Electrochemical Characterization of Marinobacter atlanticus Strain CP1 Suggests a Role for Trace Minerals in Electrogenic Activity

Abstract

The marine heterotroph, Marinobacter atlanticus strain CP1, was recently isolated from the electroautotrophic Biocathode MCL community, named for the three most abundant members: Marinobacter, an uncharacterized member of the Chromatiaceae, and Labrenzia. Biocathode MCL catalyzes the production of cathodic current coupled to carbon fixation through the activity of the uncharacterized Chromatiaceae, renamed as "Candidatus Tenderia electrophaga," but the contribution of M. atlanticus is currently unknown. Here, we report on the electrochemical characterization of pure culture M. atlanticus biofilms grown under aerobic conditions and supplemented with succinate as a carbon source at applied potentials ranging from 160 to 510 mV vs. SHE, and on three different electrode materials (graphite, carbon cloth, and indium tin oxide). M. atlanticus was found to produce either cathodic or anodic current that was an order of magnitude lower than that of the Biocathode MCL community depending on the oxygen concentration, applied potential, and electrode material. Cyclic voltammetry, differential pulse voltammetry (DPV), and square wave voltammetry (SWV) were performed to characterize putative redox mediators at the electrode surface; however no definitive redox peaks were observed. No effect on current was observed when genes encoding a putative rubredoxin (ACP86_RS07295), as well as a putative NADH:flavorubredoxin oxidoreductase (ACP86_RS07290), were deleted to evaluate their role in EET. The addition of either riboflavin or excess trace mineral solution increased anodic current by ca. an order of magnitude under the conditions in which Biocathode MCL is typically grown. These results indicate that M. atlanticus has a non-negligible ability to utilize electrodes as an electron acceptor, which can be enhanced by the presence of excess trace minerals already available in the growth medium. The ability of M. atlanticus to utilize trace minerals as electron shuttles with extracellular electron acceptors may have broader implications for its natural role in biogeochemical cycling.

Enrichment of Marinobacter sp. and Halophilic Homoacetogens at the Biocathode of Microbial Electrosynthesis System Inoculated With Red Sea Brine Pool

“…Since members of the genus Marinobacter are heterotrophs (unable to fix CO 2 ) (Onderko et al, 2019) and amplicon sequencing and metagenomics revealed that they are the dominant members at the biocathode (Figures 8, 9) where no other carbon source was provided other than CO 2 , we screened the MAGs of other members in the biocathode for the presence of genes encoding enzymes for CO 2 fixation pathways: Wood–Ljundahl pathway, reductive citric acid cycle, 3-hydroxypropionate bicycle, hydroxypropionate-hydroxybutyrate cycle, and dicarboxylate-hydroxybutyrate cycle. Of the different CO 2 fixation pathways, we only detected marker genes encoding enzymes of a complete Wood–Ljundahl pathway, including genes encoding for formyl-tetrahydrofolate ligase, which catalyzes the activation of formate utilizing ATP, methylene-tetrahydrofolate dehydrogenase/methenyl-tetrahydrofolate cyclohydrolase, 5,10- methylene-tetrahydrofolate reductase, 5-methy-tetrahydrofolate corrinoid/iron sulfur protein methyltransferase, acetyl-CoA synthase, carbon-monoxide dehydrogenase, and CO dehydrogenase/acetyl-CoA synthase (CODH/ACS) (Sewell et al, 2017).…”

“…(Proteobacteria_15 and Proteobacteria_17) revealed the presence of genes for multiheme c-type cytochromes that may participate in extracellular electron transfer (EET), however, they do not contain large multiheme c-type cytochromes similar to the ones present in known electrochemically active bacteria like Geobacter sulfurreduces and Shewanella oneidensis MR1 (Onderko et al, 2019). Recently, it was demonstrated that M. atlanticus CP1 biofilms can generate very low level of anodic and cathodic current in BES fed with oxygenated seawater medium supplemented with succinate as carbon and electron donor source, suggesting its ability to perform EET (Onderko et al, 2019). Addition of redox-active species such as riboflavin or excess trace minerals resulted in an increase in current production indicating that soluble redox mediators can facilitate EET in M. atlanticus CP1.…”

Marinobacter atlanticus electrode biofilms differentially regulate gene expression depending on electrode potential and lifestyle

“…play a major role in biogeochemical cycling [ 9 ]. The isolation of some strains from biocathodes suggested that they may be able to use the electrode as an electron donor in addition to metals [ [10] , [11] , [12] , [13] , [14] , [15] ]. However, no mechanism for either metal-oxidation or EET with electrodes has been described that might help explain the role of Marinobacter spp.…”

“…Like other Marinobacter species, M. atlanticus can oxidize or reduce iron when an organic carbon source is also provided, but does not appear to conserve energy for growth and cannot grow autotrophically [ 6 , 16 ]. M. atlanticus is a “weak electricigen” [ 17 ]; when grown on an electrode at potentials that promote electron uptake, current is at least one order of magnitude lower than the Biocathode MCL community [ 15 ]. At potentials that promote current production, current is four orders of magnitude less than what is typically observed for well-studied electricigens like Geobacter sulfurreducens [ 18 ] .…”

“…The M. atlanticus genome does not contain any large multi-heme redox proteins typically associated with EET and cells do not grow under anoxic conditions when an electrode is the sole electron acceptor [ 15 , 16 ]. Addition of excess trace minerals or riboflavin results in an increase in anodic current [ 15 ], suggesting that M. atlanticus may utilize trace redox shuttles available in the medium to facilitate EET. Previous metatranscriptomics analysis of the Biocathode MCL community indicated the gene for rubredoxin was highly expressed in M. atlanticus , suggesting it may play a role [ 19 ].…”