The adaptation to the light of the sea

Apr 29, 2008 15:14 GMT  ·  By

The water, sea water in particular, is the realm of the algae. Algae represent the generic name for several groups of inferior aquatic plants that make photosynthesis. Thus, they require light in order to live.

Part of the sunlight is reflected on the surface of the water. As we go deeper, the intensity of sunlight decreases, as the light is absorbed by the water and the bodies that are to be found inside it. Of the light spectrum, red radiations are those decreasing at the fastest rate with the increasing depths, followed by orange, yellow, green, blue and indigo light.

At the depth of 34 m (113 ft), there is practically no red light (which is the one that carries the highest load of energy for photosynthesis), but the blue and violet light penetrates into the sea to the depths of 500 m (1,660 ft). Algae can live between the depth range of 0 to 300 m (1,000 ft). However, this range varies from sea to sea and ocean to ocean, depending on factors like latitude, temperature, salinity, currents, and the values of the tides.

For instance, the limit usually goes to up 300 m in the Pacific Ocean, but in the Atlantic it is normally of 200 m (660 ft), in the Mediterranean Sea of 150 m (500 ft) and in the Black Sea of 100 m (330 ft), while in the Arctic Ocean, where light falls under sharper angles, there are no algae below 45 m (150 ft).

No matter the maximum depth, the algae have a vertical repartition pattern which is constant in most cases but that varies from group to group nonetheless, because of their photosynthetic pigments, which absorb the dominant radiation at various depths.

From the surface to depths of 6 m (20 ft), where the proportion of red light is the highest, the dominant group is made of the green algae, which have the same photosynthetic pigments like the land plants (they also being the ones from which land plants evolved).

Between 6 to 30 m (20 to 100 ft), where yellow light abounds, the dominant group is made of brown algae. At depths below 35 m (116 ft), the red algae find their optimum photosynthesis, in an abundance of green light. These algae have a red pigment, called phycoeritrin that masks the green chlorophyll and makes the absorption of green light possible.

Of course, there are species that make the exception to these rules. Cladophora, one of the most common green algae, can grow to depths of 80 m (266 ft). Some algae change pigments depending on the light they are exposed to. Oscillatoria (in fact, a photosynthetic bacterium) turns green-bluish in red light, because of the pigment phycocyanin, while under green or blue light, it becomes turns red because of the phycoeritrin.