Algae are the dominant photosynthetic organisms found in marine ecosystems. They may be tiny planktonic organisms, comprised of merely a single cell, or clusters or strands of a few dozen cells. These microscopic types are known as microalgae. Macroalgae are the more familiar types, commonly referred to as seaweeds. I’ll be dealing mainly with the latter here.
The study of algae is called phycology. Like any other discipline in biology, scientists separate their life forms into groups based on structural and phylogenetic traits that species have in common. For phycologists this means creating three main groups of macroalgae based simply on color. There are reds, greens and browns, and nearly every species worldwide falls into one of these three categories.
First off, it should be made clear that although seaweeds are often referred to as "plants", they are of course not "true" plants. I’m guilty of using this arguably wrong nomenclature myself, and I’ll justify it by saying that although most taxonomists now place algae in the kingdom Protista, others catagorize them as a subdivision of the kingdom Plantae. And since macroalgaes are "plant-like", and are also the main photosynthetic forms of life in salt water, I personally don’t mind them informally being called "plants". Purists may disagree.
If you think about the three main anatomical parts of a true plant, you’ll see that seaweeds have three corresponding parts that serve similar functions. While terrestrial plants have roots, stems and leaves, macroalgae have holdfasts, stipes and fronds.
A plant’s roots serve two main functions: to anchor the plant in place and to uptake water and nutrients. The algae’s holdfast does the first, not the second. The stipe, like a plant's stem, holds the rest of the body upright. And the fronds are where photosynthesis occurs, just like in a true plant’s leaves.
Now, getting back to the three color groups of seaweeds. The color of the algae plant is determined by the type of photosynthetic pigment found in the cells of the fronds. There is a very good adaptive reason for having different pigments that are used to capture the sun’s energy, and this has to do with the way light is absorbed as it travels through the water. I’ll go into more detail below, but basically the different pigments capture different wavelengths of light energy, which allows them to live at various depths. In other words, each group has carved out a niche for themselves based on these pigments. Keep in mind that as sunlight travels through the water column the wavelengths are filtered out in the following order: Red, orange, yellow, green, blue, indigo and violet. (Think Roy G. Biv.)
I’ve mentioned the relationship between color and light in a few other diaries. What we see as an object's color is actually that (or those) wavelength(s) of white light that is reflected off the object rather than absorbed. Red algae appear red because the main photosynthetic pigment is phycoerythrin, which absorbs all colors except for red. Because they use the shortest wavelengths to photosynthesize, red algae are able to live in much deeper water than greens or browns.
Of the three color groups, the reds are most often used as food, such as the dried seaweed wraps used in sushi (called "nori"). They are also unique in that some species, known as coralline algae, secrete a calcareous covering over the fronds. These seaweeds are major contributors to the formation of coral reefs.
After the blues, green light is filtered out rather slowly, even in the plankton rich (thus heavily turbid) cold water zones. Green algae, like terrestrial plants, use chlorophyll a and chlorophyll b to photosynthesize, and can only grow in very shallow water. Many species of green algae are able to adapt to the unforgiving intertidal zone, drying out during low tide and then rehydrating as the tide comes back in.
Unlike land vascular plants and green algae, the browns use a different version of the photosynthetic pigment, called "chlorophyll C". Combined with other pigments, like fucoxanthin (a dark brown pigment that masks the green of chlorophyll), this allows these seaweeds to survive in a variety of habitats from the intertidal zone down to several meters in depth. Browns are also the largest in size, with some species, like giant kelp, capable of reaching lengths of a hundred feet or more.
One interesting feature of many species of brown algae are the presence of air bladders. These keep the top parts of the the fronds bouyant, reaching for the sunlight. Sargassum weed, which was indirectly the subject of this diary, is an example of bladdered brown algae that lives free floating in the Sargasso Sea.
Fun Fact: Lichens, which grow on rocks and trees in damp woods throughout the world, are actually a symbiotic composite of fungi and green algae. The fungus provides nutrients by decomposition while the algae contribute nutrients through photosynthesis.
Other diaries in this series can be found here.