Absence of humic substance reduction by the acidophilic Fe(III)-reducing strain Acidiphilium SJH: implications for its Fe(III) reduction mechanism and for the stimulation of natural organohalogen formation

A vast amount of volatile organohalogens (VOX) has natural origins. Both soils and sediments have been shown to release VOX, which are most likely produced via redox reactions between Fe(III) and quinones in the presence of halide anions, particularly at acidic pH. We tested whether acidophilic Fe(III)-reducers might indirectly stimulate natural VOX formation at acidic pH by providing reactive Fe and quinone species. However, it is unknown whether acidophilic Fe(III)-reducers can reduce humic acids (HA) or fulvic acids (FA). We therefore tested the ability of the acidophilic Fe(III)-reducer Acidiphilium SJH to reduce macromolecular, suspended HA and dissolved FA at pH 3.1–3.3. We found that (i) SJH can neither reduce HA/FA nor the humic model quinone anthraquinone-2,6-disulfonic-acid (AQDS) nor stimulate the formation of FA radicals, (ii) at acidic pH, significantly more electrons are transferred abiotically both from native and reduced FA to dissolved Fe(III) than from native or reduced HA, and (iii) the presence of strain SJH does not stimulate VOX formation. Our results imply that the acidophilic Fe(III)-reducer SJH either uses an enzyme for Fe(III) reduction that can neither be used for HA/FA nor for AQDS reduction or that the location of Fe(III) reduction is inaccessible for these compounds. We further conclude that microorganisms such as strain SJH probably do not indirectly stimulate natural VOX formation at acidic pH via the formation of reactive quinone species.