Circadian patterns in key physiological processes of the marine diatom Phaeodactylum tricornutum

Ragni, Maria (2005). Circadian patterns in key physiological processes of the marine diatom Phaeodactylum tricornutum. PhD thesis The Open University.



Circadian rhythms are a widespread feature among all the levels of organismal complexity, from bacteria to humans. Nevertheless their ecological relevance in marine phytoplankton has not been clearly assessed.
In this study the diatom P. tricomutum, was used as a model for giving an integrated picture of the circadian patterns of phytoplankton photophysiology in relation to the external light field and for assessing the interaction between the cell cycle and the temporal control of processes like pigment synthesis and photosynthesis.
The results suggest that light/dark shifts are crucial for regulating the timing of different cell activities, including the synthesis of photoprotectant pigments (i.e., diadinoxanthin). The photosynthetic processes run at different paces in different stages of the cell cycle and different times of the day. This would imply a synchronization of cell processes due to both internal and external entrainer/s. When the external trigger is removed (e.g., in absence of light/dark shifts), the time distribution of the cell cycle stages quickly flattens, the efficiency of the use of light lowers, as well as the flexibility in facing light variations. Though, this does not always involve the loss of synchronization of processes like pigment synthesis, or the activation/inactivation of the photosynthetic machinery.
The results from an in situ experiment support the occurrence of a circadian regulation of photophysiological processes in natural phytoplankton, at least in the upper layer of the water column, where solar radiation can act as a signal for the entrainment of circadian rhythms, whether or not mediated by specific photoreceptors.
The overall picture is that the presence of systems for the circadian control of physiological activity would be a successful feature in marine phytoplankton, which is reasonable also in an evolutionary perspective, since the day/night cycle is a permanent pattern in the Earth system.

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