple Nonsulfur Bacteria Isolated from Acid Sulfate Soils to Act as Bioremediators and Plant Growth Promoters via Mechanisms of Resistance

Abstract

Resistance mechanisms of acid Mn2+-resistant purple nonsulfur bacteria (PNSB) were investigated to determine their ability to alleviate adverse effects of excess manganese on plant growth and productivity in acid sulfate soils (ASS). PNSB were isolated and selected based on their resistance to acid and manganese. Their resistance mechanisms were assessed on the basis of biosorption, bioaccumulation, and plant growth promoting properties. Six PNSB were selected for their resistance to Mn2+ in acidic conditions (pH 3.5–5.5). They were identified as Rhodopseudomonas palustris strains TLS12, VNS19, VNS32, VNS62, and VNW95, and Rhodopseudomonas harwoodiae strain TLW42. Under aerobic dark and microaerobic light incubating conditions in aqueous solution at pH 4.25 for 30 min, these strains adsorbed Mn2+ (1500 mg L−1) more effectively with released exopolymeric substances (EPS) than with their biomass. Under both incubating conditions, bioaccumulation of Mn2+ diminished in the following order: cell wall, cytoplasm, plasma membrane. Scanning electron microscope-energy dispersive X-ray spectrometry (SEM-EDS) found accumulated manganese (0.30–0.67% of total elements) in bacterial cells caused morphological change in the form of wrinkles and bleb-like features on exterior surfaces. All selected strains under both incubating conditions released NH4+ by N2 fixing and PO43− by solubilizing phosphate from various P-sources. Siderophores, 5-aminolevulinic acid (ALA), and indole-3-acetic acid (IAA) were also released and pH increased. The studied strains showed potential as bioremediators that could reduce Mn2+ toxicity using EPS and effectively release nutrients and plant growth promoting substances to improve cultivation and fertility in acidic conditions.