Saturday, 27 May 2017

Plant pigments of all colors

I love seeing all the plants getting green and filled out in the spring. Suddenly the landscape goes from bare and brown to many shades of green. This got me thinking: I know that leaves are green because of their photosynthetic pigments, but what about red and brown algae in ponds? If they're photosynthetic too, why aren't they green? So, today, a look at plant light-harvesting pigments - of all colors! - and what they're good for in a plant's life and human use.

Figure 1: Light that can penetrate Earth's atmosphere. Visible light can make it through the atmosphere (so it moves further down the y-axis in the diagram) while UV and infrared light is blocked (does not move far down the y-axis).
Visible light makes it through the atmosphere to create color - The sun emits many forms of energy, including UV, visible, and infrared light. Earth's atmosphere blocks much of the UV and infrared light, leaving visible light to reach the surface (Figure 1). Remember ROY G BIV? These are the components (red, orange, yellow, green, blue indigo-formerly-and violet) of visible light. The way surfaces absorb and reflect these different types of light creates their color.

Leaves harvest light with pigments - Plants are masters at capturing visible light and transforming it into sugar - a source of chemical energy that feeds all the animals on the planet, including humans. But leafy plants really only absorb the red/orange/yellow and blue/purple light for sugar production - the green light is reflected away, which makes most leaves appear green. To harvest light, plants use specialized chemicals called pigments. Each pigment can only capture a certain color of light and transfer the light's energy into the plant's sugar-making processes. This means that the plant has to make a few different types of pigments if it wants to harvest energy from as many colors of light as possible. Most land plants make chlorophyll A, which collects purple and orange/red light, and chlorophyll B, which harvests blue and yellow light.

Figure 2: Plant pigments that harvest green light. Fucoxanthin and phycobilins are two examples of light harvesting chemicals that water plants make to harvest the green light that they have access to.
Not all plants use chlorophyll - Chlorophyll makes most land plants green - but what about water plants? Could water, like the atmosphere, also filter out some light, leaving even less for water plants to feed on? Yes! Water absorbs lots of red/orange/yellow and purple/blue light, allowing mainly green light through (Figure 2). Since the chlorophyll that land plants use isn't very good for harvesting this green light, water plants make light-collecting pigments called fucoxanthin or phycobilins instead. These specialize in collecting green light, leaving the small amount of red and blue/purple light that does make it through the water to be reflected away from the water plant, resulting in its brown/red appearance. Nifty!

Fucoxanthin is bioactive! - As with many plant chemicals, fucoxanthin (the brown algae pigment) has beneficial effects on the human body. There is some evidence that this chemical promotes fat burning in fatty tissues, that it is an antioxidant, and a cancer-killing compound. This has led to its sale as a dietary supplement.

The next time you are admiring plants in the park, or swimming with aquatic plants, perhaps you will think about the wonderful variety of plant pigments and how they create the vibrant colors we see.