Sulfur-oxidizing chemoautotrophs such as the
Thiomicrospiras, which are important in marine habitats,
can use the oxidation of reduced sulfur compounds
to fuel carbon fixation (and growth).

Depicted here is a culture plate filled with culture medium
that includes the pH indicator phenol red.
This compound is red/pink at neutral pH, and yellow at acidic pH.

Thiomicrospira crunogena cells are growing at the top,
and the medium is turning yellow as a result.
Like many sulfur-oxidizing chemoautotrophs,
this organism acidifies its medium as it grows,
due to the accumulation of sulfuric acid
produced as a result of sulfur oxidation.