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RESEARCH PRODUCT

The Variability of the Photosynthetic Unit in Chlorella Fusca

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The concept of the photosynthetic unit (PSIT) was introduced by Gaffron and Wohl (1936) on the basis of Emerson and Arnold’s flash light experiments (1932). The PSU was defined as the minimum of chlorophyll molecules required for the evolution of one O2. Later on, the PSU was considered to be the complex of one reaction center and its appropriate antenna of light harvesting chlorophyll molecules. Assuming that there is an equal number of the PS I and PS II reaction centers the PSU was mostly expressed as Chl/P-700. However, recently it could be shown that the ratio of PS II/PS I is variable depending on the light conditions during growth (Kawamura et al., 1979; Myers, Graham, 1982). Furthermore, there are contradictory results about the relationship between the level of light saturated photosynthesis (Pmax) and the ratio of Chl/P-700. It has been claimed that the quotient Chl/P-700 remains almost unchanged during high light adaptation (Bjorkman, 1981), Prezelin (1981) reports a strong correlation between the ratio of Chl/P-700 and Pmax in some species of algae, supposing the existence of different growth strategies. Referring to the physiological meaning of the PSU, it must be emphasized* that it decreases the discrepancy between light absorption and dark reaction capacity. Thus, Wild (1979) proposed the define the PSU as the molar ratio of chlorophyll per rate limiting component of the electron transport chain. In this work we report a correlation analysis between Pmax and the concentrations of P-700 and cytochrome f in order to investigate the component which is able to indicate the capacity of the rate limiting step in photosynthesis.