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12.4: Pigments and Evolutionary Adaptations

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  • Cyanobacteria were potentially the first organisms to do oxygenic photosynthesis -- the variety of photosynthesis that produces oxygen as a waste product. To do this, cyanobacteria use the pigment chlorophyll a. This is the only pigment directly involved in photosynthesis, but other pigments called accessory pigments can absorb wavelengths of light, then transfer this energy to chlorophyll a. Cyanobacteria have accessory pigments called phycobilins that allow them to absorb more of the blue and red portions of the spectrum of light. The pigment phycocyanin, a blue phycobilin, resulted in the cyanobacteria’s more common name: blue-green algae.

    Which part of the spectrum of light do you think phycocyanin absorbs?

    The red algae were the first lineage of organisms to have true chloroplasts, derived from the endosymbiosis of a cyanobacterium, and so they have the same pigments: chlorophyll a and phycobilins. In particular, the red algae derive most of their coloring from a red phycobilin called phycoerythrin. This pigment reflects red, meaning it absorbs the blue portion of the spectrum.

    Red algae are almost entirely marine and have been found deep in the water column. The record for algal depth was set by a coralline red alga found 268 m (about 880 feet) below the surface. Why is this so impressive? As light travels through water molecules, much of it is reflected back or refracted off course. High-energy blue light is more likely to penetrate deeper into the water column, so it is essential that the algae are able to capture it. Because both chlorophyll a and phycocyanin would reflect much of this blue light instead of absorbing it, red algae make use of phycoerythrin to capture it, making them appear red.

    Green algae are more likely to live in shallow marine waters, freshwater, or even terrestrially. This group of organisms has lost the phycobilins present in its red algal ancestors and has adapted chlorophyll b and carotenoids, instead. Carotenoids can help mitigate damage caused by solar radiation, as well as absorb some of the green parts of the spectrum. These pigments are reds, oranges, and yellows..

    As you saw in your TLC strip, chlorophyll b appears more yellow while chlorophyll a appears more blue. What portion of the spectrum is chlorophyll b absorbing that chlorophyll a is not?

    What color were the carotenoids on your TLC strip?

    Where might you see carotenoids in your daily life?

    Brown algae have chlorophyll a and chlorophyll c, as well as the carotenoid fucoxanthin, which gives them their golden brown color. Compare the available specimens of cyanobacteria, red algae, green algae, and brown algae. What do the colors of these organisms tell you about where they live?

    Which pigment do all of these organisms have?

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