Iron plaque crystallinity, heavy metal toxicity, and metal translocation in Kandelia obovata seedlings as altered by an iron-reducing bacterium under different flooding regimes

Purpose: Most Pseudomonas spp. are iron (Fe) redox cycling bacteria, which are common in the rhizosphere of wetland plants. The purpose of this work was to investigate the influence of P. sp. SCSWA09, a Fe reducing bacterium isolated from mangrove plant Kandelia obovata, on the development of Fe plaques as well as the accumulation and translocation of heavy metals (HMs) in K. obovata. Methods: A controlled experiment was conducted in a climate chamber using K. obovata as model plants to evaluate the effect of P. sp. SCSWA09 on the development of Fe plaque and the plants' response to HM stress under periodic flooding conditions. Results: The results demonstrated that SCSWA09 had a clear capacity to reduce Fe (III). K. obovata seedlings inoculated with SCSWA09 showed considerably lower levels of Fe plaque production than uninoculated seedlings, notably the amorphous Fe plaque. Periodic flooding accelerated the transformation of amorphous Fe (hydrogen) oxide into crystalline Fe (hydrogen) oxide in Fe plaque, and SCSWA09 inoculation accelerated this process. The buildup of Cr and Cu in K. obovata roots steadily reduced as the flooding period rose, but SCSWA09 inoculation significantly increased Cr translocation to K. obovata leaves. Conclusion: Both SCSWA09 and periodic flooding enhanced the crystallinity of iron (hydr-) oxides in the treatment solution and Fe plaque, which influenced HM absorption and translocation in plants.