PHOTOACCLIMATION IN PHOTOSYNTHETIC MICROORGANISMS: AN ULTRASTRUCTURAL RESPONSE

Abstract

Photosynthetic microorganisms are able to modify their chemical composition, cellular structure, and organization of their chloroplasts in response to the level of irradiance. The photosynthetic apparatus adjusts itself to any new light regime by changing the ultrastructural properties of the chloroplast to provide space and area needed to match other biochemical changes in order to optimize light harvesting and utilization. Acclimation to low light intensity is characterized by an increase in thylakoid number in cyanobacteria, and in the chloroplast volume in eukaryotic plants. In the Eukaryota, these changes allow the packaging of more thylakoids within this organelle to harbor the addition of photosynthetic complexes, i.e., light harvesting antennae, reaction centers, and electron transport components. These changes are essential for optimal operation of the photosynthetic apparatus at low light intensity, mainly to increase the absorption of light energy. Acclimation to high irradiance is characterized by a reduction of the surface density of thylakoid membranes and reduction in the specific volume of the chloroplast. The accumulation of storage bodies containing starch and lipids is yet another typical feature of high light acclimated cells in response to the high rate of photosynthetic activity.