Dive Dry with Dr. Bill

#167: Why Red and Brown vs. Green Algae?

I rarely write about one of the most important groups of biological organisms in the ocean, so it is time to focus on them. I'm referring to the base of most of the marine food chains, the seaweeds or algae. Personally I have great disdain for the term "seaweed." A weed is a plant growing "out of place" such as a native plant ("weed") in one's exotic flower bed for some gardeners, or a Mediterranean exotic like the Dyer's greenwold ("broom") growing on Catalina's hillsides for Conservancy biologists. By contrast the so-called "seaweeds" are growing exactly where they should be... in the sea! I would make exceptions for introduced species like the Caribbean alga Caulerpa taxifolia or the Japanese kelp Undaria pinnatifida seen in our waters, and would consider them weeds in the usual sense of the word.

"Seaweeds" should be referred to as algae, simple photosynthetic organisms. Algae were once referred to as "plants," but in this biologically correct world they are no longer considered such due to significant differences at the cellular level. The old definition of plant (still in use according to my handy Funk & Wagnalls) is a living organism which grows based on food produced by photosynthesis. This very basic definition would apply to the algae, but we scientists tend to be a bit more precise in our categorizations of living things... sometimes too precise! Of course there are also land "plants" which do not photosynthesize, but I digress (as usual).

Algae, like Catalina's giant sea dahlia or Coreopsis (reportedly W. C. Fields favorite plant for reasons I won't go into here but it has to do with "mating"), Catalina ironwood trees, or even roses use their pigments to capture light energy from the sun, then use the energy to create carbohydrate foods from water and carbon dioxide. Just like land plants, they "fix" the energy of sunlight into a form of chemical energy that can be used to sustain not only their life, but the lives of the many plant-eaters or herbivores in their ecosystem, and ultimately the vicious carnivores like myself.

Back in the days when I was vice president of the Catalina Conservancy and was trying to explain our ecological restoration programs, I was astounded that many of our critics placed so little value on "plants." Without a means of converting abundant solar energy into food, there would be almost no base for the wonderful natural ecosystems of our island. After all, what would happen if, as the island emerged from the sea, a butterfly flew over from the mainland and there were no plants yet established? Put simply, it would die without a food source for it (or its caterpillars assuming a mate flew over with it).

Algae, including not only our magnificent giant kelp but also the many smaller forms attached to the bottom or drifting in the currents as "plant" or phytoplankton, likewise serve as the basis for our marine food chains (and ultimately the sushi we eat at Flip's!). There are filter feeders which capture the phytoplankton, and snails that munch directly on the larger algae. These in turn become food for other marine critters like fish until we get to the apex predators like the great white shark... and me (or you, unless you're vegetarian)! Without photosynthesis in the sea to convert solar energy into a form usable by marine life, we'd be limited to critters like bacteria that can produce their own food through other chemical pathways (referred to as chemosynthesis).

Algae have an interesting aspect missing from most land plants. Everyone loves the "greenery" produced by plants due to the green photosynthetic pigment chlorophyll in their leaves. Our undersea algae "gardens" may lack the beauty seen in colorful flowering plants, but they make up for it by incorporating pigments of different colors than just monotonous green! We have reds, browns, and golden colors in addition to the relatively few truly green algae in Catalina waters. Why is this?

On land, green is the perfect color to capture sunlight. Remember in elementary school science (or art) class when you learned the colors of the spectrum, or rainbow? ROY G BIV was the acronym I was introduced to... red, orange, yellow, green, blue, indigo and violet. Notice that green is right in the middle of the spectrum. A green pigment receiving sunlight reflects back the green portion of the spectrum to our eyes, and absorbs both the lower energy red end and the higher energy blue end. This is a perfect adaptation for a plant on land to optimize its ability to gather sunlight, especially since sunlight peaks in the lower energy yellow region.

In the sea, light is quite different. Within the first 30 feet most of the lower energy reds, oranges and yellows have been filtered out by the water. Therefore they are not available for photosynthesis. The dominant light underwater is green through the higher energy blues. If algae only had chlorophyll to capture available light underwater, they would be limited in what colors they could absorb (and therefore what energy could be captured). To deal with the changing quality (color) of light underwater, algae often use accessory pigments including reds (phycobilins) and browns (fucoxanthin). These pigments absorb everything from the green end up through the blues. Unless you use a dive light underwater, there is usually very little red light to be reflected back to your eyes, so you may see the red algae as dark in color.

So algae come in a wider variety of colors than do land plants. You will see "brown" algae (like the giant kelp) and red algae (including some that seem to be purple) in addition to the greens. Now I must admit that when I make a salad, I prefer most of my leafy components to be green (but do add some red onion for taste). Underwater, snails and other plant-eating herbivores are not so picky... they'll munch away on the red, brown or green (especially if they are hungry!).

© 2005 Dr. Bill Bushing. Watch the "Dive Dry with Dr. Bill" underwater videos on Catalina Cable TV channel 49, 10:00 AM and 5:00 PM weekdays. Buy my "Munching and Mating in the Macrocystis" DVD so you can take Dr. Bill home with you... we'll both be glad you did!

A green and a brown alga; two red algae.

This document maintained by Dr. Bill Bushing.
Material and images © 2005 Star Thrower Educational Multimedia