The colors of plants

A guest article by Dr. Katja Rembold, Botanical Garden of the University of Bern

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Life on our planet originated in the water and colonized the land from there. When we look around in nature today, the vegetation is mostly dominated by the color green. This is because it was the green algae that conquered the land around 500 million years ago and from which our present-day land plants are descended. Now imagine if it had been the red algae that had prevailed on land… then our earth would look very different today.

But what are algae anyway?

“Algae” is a collective term for mostly aquatic organisms that are able to photosynthesize - i.e. convert water and carbon dioxide into oxygen and sugar with the help of solar energy. This applies both to microscopically small unicellular organisms (microalgae) and to 60-meter-long algae (macroalgae). However, algae differ from plants in terms of their structure and, with the exception of green algae, they are not closely related to plants. Blue-green algae” are actually bacteria (= cyanobacteria) which, unlike all other “algae”, have no cell nucleus at all (prokaryotes).

Together with the red algae, green algae, brown algae, golden algae, yellow-green algae, etc., this results in a whole rainbow of algae colors, which are derived from the external appearance of the respective group. In addition, there are numerous other groups of algae that are not named after their color, such as diatoms, decorative algae, yoke algae and many more.

Why are green algae green and red algae red?

Both green algae and red algae photosynthesize, but they use different organelles and pigments to do so. The chloroplasts of green algae contain chlorophyll a and chlorophyll b - the so-called “leaf green”, i.e. green pigments. These fulfill important functions, absorb light and regulate the transfer of energy and electrons during photosynthesis.

The rhodoplasts of red algae also contain chlorophyll a, but no other chlorophylls. Instead, they contain phycobilins, which in turn contain the red pigment phycoerythrin, which gives red algae their red color. However, phycoerythrin is light-sensitive and is destroyed with increasing light intensity. Red algae that grow close to the water surface therefore often look brownish rather than red.

Red-leaved land plants

There are also land plants with red leaves or trees that turn red in the fall. However, this has nothing to do with red algae and phycoerythrin. In addition to chlorophyll, land plants also contain other colorants such as yellow-red carotenoids or red anthocyanins. However, these are usually overlaid by green chlorophyll. Before the leaves fall off in the fall, the plants first extract their proteins and valuable nutrients in order to store them temporarily until the next growing season. The green chlorophyll is also broken down in the process, causing the leaves to turn yellow to red depending on their red pigment content.

Plants that have red leaves all year round contain so many red pigments that the green chlorophyll is overlaid and the leaves appear red. Red coloration can often be observed in plants in very sunny locations in particular. The red pigments act like a sunscreen here, protecting the leaves from burns. They can still photosynthesize with their chlorophyll.