Microbial electroactive biofilms dominated by Geoalkalibacter spp. from a highly saline–alkaline environment

Abstract

Understanding of the extreme microorganisms that possess extracellular electron transfer (EET) capabilities is pivotal to advance electromicrobiology discipline and to develop niche-specific microbial electrochemistry-driven biotechnologies. Here, we report on the microbial electroactive biofilms (EABs) possessing the outward EET capabilities from a haloalkaline environment of the Lonar lake. We used the electrochemical cultivation approach to enrich haloalkaliphilic EABs under 9.5 pH and 20 g/L salinity conditions. The electrodes controlled at 0.2 V vs. Ag/AgCl yielded the best-performing biofilms in terms of maximum bioelectrocatalytic current densities of 548 ± 23 and 437 ± 17 µA/cm2 with acetate and lactate substrates, respectively. Electrochemical characterization of biofilms revealed the presence of two putative redox-active moieties with the mean formal potentials of 0.183 and 0.333 V vs. Ag/AgCl, which represent the highest values reported to date for the EABs. 16S-rRNA amplicon sequencing of EABs revealed the dominance of unknown Geoalkalibacter sp. at ~80% abundance. Further investigations on the haloalkaliphilic EABs possessing EET components with high formal potentials might offer interesting research prospects in electromicrobiology.

Electrochemical enrichment of haloalkaliphilic nitrate-reducing microbial biofilm at the cathode of bioelectrochemical systems

“…A modified M9 medium with 20 g/L salinity and 9.5 pH was used as a catholyte or growth medium (200 mL) in the cathode chamber. Its composition is as follows (per L of distilled water): 4.33 g Na 2 HPO 4 , 2.69 g NaH 2 PO 4 , 20 g NaCl, 4.3 g Na 2 CO 3 , 0.13 g KCl, 0.31 g NH 4 Cl, 12.5 mL vitamins, and 12.5 mL trace elements ( Yadav and Patil, 2020 ). Sodium nitrate (10 mM) and sodium bicarbonate (10 mM) were used as the terminal electron acceptor (TEA) and carbon source, respectively.…”

“…Several studies have reported on the electrotrophic mixed-culture microbial biofilms, grown with electron acceptors such as oxygen ( Rabaey et al., 2008 ), H + ions ( Aulenta et al., 2008 ), CO 2 ( Aryal et al., 2017 ), heavy metals, polyaromatic hydrocarbons, dyes, and so on ( Gregory and Lovley, 2005 ; Tandukar et al., 2009 ; Sharma et al., 2020 ; Fang et al., 2016 ). Notably, EAMs have been studied mostly from nonextreme environments, and only a few studies have reported the diversity of these microbes inhabiting extreme habitats ( Koch and Harnisch, 2016 ; Rowe et al., 2017 ; Logan et al., 2019 ; Yadav and Patil, 2020 ). In particular, electrotrophs from extreme habitats have been barely studied, limiting our understanding of EET-capable microbes and their role in element cycling under specific environmental conditions and their use for various BES applications.…”

Protocol for bioelectrochemical enrichment, cultivation, and characterization of extreme electroactive microorganisms

“…Depending upon the mode of EET, EAMs are categorized as exoelectrogens and electrotrophs. Exoelectrogens can transfer electrons to a solid-state electron acceptor, and electrotrophs can oxidize a solid-state electron donor ( Logan et al., 2019 ; Yadav and Patil, 2020 ). Bioelectrochemical systems (BESs) are routinely used in laboratories to study the electroactive nature of microorganisms and explore them for various applications ( Yee et al., 2020 ).…”

“…This protocol describes the electrochemical enrichment and characterization of nitrate-reducing electrotrophs from the highly saline and alkaline environment of Lonar Lake. The protocol has been adapted from various other papers on the enrichment of different types of EAMs (e.g., Karbelkar et al., 2019 ; Gregory et al., 2004 ; Yadav and Patil, 2020 ). It mainly targets the growth of cathodic microbial biofilm, provided with a suitable terminal electron acceptor (TEA), i.e., nitrate.…”

“… ∗∗∗ Note: All the instruments mentioned above can be of any reliable make. Procedure: The medium composition is presented in Table 1 ( Yadav and Patil, 2020 ; Chaudhary et al., 2021 ). Dissolve the components in 500 mL distilled water, and then make the final volume to 1000 mL.…”