  A production of heat by mitochondria is critical for maintaining body
temperature, regulating metabolic rate and preventing oxidative damage to
mitochondria and cells. Up to now mitochondrion heat production was
characterized only by methods based on fluorescent probes which are sensitive
to environmental variations (viscosity, pH, ionic strength, quenching etc.).
Herein, for the first time the heat release of isolated mitochondria was
unambiguously measured by a diamond thermometer (DT) which is absolutely
indifferent to external non-thermal parameters. We show that during total
uncoupling of transmembrane potential by CCCP application the temperature near
mitochondria rises by 4-22 {\deg}C above the ambient temperature, with an
absolute maximum of 45 {\deg}C. Such a broad temperature response may be
associated with the heterogeneity of the mitochondria themselves as well as
their aggregations in the isolated suspension. It also revealed spontaneous
temperature bursts prior to CCCP application that can reflect involvement of
some mitochondria to ATP synthesis or membrane potential leaking to avoid
reactive oxygen species hyperproduction. The used temperature sensor and the
data obtained shed light to the thermodynamics on the subcellular level.
