Light is a form of energy that travels in waves. When the Sun’s light arrives at Earth, it has a white quality to it. White light is made up of the colors red, orange, yellow, green, blue, indigo, and violet. The color of light is dependent on the length of the light wave. Light in the visible spectrum contains colors and has wavelengths between 0.4 and 0.8 microns (1 micron equals 1/1000000 m; a micron is also known as a micrometer). Violet light has the shortest wavelength in the visible spectrum and red has the longest.
Light is affected differently by water than it is by air. Air transmits light, but water can transmit, absorb, and reflect light. Water’s ability to transmit light makes it possible for photosynthesis to take place beneath the surface. All of the wavelengths of visible light are not transmitted equally, however; some penetrate to greater depths than others.
Light on the red side of the spectrum is quickly absorbed by water as heat, so red only penetrates to 49.2 feet (15 m). Blue light is not absorbed as much, so it penetrates the deepest, reaching 100 feet (33 m). Green light, in the middle of the spectrum, reaches intermediate depths. When light enters water that is filled with particles such as dirt and plant matter, as in an estuary, it takes on a greenish brown hue because it only penetrates far enough to strike, and be reflected from, the particles. In tropical water where particulate levels are very low, light travels much deeper before it reaches enough particles to be reflected back to the surface, so tropical water appears blue. Below 1,500 feet (457.2m), no light is able to penetrate.
Because of the way light behaves in water, aquatic plants do not receive as much of the Sun’s energy as do plants on land. To compensate, most species contain some accessory pigments, chemicals that are adept at capturing blue and green light. These accessory pigments provide the plants additional light and thereby help macroalgae increase their rate of photosynthesis. Some of these pigments mask the green of chlorophyll and give colors to macroalgae that are not usually associated with plants. Accessory pigments explain why seaweed occurs in shades of brown, gold, and red. Green algae contain accessory pigments, too, but they do not mask the color of chlorophyll as the pigments in other kinds of algae do.