Coenzyme Q (CoQ; ubiquinone) and plastoquinone (PQ) are metabolic electron carriers that, in their reduced
state, are powerful antioxidants for cellular protection against oxidative damage. Although damage resulting
fromgeneration of reactive oxygen species (ROS) is strongly implicated in the initiation of symbiotic dysfunction
that leads to coral bleaching, very little is known about the redox state of these two prenylquinone pools during
the stress response. Here we describe a quantitative liquid chromatography–mass spectrometry (LC–MS)method
that permits simultaneous measurement of the physiological redox state of both CoQ and PQ in whole corals.
The application of this method indicates that the CoQ and PQ pools in the coral–Symbiodinium symbiosis are
maintained predominantly in their reduced (antioxidant) forms, and it is the coral CoQ redox state that is
most affected by acute thermal stress.