Laboratory Simulation of an Iron(II)-rich Precambrian Marine Upwelling System to Explore the Growth of Photosynthetic Bacteria

Abstract

A conventional concept for the deposition of some Precambrian Banded Iron Formations (BIF) proceeds on the assumption that ferrous iron [Fe(II)] upwelling from hydrothermal sources in the Precambrian ocean was oxidized by molecular oxygen [O2] produced by cyanobacteria. The oldest BIFs, deposited prior to the Great Oxidation Event (GOE) at about 2.4 billion years (Gy) ago, could have formed by direct oxidation of Fe(II) by anoxygenic photoferrotrophs under anoxic conditions. As a method for testing the geochemical and mineralogical patterns that develop under different biological scenarios, we designed a 40 cm long vertical flow-through column to simulate an anoxic Fe(II)-rich marine upwelling system representative of an ancient ocean on a lab scale. The cylinder was packed with a porous glass bead matrix to stabilize the geochemical gradients, and liquid samples for iron quantification could be taken throughout the water column. Dissolved oxygen was detected non-invasively via optodes from the outside. Results from biotic experiments that involved upwelling fluxes of Fe(II) from the bottom, a distinct light gradient from top, and cyanobacteria present in the water column, show clear evidence for the formation of Fe(III) mineral precipitates and development of a chemocline between Fe(II) and O2. This column allows us to test hypotheses for the formation of the BIFs by culturing cyanobacteria (and in the future photoferrotrophs) under simulated marine Precambrian conditions. Furthermore we hypothesize that our column concept allows for the simulation of various chemical and physical environments -including shallow marine or lacustrine sediments. Keywords AbstractA conventional concept for the deposition of some Precambrian Banded Iron Formations (BIF) proceeds on the assumption that ferrous iron [Fe(II)] upwelling from hydrothermal sources in the Precambrian ocean was oxidized by molecular oxygen [O 2 ] produced by cyanobacteria. The oldest BIFs, deposited prior to the Great Oxidation Event (GOE) at about 2.4 billion years (Gy) ago, could have formed by direct oxidation of Fe(II) by anoxygenic photoferrotrophs under anoxic conditions. As a method for testing the geochemical and mineralogical patterns that develop under different biological scenarios, we designed a 40 cm long vertical flow-through column to simulate an anoxic Fe(II)-rich marine upwelling system representative of an ancient ocean on a lab scale. The cylinder was packed with a porous glass bead matrix to stabilize the geochemical gradients, and liquid samples for iron quantification could be taken throughout the water column. Dissolved oxygen was detected non-invasively via optodes from the outside. Results from biotic experiments that involved upwelling fluxes of Fe(II) from the bottom, a distinct light gradient from top, and cyanobacteria present in the water column, show clear evidence for the formation of Fe(III) mineral precipitates and development of a chemocline between Fe(II) and O 2 . This column allows us to test hypotheses for t...

Oxic Fe(III) reduction could have generated Fe(II) in the photic zone of Precambrian seawater

“…On day 12, the Fe(II) concentration in the bottom of the column (266 µM at −27 cm; Fig. 2 ) was reflective of this new input, with some loss of Fe(II) due to abiotic oxidation with oxygen that diffused into the medium reservoir and column, such as was observed in an abiotic column experiment 16 . However, day 12 Fe(II) concentrations were highest in the top of the column (338 μM at −0.9 cm).…”

“…The abiotic experiment (described in ref. 16 and Supplementary Information) maintained O 2 concentrations at or below 4.7 μM, near the optimal resolution of the oxygen sensor (2.83 ± 0.14 µmol) and so 4.7 μM is our operationally-defined anoxic threshold for the column. This low level of oxygen was likely sustained through fittings, tubing, and connection, despite using the lowest oxygen diffusivity materials reasonable 16 .…”

“…Extensive details of column design, setup, and operation have been published 16 , and so are described only briefly here. The column was a modified 1 L glass graduated cylinder (Fig.…”

“…Optodes glued to the inner wall of the column were used to non-invasively monitor the dissolved oxygen concentration in the headspace, water-saturated column, and medium reservoirs with a fiber-optic light source (FiBox, PreSens, GmbH) 16 . Two optodes were placed in the headspace, and eleven more in the saturated part of the column, between −0.2 and −18.3 cm (Fig.…”

“…To address these questions, we utilized a flow-through column where a marine planktonic cyanobacterium was grown in a gradient of Fe(II)-rich seawater medium advectively supplied from the bottom, and light provided from the top 16 . This column is a laboratory-scale analogue to both the water column and shallow microbial mat environments that would have been present along a Precambrian continental margin exposed to upwelling Fe(II)-rich waters (Fig.…”