|Bielefeld University||Department of Chemistry||Physical and Biophysical Chemistry||deutsch|
Algae as Model Organisms
The Green Alga Chlamydomonas
Chlamydomonas reinhardtii is a unicellular green alga with a size of about 10 micrometers and is found all over the world. It serves as a simple model organism for the investigation of photosynthesis and processing of light signals. With the help of two flagella it reacts to the light conditions of the environment (phototaxis). It uses a biological clock for daily time keeping, which is synchronized by light (circadian rhythm). To investigate the entrainment of this rhythm Chlamydomonas has already been sent to space on the Space Shuttle.
The Diatom Phaeodactylum
Diatoms are unicellular, photosynthetic organisms, which are found in marine and freshwater environments, for example as part of the plankton. Diatoms contribute significantly (around one fifth) to primary productivity on Earth. Phaeodactylum tricornutum is one of the best characterized diatoms. Its genome has been fully sequenced.
Typical for a member of the plant family, Chlamydomonas contains phototropin und cryptochromes. However, it is lacking the whole family of the red light sensitive phytochromes. Instead, the alga contains an animal-like cryptochrome (aCRY), which is the only flavoprotein so far that is activated by red light (see Plant Cell 2012). Additionally, rhodopsins are found in the green alga, some of them with homology to archaebacterial receptors. The latter include the channelrhodopsins, which are responsible for phototaxis. Phototropin plays a role in the process of switching from vegetative to sexual reproduction. aCRY regulates the transcription of a series of genes.
The green alga thereby offers the opportunity to study the mechanism of almost all common photoreceptors within a comparatively simple organism. For our biophysical investigations, we overproduce the photoreceptors or segments thereof in larger amounts in E. coli (such as the cryptochrome, see J. Biol. Chem. 2007). From similarities or differences in the properties of the Chlamydomonas photoreceptors as compared to those of higher plants, one can draw conclusions on the relevant processes and observe evolutionary developments.
Phaeodactylum contains different photoreceptors than plants, because it is a secondary endosymbiont. Cryptochromes and probably also phytochromes are still found in the diatom. However, phototropin is not produced. Instead, aureochrome regulates some of the blue light responses such as the acclimation to high light.
Impact in Biotechnology, Medicine and Climate Change
-Genetically modified Chlamydomonas cells produce hydrogen or isoprene upon illumination under suitable conditions.
They are candidates for a direct, environmentally sound conversion of solar energy into fuel.