Iron plaque formation and rhizosphere iron bacteria in Spartina alterniflora and Phragmites australis on the redoxcline of tidal flat in the Yangtze River Estuary

Abstract

As two common estuarine wetland plants in China, Spartina alterniflora and Phragmites australis significantly differ in the mechanism of gas transportation and radial oxygen loss, and this difference can lead to significant differences in the shaping of the rhizosphere bacterial community and the formation of root iron (Fe) plaque. In this study, the difference in root Fe plaque formation between S. alterniflora and P. australis on the redoxcline of tidal flat was determined, and the composition of Fe-oxidizing bacteria (IOB) and Fe-reducing bacteria (IRB) communities in the rhizosphere and Fe plaque of the two plants was compared. With the increase in soil redox potential (Eh), the Fe plaque (amorphous and crystalline forms) content in the roots of S. alterniflora and P. australis decreased significantly. The Fe plaque content in the roots of P. australis was significantly higher than that of S. alterniflora, regardless of the tidal level. High soil Eh improved the formation of crystallized Fe plaque, as the proportion of crystalline Fe plaque increased significantly with the increase in soil Eh. Under similar soil Eh, the microbial diversity of the P. australis rhizosphere was higher than that of the S. alterniflora rhizosphere. The relative abundances of the dominant IOB and IRB, such as the genera Desulfuromonas, Geothermonbacter, Pseudomonas, Paracoccus, Geobacter, Amaricoccus, Pelobacter, and Gallionella, in the P. australis rhizosphere in low-tidal flat are generally higher than those in the S. alterniflora rhizosphere. However, the relative abundances of primary IRB (such as Desulfuromonas) and IOB (such as Pseudomonas) in the rhizosphere of the two plants increased with the increase in soil Eh and soil total Fe content, indicating that the enrichment of Fe bacteria in the S. alterniflora and P. australis rhizospheres may not be the primary determinant of Fe plaque formation.