Light-dependent induction of strongly increased microalgal growth by methanol

Low methanol concentrations (about 0.5% v/v) induce biomass production in cultures of the unicellular green alga Scenedesmus obliquus by more than 300%, compared to controls without this solvent. This effect on the microalgal growth was found to be dependent on the solvent concentration, the packed cell volume (PCV), light intensity and light quality. It could be shown that methanol addition leads to a decrease in size of the light harvesting complex (LHC) on the basis of chlorophylls and proteins, and thus to changes in structure and functioning of the photosynthetic apparatus. These alterations lead to enhanced photosynthesis and respiration rates. The action of methanol on the photosynthetic apparatus is thus comparable to the effect of enhanced CO(2) concentrations. These findings support the previously proposed pathway for methanol metabolization with CO(2) as the final product. We conclude that the subsequent assimilation of the increased CO(2) amounts by the Calvin-Benson cycle is a possible explanation for the methanol-mediated increase in biomass production in terms of PCV. The methanol effect is observed only in the light and in the presence of a functioning photosynthetic apparatus. Preliminary action spectra suggest that the primary photoreceptor is a chlorophyll-protein complex with two absorption maxima at 680 and 430 nm, which may possibly be attributed to the reaction center of photosystem II (PSII).