The Variability of the Photosynthetic Unit in Chlorella I. The Effect of Vanadium on Photosynthesis, Productivity, P-700 and Cytochrome f in Undiluted and Homocontinuous Cultures of Chlorella.

Chlorella fusca was grown autotrophically in undiluted cultures over a period of seven days under two different light intensities (5, 26 W · m(-2)) and under four concentrations of vanadate (1, 20, 100, 500 μg·l(-1)) in the nutrient medium. It was shown that vanadium enhances the chlorophyll content under both light conditions depending on the age of the cultures. A stimulating effect of vanadium on the dry weight production and the photosynthetic activity can be observed only in strong light. In a second series of experiments, cells were grown under homocontinuous conditions. In this case the chlorophyll content · l(-1) suspension was kept constant by automatic dilution at a very low cell density. The results demonstrate that vanadium enhances the ratio of chlorophyll per dry weight only in low light. The light saturating curves show that vanadium induces a higher photosynthetic activity in high light, but not in low light. Vanadium stimulates the productivity in strong light by 50%. The ratio of P-700 per 1000 chlorophylls rises from 1.4 to 2.0 in high light and from 0.9 to 1.2 in low light. Contrary to this, the quotient cytochrome f per 1000 chlorophylls is affected only in high light. It runs up from 2.6 to 4.8. In a third experimental run, the measurements were repeated when EDTA (20 μmol·l(-1)) was added to the nutrient medium. It is shown that EDTA can reduce the effects of sublethal iron stress, which is the condition when the vanadium effect becomes most obvious. Under conditions of facilitated iron uptake the vanadium effect on photosynthesis, P-700 and cytochrome f disappears. The regulation capacity of vanadium on the photosynthetic apparatus is discussed on the basis of the variability of the photosynthetic unit. It is shown that vanadium can be used as an environmental factor to control different components of the photosynthetic apparatus in a different way. The photosynthetic capacity and the content of cytochrome f are strongly correlated; on the other hand, P-700 is changed independently from photosynthesis. This gives evidence to define the PSU as the ratio of chlorophyll to cytochrome f.