Electrotrophs are microbes that can take up electrons into their cells from external solid-phase conductive substrates (i.e., ferrous iron [Fe(II)]-minerals and electrodes) to obtain energy for growth, and transfer the electrons to various terminal electron acceptors, such as carbon dioxide (CO2) and nitrate (NO3-). The electrotrophic metabolism coupling with the transformations of nutrient elements can affect the biogeochemical cycles in natural environments. Paddy soil is one of the typical and artificial wetlands. The frequent alterations of wetting and drying cause the gradient of redox potential and mineral production which could provide a suitable habitat for electrotroph. However, the community and structure of electrotrophs in paddy soils remain poorly understood. Additionally, fertilization is one of the common practices to improve crop yields, but the influences of long-term different fertilization practices on electrotrophic community in paddy soils remain unclear.
In this study, we selected five paddy soils with various physicochemical properties and one paddy soil receiving different fertilization, respectively. The poised cathodes in microbial electrosynthesis systems (MESs) were used as proxies of electron donors to enrich the electrotrophic communities from these paddy soils. The Illumina sequencing and high-through quantative PCR (HT-qPCR) technology were used to: (1) analysis the communities of potentital electrotrophic microorganisms enriching from paddy soils; (2) examine the contributions of driving soils properties on the variations of electrotrophic communities; (3) explore the responses of electrtrophic communities to long-term different fertilization practies in paddy soils. The main content and results are as followings:
1) the putative electrotrophs were enriched by cathodes of MESs constructed from five paddy soils with various properties using bicarbonate as an electron acceptor, and identified by 16S rRNA-gene based Illumina sequencing. The electrons were gradually consumed on the cathodes, and 25%–45% of which were recovered to reduce bicarbonate to acetic acid by the potential electrotrophs during MES operation. Firmicutes was the dominant bacterial phylum on the cathodes, and Bacillus genus within this phylum was greatly enriched and was the most abundant population among the detected putative electrotrophs for almost all soils. Furthermore, several other members of Firmicutes and Proteobacteria may also participate in electrotrophic process in different soils. Soil pH, amorphous iron and electrical conductivity significantly influenced the putative electrotrophic bacterial community, which explained about 33.5% of the community structural variation.
2) The poised cathodes (−0.4 V versus Ag/AgCl) in microbial electrosynthesis systems (MESs) were used as proxies of electron donors and NO3- as an electron acceptor to enrich the electrotrophic communities in four paddy soils receiving different fertilization treatments since 1982, including no fertilization (CK), compost cattle manure only (M), nitrogen, phosphorus, and potassium (NPK) chemical fertilizers, and a combination of M and NPK fertilizers (MNPK). Illumina MiSeq sequencing was used for electrotrophic community analyses and HT-qPCR was performed to explore the genes involving with nitrate reduction. The results showed that compared CK, the soils of M, NPK, and MNPK significantly increased the taxonomic and phylogenetic diversity of the putative electrotrophic microbial community. Actinobacteria and Proteobacteria were the dominant electrotrophic microbes in the unfertilized soil. NPK use favored electrotrophic populations belonging to Firmicutes, whereas M and MNPK use favored Chloroflexi, Deinococcus-Thermus and Gemmatimonadetes populations. Furthermore, Streptomyces, Truepera, or Bacillus genera were putative electrotrophs enriched by all fertilization practices. These putative electrotrophic microbial populations consumed the biocathodic currents coupling with nitrate reduction when nitrate served as an electron acceptor, and 18–35% of electrons were recovered by electrotrophs to reduce nitrate to ammonium. Soil total carbon, nitrogen, and dissolved organic carbon were major factors influencing the compositions of electrotrophic communities in soils receiving different fertilizers, accounting for 53% of structural variations.
3) To investigate the impacts of long-term fertilization on the community of electrotrophic Geobacteraceae family, the extacted DNA in unfertilized and fertilized treatments of the section 2 was amplified to target the Geobacteraceae 16S rRNA genes using the primer pairs Geo494F and Geo825R for Illumina sequencing. The results showed that the species richness and diversity of Geobacteraceae community were not significantly changed by fertilizer treatments. Geobacteraceae in the treatments consisted of Geobacter (accounting for 90%–95% of total reads) and Geothermobacter genera (5%–10%), and all fertilizer treatments induced a significant (P < 0.05) decline in Geobacter and a marked enrichment of Geothermobacter. The taxonomic (based on Bray-Curtis distance) and phylogenetic structures (weighted-UniFrac distance) of the Geobacteraceae communities in all fertilizer treatments were clearly different from those in the non-fertilizer treatment; however, there were no significant changes among the different fertilization treatments. The variations in the Geobacteraceae community induced by long-term fertilization were mainly determined by changes in soil pH, total carbon, and total nitrogen.
The above content indicated that the diverisity of potential electrotrophic communitis in paddy soils were mainly influenced by the soil pH and the contents of carbon and nitrogen, and the microbes in Firmicutes and Proteobacteria were the dominate populations. The metabolisms of electrtrophs also influence the cycles of carbon and nitrogen in paddy soils. This study expands our knowledge about the diversity and compositions of electrotrophic communities and their roles on elemental cycles in paddy soils, it also provides theoretical basis for regulating the nitrogen fertilizer efficiency by electrotrophs.