Growth rate and control of development of the photosynthetic apparatus in Chloroflexus aurantiacus

Abstract

Chloroflexus aurantiacus was grown photoheterotrophically in a chemostat in order to study the influence of growth rate on the formation of bacteriochlorophyll a (Bchl a) which represents the membrane-bound photosynthetic pigment complexes, and of Bchl c which represents the light harvesting pigment-proteins of the chlorosome. Steady state cell protein levels as well as specific Bchl a contents increased linearly and specific Bchl c contents exponentially when the dilution rate, representing growth rate, was decreased. In spite of differences in the light intensities, continuous cultures growing at comparable growth rates and densities exhibited comparable specific contents of both Bchls and largely identical molar ratios of Bchl c/Bchl a. The growth rate of constantly illuminated batch cultures was varied by changing the concentration of growth-limiting nutrients. Cultures growing at higher growth rates showed higher cell densities but lower specific Bchl levels as well as lower molar ratios of Bchl c/Bchl a than cultures growing at low growth rate. Determination of the light energy flux required for half-maximal saturation of photosynthetic activity (light dependent proton extrusion) by chemostat cultures showed a dependency of that activity by the content of cellular Bchl c. In summary, the results suggest that, growth rate or a factor regulating growth rate, rather than light affected specific Bchl levels and because of the increasing molar ratio of Bchl c to Bchl a, the light harvesting capacity and photosynthetic efficiency of the photosynthetic apparatus.