The temperature-dependent accumulation of Mg-protoporphyrin IX and reactive oxygen species in Chlorella vulgaris

Photosynthetic organisms undergo photoacclimation in response to changes in environmental conditions to maximize energy production and at the same time protect the light-sensitive pigments and proteins from excess light. Low temperature and high irradiance both cause the electron transport chain to become more reduced which can result in the production of damaging reactive oxygen species. In the unicellular green alga Chlorella vulgaris Beij., light and temperature regulate light-harvesting protein accumulation via the redox state of the plastoquinone pool. To investigate temperature-dependent factor (s) regulating light-harvesting protein accumulation we measured the abundance of chlorophyll biosynthetic precursors and reactive oxygen species production in C. vulgaris cells acclimated to a series of growth conditions. We observed that Mg-protoporphyrin accumulates in response to low temperature, but its abundance does not correlate with light-harvesting protein levels. Reactive oxygen levels measured under the same growth conditions strongly correlated with light-harvesting protein levels. Therefore, we suggest that reactive oxygen species may act as part of both a temperature- and irradiance-dependent signalling mechanism in the regulation of light-harvesting protein accumulation in response to growth conditions.