Microbial metabolism plays a key role in controlling the fate of toxic groundwater
contaminants such as arsenic. Dissimilatory metal reduction catalysed by subsurface bacteria
can facilitate the mobilisation of arsenic via the reductive dissolution of As(V)-bearing Fe(III)
mineral assemblages. The mobility of liberated As(V) can ...
Microbial metabolism plays a key role in controlling the fate of toxic groundwater
contaminants such as arsenic. Dissimilatory metal reduction catalysed by subsurface bacteria
can facilitate the mobilisation of arsenic via the reductive dissolution of As(V)-bearing Fe(III)
mineral assemblages. The mobility of liberated As(V) can then be amplified via reduction to
the more soluble As(III) by As(V)-respiring bacteria. This investigation focused on the
reductive dissolution of As(V) sorbed onto Fe(III)-(oxyhydr)oxide by model Fe(III)- and
As(V)-reducing bacteria, to elucidate the mechanisms underpinning these processes at the
single cell scale. Axenic cultures of Shewanella sp. ANA-3 wild-type cells (able to respire both
Fe(III) and As(V)) were grown using C-labelled lactate on an arsenical Fe(III)-
(oxyhydr)oxide thin film, and after colonisation, the distribution of Fe and As in the solid phase
was assessed using nanoscale secondary ion mass spectrometry (NanoSIMS), complemented
with aqueous geochemistry analyses. Parallel experiments were conducted using an arrA
mutant, able to respire Fe(III) but not As(V). NanoSIMS imaging showed that most
metabolically active cells were not in direct contact with the Fe(III) mineral. Flavins were
released by both strains, suggesting that these cell-secreted electron shuttles mediated
extracellular Fe(III)-(oxyhydr)oxide reduction, but did not facilitate extracellular As(V)
reduction, demonstrated by the presence of flavins yet lack of As(III) in the supernatants of the
arrA deletion mutant strain. 3D reconstructions of NanoSIMS depth-profiled single cells
revealed that As and Fe were associated with the cell surface in the wild-type cells, whereas
for the arrA mutant only Fe was associated with the biomass. These data were consistent with
Shewanella sp. ANA-3 respiring As(V) in a multistep process; first the reductive dissolution
of the Fe(III) mineral released As(V), and once in solution, As(V) was respired by the cells to
As(III). As well as highlighting Fe(III) reduction as the primary release mechanism for arsenic,
our data also identified unexpected cellular As(III) retention mechanisms that require further
investigation.