  We report on the modification of the spectrum of a passive scalar inside a
turbulent flow by the injection of large bubbles. While the spectral
modification through bubbles is well known and well analyzed for the velocity
fluctuations, little is known on how bubbles change the fluctuations of an
approximately passive scalar, in our case temperature. Here we uncover the
thermal spectral scaling behavior of a turbulent multiphase thermal mixing
layer. The development of a $-3$ spectral scaling is triggered. By injecting
large bubbles ($\text{Re}_{\text{bub}} = \mathcal{O}(10^2)$) with gas volume
fractions $\alpha$ up to 5\%. For these bubbly flows, the $-5/3$ scaling is
still observed at intermediate frequencies for low $\alpha$ but becomes less
pronounced when $\alpha$ further increases and it is followed by a steeper $-3$
slope for larger frequencies. This $-3$ scaling range extends with increasing
gas volume fraction. The $-3$ scaling exponent coincides with the typical
energy spectral scaling for the velocity fluctuations in high Reynolds number
bubbly flows. We identify the frequency scale of the transition from the $-5/3$
scaling to the $-3$ scaling and show how it depends on the gas volume fraction.
