  Life in the clouds of Venus, if present in sufficiently high abundance, must
be affecting the atmospheric chemistry. It has been proposed that abundant
Venusian life could obtain energy from its environment using three possible
sulfur energy-metabolisms. These metabolisms raise the possibility of Venus's
enigmatic cloud-layer SO$_2$-depletion being caused by life. We here couple
each proposed energy-metabolism to a photochemical-kinetics code and
self-consistently predict the composition of Venus's atmosphere under the
scenario that life produces the observed SO$_2$-depletion. Using this
photo-bio-chemical kinetics code, we show that all three metabolisms can
produce SO$_2$-depletions, but do so by violating other observational
constraints on Venus's atmospheric chemistry. We calculate the maximum possible
biomass density of sulfur-metabolising life in the clouds, before violating
observational constraints, to be $\sim10^{-5}\,-\,10^{-3}\,{\rm mg\,m^{-3}}$.
The methods employed are equally applicable to aerial biospheres on Venus-like
exoplanets, planets that are optimally poised for atmospheric characterisation
in the near future.
