This study was motivated by surprising gaps in the current knowledge of
microbial inorganic carbon (Ci) uptake and assimilation at acidic pH values (pH
< 3). Particularly striking is the limited understanding of the differences
between Ci uptake mechanisms in acidic versus circumneutral environments where
the Ci predominantly occurs either as a dissolved gas (CO2) or as bicarbonate
(HCO3 -), respectively. In order to gain initial traction on the problem, the
relative abundance of transcripts encoding proteins involved in Ci uptake and
assimilation was studied in the autotrophic, polyextreme acidophile
Acidithiobacillus ferrooxidans whose optimum pH for growth is 2.5 using ferrous
iron as an energy source, although they are able to grow at pH 5 when using
sulfur as an energy source. The relative abundance of transcripts of five
operons (cbb1-5) and one gene cluster (can-sulP) was monitored by RT-qPCR and,
in selected cases, at the protein level by Western blotting, when cells were
grown under different regimens of CO2 concentration in elemental sulfur. Of
particular note was the absence of a classical bicarbonate uptake system in A.
ferrooxidans. However, bioinformatic approaches predict that sulP, previously
annotated as a sulfate transporter, is a novel type of bicarbonate transporter.
A conceptual model of CO2 fixation was constructed from combined bioinformatic
and experimental approaches that suggests strategies for providing ecological
flexibility under changing concentrations of CO2 and provides a portal to
elucidating Ci uptake and regulation in acidic conditions. The results could
advance the understanding of industrial bioleaching processes to recover metals
such as copper at acidic pH. In addition, they may also shed light on how
chemolithoautotrophic acidophiles influence the nutrient and energy balance in
naturally occurring low pH environments.
