In contrast, pMn showed no gravitational settling behavior (Figures 1, 4b). Instead, peak 116 pMn concentrations remained close in depth to dMn and 3 He xs and followed density surfaces, 117 indicative of isopycnal mixing. Thus, while most pMn and pFe are removed exponentially from (Figures 1, S1). 159The physicochemical form of hydrothermal dissolved iron -A key finding of this study, 160 not discussed previously 8 but critical to the fate of Fe in the SEPR hydrothermal plume, is that 161 the maximum dissolved Fe also deepens by ~350 m by Sta. 36, mimicking pFe (Figures 1, 4a). is not isotopically resolvable in the near-field plume (dFe >5 nM). would not apply to dMn because dMn is minimally complexed by organic matter in natural 233 waters 2 , and negligible Mn was observed in the colloidal fraction ( Figure S4).

Geobacter sulfurreducens uses at least two different pathways to transport electrons out of the inner membrane quinone pool before reducing acceptors beyond the outer membrane. When growing on electrodes poised at oxidizing potentials, the CbcL-dependent pathway operates at or below redox potentials of –0.10 V vs the standard hydrogen electrode, whereas the ImcH-dependent pathway operates only above this value. Here, we provide evidence that G. sulfurreducens also requires different electron transfer proteins for reduction of a wide range of Fe(III)- and Mn(IV)-(oxyhydr)oxides, and must transition from a high- to low-potential pathway during reduction of commonly studied soluble and insoluble metal electron acceptors. Freshly precipitated Fe(III)-(oxyhydr)oxides could not be reduced by mutants lacking the high-potential pathway. Aging these minerals by autoclaving did not change their powder X-ray diffraction pattern, but restored reduction by mutants lacking the high-potential pathway. Mutants lacking the low-potential, CbcL-dependent pathway had higher growth yields with both soluble and insoluble Fe(III). Together, these data suggest that the ImcH-dependent pathway exists to harvest additional energy when conditions permit, and CbcL switches on to allow respiration closer to thermodynamic equilibrium conditions. With evidence of multiple pathways within a single organism, the study of extracellular respiration should consider not only the crystal structure or solubility of a mineral electron acceptor, but rather the redox potential, as this variable determines the energetic reward affecting reduction rates, extents, and final microbial growth yields in the environment.

Sulfide mineral precipitation occurs at mid-ocean ridge (MOR) spreading centers, both in the form of plume particles and seafloor massive sulfide structures. A common constituent of MOR is the iron-bearing sulfide mineral pyrrhotite, which was chosen as a substrate for in-situ incubation studies in shallow waters of Catalina Island, CA to investigate the colonization of iron-oxidizing bacteria. Microbial community datasets were obtained from in-situ incubated pyrrhotite, allowing for direct comparison to microbial communities of iron-sulfides from active and inactive chimneys in deep-sea environments. Unclassified Gammaproteobacteria and Alphaproteobacteria (Magnetovibrio) largely dominated the bacterial community on pyrrhotite samples incubated in the water column while samples incubated at the surface sediment showed more even dominance by Deltaproteobacteria (Desulfobulbus), Gammaproteobacteria (Piscirickettsiaceae), Alphaproteobacteria (Rhodobacteraceae), and Bacteroidetes (Flavobacteriia). Cultivations that originated from pyrrhotite samples resulted in the enrichment of both, sheath-forming and stalk-forming Zetaproteobacteria. Additionally, a putative novel species of Thiomicrospira was isolated and shown to grow autotrophically with iron, indicating a new biogeochemical role for this ubiquitous microorganism.

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A facultatively anaerobic bacterium, designated strain 1MBB1 T , was isolated from basaltic breccia collected from 341 m below the seafloor by seafloor drilling of Rigil Guyot during Integrated Ocean Drilling Program Expedition 330. The cells were straight rods, 0.5 mm wide and 1-3 mm long, that occurred singly and in chains. Strain 1MBB1 T stained Gram-positive. Catalase and oxidase were produced. The isolate grew optimally at 30 8C and pH 7.5, and could grow with up to 12 % (w/v) NaCl. The DNA G+C content was 40.5 mol%. The major cellular fatty acids were C 16:1 v11c (26.5 %), anteiso-C 15:0 (19.5 %), C 16:0 (18.7 %) and iso-C 15:0 (10.4 %), and the cell-wall diamino acid was meso-diaminopimelic acid. Endospores of strain 1MBB1 T oxidized Mn(II) to Mn(IV), and siderophore production by vegetative cells was positive. Phylogenetic analysis of the 16S rRNA gene indicated that strain 1MBB1 T was a member of the family Bacillaceae, with Bacillus foraminis CV53 T and Bacillus novalis LMG 21837 T being the closest phylogenetic neighbours (96.5 and 96.2 % similarity, respectively). This is the first novel species described from deep subseafloor basaltic crust. On the basis of our polyphasic analysis, we conclude that strain 1MBB1 T represents a novel species of the genus Bacillus, for which we propose the name Bacillus rigiliprofundi sp. nov. The type strain is 1MBB1 T (5NCMA B78 T 5LMG 28275 T ).