5.11E: Carotenoids and Phycobilins

Learning Objectives

• Distinguish between carotenoids and phycobilins

Figure: Microbial Mats Around the Grand Prismatic Spring: Thermophiles produce some of the bright colors of Grand Prismatic Spring, Yellowstone National Park. The color of the mats of algae and bacteria is due to the ratio of chlorophyll to carotenoid molecules produced by the organisms. During summertime the chlorophyll content of the organisms is low and thus the mats appear orange, red, or yellow. However during the winter, the mats are usually dark green, because sunlight is more scarce and the microbes produce more chlorophyll to compensate, thereby masking the carotenoid colors.

Photosynthesis in many plants and algae depend on chlorophylls which absorb light closer to the ultraviolet side of the spectrum, and emit light in the green end of the spectrum. However during certain times of the year or in various location most of the light may be shifted to other wavelengths away from the ultraviolet spectrum. To deal with these problems, organisms dependent on photosynthesis express various compounds that allow them to absorb different spectrum of light. Notably are carotenoids and phycobilins.

Chromoplasts of plants and some other photosynthetic organisms like algae, some bacteria, and some fungi. Carotenoids can be produced from fats and other basic organic metabolic building blocks by all these organisms. Carotenoids generally cannot be manufactured by species in the animal kingdom so animals obtain carotenoids in their diets, and may employ them in various ways in metabolism.There are over 600 known carotenoids; they are split into two classes, xanthophylls (which contain oxygen) and carotenes (which are purely hydrocarbons, and contain no oxygen). All carotenoids are tetraterpenoids, meaning that they are produced from 8 isoprene molecules and contain 40 carbon atoms. Carotenoids in general absorb blue light. They serve two key roles in plants and algae: they absorb light energy for use in photosynthesis, and they protect chlorophyll from photodamage.

Phycobilins (from Greek: φ (phykos) meaning “alga”, and from Latin: bilis meaning “bile”) are chromophores (light-capturing molecules) found in cyanobacteria and in the chloroplasts of red algae, glaucophytes and some cryptomonads (though not in green algae and higher plants). They are unique among the photosynthetic pigments in that they are bonded to certain water-soluble proteins, known as phycobiliproteins. Phycobiliproteins then pass the light energy to chlorophylls for photosynthesis.The phycobilins are especially efficient at absorbing red, orange, yellow, and green light, wavelengths that are not well absorbed by chlorophyll a. Organisms growing in shallow waters tend to contain phycobilins that can capture yellow/red light, while those at greater depth often contain more of the phycobilins that can capture green light, which is relatively more abundant there.

Key Points

• Chlorophylls absorb light most efficiently at the ultraviolet end of the spectrum, however not all light that an organism gets is at that wavelength. Thus many photosynthetic organisms rely on accessory compounds to get light from different spectrums.
• Caretenoids aid in the absorption of light in the blue-range spectrum, while at the same time help with the oxidative stress due to the photosynthetic process.
• Phycobilins aid in the absorption of light in the red, orange, yellow, and green light, wavelengths.

Key Terms

• isoprene: An unsaturated hydrocarbon, C5H8, that is readily polymerized; natural rubber (caoutchouc) is cis-1,4-polyisoprene, and trans-1,4-polyisoprene is present in gutta-percha and balata; it is the structural basis for the terpenes.
• photosynthesis: The process by which plants and other photoautotrophs generate carbohydrates and oxygen from carbon dioxide, water, and light energy in chloroplasts.