Q-291. Coupling Microbial Oxidation of FE(II) to Photooxidation of Natural Organic Matter as a Straregy for Growth by Rhodobacter capsulatus SB1003

A. J. Poulain, D. K. Newman;
Massachusetts Inst. of Technology, Cambridge, MA.

The metabolically diverse purple non-sulfur bacterium, Rhodobacter capsulatus SB1003, grows poorly photolithotrophically when ferrous iron is the sole electron source, and hence, might be expected to contribute negligibly to Fe(II) oxidation in anoxic environments. Recent work showed that R. capsulatus can grow photoheterotrophically using the products of the photooxidation of organic compounds it would typically be unable to metabolize (i.e. citrate and nitrilotriacetate). This process is enabled via light-dependent enzymatic oxidation of Fe(II) providing the ferric iron involved in the photochemical reaction. To further understand this coupling between microbial oxidation of Fe(II) and the subsequent photoreduction of Fe(III) as a way for bacteria to access a pool of otherwise unavailable organic carbon, we are: i) identifying the genetic basis of this process using random transposon mutagenesis, ii) assessing the potential of more complex natural organic matter to be metabolized after Fe(III)-dependent photodegradation by testing a suite of IHSS fulvic acids and iii) characterizing the kinetics of growth, as well as that of Fe(II) oxidation and Fe(III) photoreduction. Preliminary results suggest that fulvic acids have the potential to enable Fe(II) oxidation. Blue to UV radiations are most likely involved in the photochemical reaction and further experiments will determine the optimum light regimen for both microbial and photochemical reactions and assess whether these reactions require spatial coupling. So far, the genes identified as being involved in Fe(II) oxidation by SB1003 appear to encode a cytochrome-like protein involved in anoxygenic photosynthesis but await confirmation via complementation analysis. The results of these studies will allow us to: i) better characterize the environments in which Fe(II) biooxidation is subsequently coupled to the photooxidation of organic carbon thus affecting both the C and Fe biogeochemical cycles; ii) provide rates of Fe redox transformations and thus better model Fe fluxes from anoxic environments or at the oxic/anoxic interfaces and iii) assess the diversity of molecular pathways involved in phototrophic Fe(II) oxidation.