Study on diversity, nitrogen-fixing capacity, and heavy metal tolerance of culturable Pongamia pinnata rhizobia in the vanadium-titanium magnetite tailings

Tian Shen; Ruimin Jin; Jing Yan; Xiran Cheng; Lan Zeng; Qiang Chen; Yunfu Gu; Likou Zou; Ke Zhao; Quanju Xiang; Petri Penttinen; Menggen Ma; Shuangcheng Li; Ting Zou; Xiumei Yu

doi:10.3389/fmicb.2023.1078333

Study on diversity, nitrogen-fixing capacity, and heavy metal tolerance of culturable Pongamia pinnata rhizobia in the vanadium-titanium magnetite tailings

Front Microbiol. 2023 Jun 19:14:1078333. doi: 10.3389/fmicb.2023.1078333. eCollection 2023.

Authors

Tian Shen¹, Ruimin Jin¹, Jing Yan¹, Xiran Cheng¹, Lan Zeng¹, Qiang Chen^{1

2}, Yunfu Gu¹, Likou Zou¹, Ke Zhao¹, Quanju Xiang¹, Petri Penttinen¹, Menggen Ma¹, Shuangcheng Li¹, Ting Zou¹, Xiumei Yu^{1

2}

Affiliations

¹ College of Resources, Sichuan Agricultural University, Chengdu, China.
² Key Laboratory of Investigation and Monitoring, Protection and Utilization for Cultivated Land Resources, Ministry of Natural Resources, Chengdu, China.

Abstract

Introduction: The diversity, nitrogen-fixing capacity and heavy metal tolerance of culturable rhizobia in symbiotic relationship with Pongamia pinnata surviving in vanadium (V) - titanium (Ti) magnetite (VTM) tailings is still unknown, and the rhizobia isolates from the extreme barren VTM tailings contaminated with a variety of metals would provide available rhizobia resources for bioremediation.

Methods: P. pinnata plants were cultivated in pots containing the VTM tailings until root nodules formed, and then culturable rhizobia were isolated from root nodules. The diversity, nitrogen-fixing capacity and heavy metal tolerance of rhizobia were performed.

Results: Among 57 rhizobia isolated from these nodules, only twenty strains showed different levels of tolerance to copper (Cu), nickel (Ni), manganese (Mn) and zinc (Zn), especially strains PP1 and PP76 showing high tolerance against these four heavy metals. Based on the phylogenetic analysis of 16S rRNA and four house-keeping genes (atpD, recA, rpoB, glnII), twelve isolates were identified as Bradyrhizobium pachyrhizi, four as Ochrobactrum anthropic, three as Rhizobium selenitireducens and one as Rhizobium pisi. Some rhizobia isolates showed a high nitrogen-fixing capacity and promoted P. pinnata growth by increasing nitrogen content by 10%-145% in aboveground plant part and 13%-79% in the root. R. pachyrhizi PP1 showed the strongest capacity of nitrogen fixation, plant growth promotion and resistance to heavy metals, which provided effective rhizobia strains for bioremediation of VTM tailings or other contaminated soils. This study demonstrated that there are at least three genera of culturable rhizobia in symbiosis with P. pinnata in VTM tailings.

Discussion: Abundant culturable rhizobia with the capacity of nitrogen fixation, plant growth promotion and resistance to heavy metals survived in VTM tailings, indicating more valuable functional microbes could be isolated from extreme soil environments such as VTM tailings.

Keywords: Pongamia pinnata; VTM tailings; nitrogen fixation; plant growth promotion; rhizobia; tolerance.