Photoferrotrophy and phototrophic extracellular electron uptake is common in the marine anoxygenic phototroph Rhodovulum sulfidophilum

Abstract

Photoferrotrophy allows anoxygenic phototrophs to use reduced iron as an electron donor for primary productivity. Recent work shows that freshwater photoferrotrophs can use electrons from solid-phase conductive substances via phototrophic extracellular electron uptake (pEEU), and the two processes share the underlying electron uptake mechanism. However, the ability of marine phototrophs to perform photoferrotrophy and pEEU, and the contribution of these processes to primary productivity is largely unknown. To fill this knowledge gap, we isolated 15 new strains of the marine anoxygenic phototroph Rhodovulum sulfidophilum on electron donors such as acetate and thiosulfate. We observed that all of the R. sulfidophilum strains isolated can perform photoferrotrophy. We chose strain AB26 as a representative strain to study further, and find that it can also perform pEEU from poised electrodes. We show that during pEEU, AB26 transfers electrons to the photosynthetic electron transport chain. Furthermore, systems biology-guided mutant analysis shows that R. sulfidophilum AB26 uses a previously unknown diheme cytochrome c protein, which we call EeuP, for pEEU but not photoferrotrophy. Homologs of EeuP occur in a range of widely distributed marine microbes. Overall, these results suggest that photoferrotrophy and pEEU contribute to the biogeochemical cycling of iron and carbon in marine ecosystems.

Taxonomic Re-Evaluation and Genomic Comparison of Novel Extracellular Electron Uptake-Capable Rhodovulum visakhapatnamense and Rhodovulum sulfidophilum Isolates

“…As previously described [12], Cell cultures were sampled anaerobically and immediately mixed with 1:1 with RNAlater ® (Qiagen, Germantown, MD, USA). RNA was extracted from cells using the RNeasy Mini Kit (Qiagen, USA) and DNA removal performed using Turbo DNA-free Kit (Ambion, Austin, TX, USA).…”

“…A total of 15 isolates were cultured from a marine microbial mat in Woods Hole, MA, USA and identified as belonging to R. sulfidophilum through 16S sequencing [12,15].…”

“…One well-characterized electron uptake system is known; the PioABC operon studied in TIE-1 [11]. Studies of pEEU in TIE-1 revealed the Calvin-Benson-Bassham cycle as an electron sink during pEEU [12,13]. This link to carbon dioxide fixation connects pEEU to the carbon cycle.…”

“…We sequenced and assembled the genomes of 12 of the 15 isolates based on these data (3 isolates, namely Rhodovulum sulfidophilum strain AB14, strain AB26, AB30, were assembled previously [15]). We have previously reported that all 15 strains were capable of heterotrophic, photoautotrophic, and photoheterotrophic growth as well as photoferrotrophy [12]. A representative strain, R. sulfidophilum AB26, was selected for further growth studies and found to be capable of pEEU in bioelectrochemical systems (BES) using a novel uptake pathway consisting of a di-heme c-type cytochrome (EeuP), with carbon dioxide fixation as a cellular sink for acquired electrons [12].…”

“…A representative strain, R. sulfidophilum AB26, was selected for further growth studies and found to be capable of pEEU in bioelectrochemical systems (BES) using a novel uptake pathway consisting of a di-heme c-type cytochrome (EeuP), with carbon dioxide fixation as a cellular sink for acquired electrons [12]. EeuP is found in diverse bacterial lineages without other characterized pEEU uptake systems in their genomes, which may suggest its role as a novel uptake system [12].…”