‘Plants’ of a Different Color

March 1, 2011
By Mandy Calkins

In the ocean’s dim depths, it’s not easy being green; that’s why some plant-like organisms use different pigments to survive.

Red Algae appears red due to accesory pigments. Credit: National Oceanic and Atmospheric Administration

Like terrestrial plants, marine phytoplankton and algae use the pigment chlorophyll to absorb light energy. Pigments take on the color of the light they reflect, not the color they absorb. Chlorophyll appears green because it best absorbs light from the red and violet ends of the spectrum, while reflecting green light.

In the ocean, however, most of this red and violet light is absorbed by the water fairly close to the surface. At
greater depths, algae and phytoplankton need help from accessory pigments.

Accessory pigments are compounds that can harvest blue and green light − precisely the wavelengths that chlorophyll can’t absorb well. They then transfer this light energy to chlorophyll molecules for use in photosynthesis.

These helpful compounds are also known as “masking pigments” because they conceal the green appearance of chlorophyll, giving an organism brown, yellowish, or even red coloring. So, accessory pigments determine both an organism’s color and its ideal depth.

Brown algae, such as giant kelp and sargassum seaweed, contain a brownish accessory pigment and can live up to 115 feet deep. Red algae can thrive at even greater depths. This group’sscientific name, Rhodophyta, comes from the Greek word for “red,” indicating the color of the accessory pigment. The light-absorbing ability of some red algae is so efficient that they can thrive at depths of more than 800 feet! That means they can live on light in places where humans can’t see any light at all.