Phototrophic oxidation of ferrous iron by a Rhodomicrobium vannielii strain

Abstract

Oxidation of ferrous iron was studied with the anaerobic phototrophic bacterial strain BS-1 . Based on morphology, substrate utilization patterns, arrangement of intracytoplasmic membranes and the in vivo absorption spectrum, this strain was assigned to the known species Rhodomicrobium vannielii. Also, the type strain of this species oxidized ferrous iron in the light. Phototrophic growth of strain BS-1 with ferrous iron as electron donor was stimulated by the presence of acetate or succinate as cosubstrates. The ferric iron hydroxides produced precipitated on the cell surfaces as solid crusts which impeded further iron oxidation after two to three generations. The complexing agent nitrilotriacetate stimulated iron oxidation but the yield of cell mass did not increase stoichiometrically under these conditions. Other complexing agents inhibited cell growth. Ferric iron was not reduced in the dark, and manganese salts were neither oxidized nor reduced. It is concluded that ferrous iron oxidation by strain BS-1 is only a side activity of this bacterium that cannot support growth exclusively with this electron source over prolonged periods of time.

Fossilized iron bacteria reveal a pathway to the biological origin of banded iron formation

“…3c-e, and Supplementary Movies 1 and 2 give a deeper appreciation of depth. We did not observe such geometries in portions of the rock populated by single cells, suggesting that a strong prerequisite for the formation of these 3D micro-fabrics was that the cells were immobilized in the stalk production phase and fossilized in their original spatial position, which is possible with R. vanielii 7,8 .…”

“…Three major mechanisms are currently implicated in the oxidation of Fe 2 þ to Fe 3 þ : abiotic O 2 precipitation 1,26 , UVdriven photochemical Fe 2 þ oxidation 1,2 and microbial transformation 1,3,[6][7][8][9][10][11][12][13][14]28 . In addition to the low Archaean O 2 levels, recent experimental studies have rejected photochemical Fe 2 þ oxidation as a potential major contributor to BIF formation 1,3 .…”

“…4a-c), exospore-like features (Fig. 4b,d) and fossilization mechanism, we interpret as fossil relatives of the anoxygenic photoferrotroph, Rhodomicrobium vanielii 7,8,23,24 . Similar to their modern counterparts, these microorganisms grew by polar budding, forming vast networks of cells connected end-to-end by stalk appendages, when transforming Fe 2 þ to Fe 3 þ during anoxygenic photosynthesis.…”

“…However, R. vanielii becomes completely encrusted during anaerobic phototrophic growth on Fe, leading to the arrest of cell growth and Fe metabolism 7,8 . This probably accounts for the remarkable preservation we report here and for the fact that the cells and their stalks were heavily encrusted by haematite.…”

“…Instead, biotic, anoxygenic photoferrotrophic precipitation according to the equation 4Fe 2 þ þ CO 2 þ 11H 2 O þ light-[CH 2 O] þ 4Fe(OH) 3 þ 8H þ has been proposed [6][7][8][9][10][11][12][13][14] . With the exemption of proxies such as iron isotopes 12,13 , there exists no direct environmental evidence-neither in ancient nor in modern ecosystems-demonstrating how photoferrotrophs could have accounted for vast-scale biological BIF deposition, including the formation of their spectacular banded consistency.…”