Photosynthetic organisms are brilliant artists when it comes to their use of colors. Most people are familiar with the wide range of colors plants display. Roses are red, violets are blue, anyone? Well, cyanobacteria package prodigious amounts of pigments in prokaryotic form. Their name is derived from their blue-green color. It’s kinda their trademark. As photosynthetic organisms, they have plenty of chlorophyll in their photosynthetic machinery. Their blue color comes from their light-harvesting antenna, the phycobilisomes (PBS), which are comprised of proteins with blue pigments.
Phycobiliproteins bind blue bilin pigments and assemble into discs; these discs assemble into the large phycobilisome complexes that sit on top of the photosystems and funnel light energy to them. The PBS looks like a giant blue octopus with a central core and a number of ‘arms’ of phycobiliprotein discs that extend out in all directions. Because these complexes are particularly good at absorbing orange light, which chlorophyll does not, the PBSs extend the wavelengths of light (and therefore the energy) available to cyanobacterial photosystems.
Some cyanobacteria like Fremyella diplosiphon can take color to another level. They can change the composition of their PBSs according to the color of light available to them in a process called complementary chromatic adaptation (CCA). This is possible because some species of cyanobacteria have two varieties of phycobiliproteins, each of which coordinates a distinct pigment. Phycocyanin binds the blue phycocyanobilin chromophore, which efficiently absorbs orange and red light and appears blue in color. Phycoerythrin binds the pigment phycoerythrobilin, which efficiently absorbs green light and appears pink to red in color. Thus, these cyanobacteria have blue PBSs in red light and have a typical blue-green appearance. However, in green light, they produce PBSs containing phycoerythrin and have a rose or red-colored appearance.To achieve this dramatic color change, the cells have elaborate systems for sensing the incident light wavelengths in their environment and eliciting the appropriate response. Changing out the PBS antenna requires changes in gene expression of the protein components as well as the pathways that synthesize the necessary pigment (phycocyanin vs. phycoerythrin). It also means that the existing phycobiliproteins and their pigments must be degraded and their components recycled. The PBS antenna complexes comprise a significant portion of soluble protein within cyanobacterial cells, so it is important that the pieces be reused, no matter the light condition. All of these requirements add up to a sophisticated system of multiple players working to make this simple-looking color change happen on cue. This phenomenon is the research area of a number of groups working on cyanobacteria and photosynthesis.
In this way, cyanobacteria are similar to Picasso, going through a blue period and later a rose period. In the case of Fremyella, the change is not due to swings in mood from melancholy to cheer, nor is it purely aesthetic. In biology, even beauty is functional. In the case of complementary chromatic adaptation, it allows cyanobacteria to most efficiently harvest the wavelengths of light available to them. You may be wondering when cyanobacterial cells would be exposed to differently colored light sources outside of an elementary school science fair project. Well, this scenario is more commonplace than you would think. For example, cyanobacteria cells close to the surface of a pond or ocean would be exposed to higher amount of red light compared to those deeper in the water column.* Those cyanobacterial cells that find themselves deeper in the water column would have much less red light because it does not penetrate as well as green light and other photosynthetic organisms living closer to the surface have already absorbed all of the red wavelengths of light allowing only green to pass through.**
The image on the left may appear to be some kind of impressionism masterpiece or abstract work by a yet-to-be-discovered artist, but really it’s the combination of two cultures of Fremyella. The green side is the normal strain, while the red side is a mutant that contains only the red phycoerythrin pigment.
This post was written for the April Berry Go Round blog Carnival. Plants aren’t the only photosynthetic organisms that use color beautifully and efficiently.
*For those of you paying attention, this means they would use the blue phycocyanin-pigmented PBSs and appear blue-green.
**This means that cyanobacterial cells deeper in the water would adapt to use phycoerythrin in their PBSs and appear red.
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