I had never thought about this before, but perhaps it's due to how light is influenced by water. Light passes through air pretty freely, but gets absorbed and refracted a lot more in water. As you know, green plants are absorbing pretty much all the non-green wavelengths of light.Maybe other pigments are more effective at absorbing other wavelengths.If that's true, I'd expect the colors of plants to vary predictably with depth, water clarity, etc.

It could also be due to the availability of raw materials present to make the pigments, I guess.

My third hypothesis is that it doesn't have anything to do with the water itself, but that some of those aquatic plants are on an evolutionary branch that uses different pigments, so that it's sort of an accident of history. Not being a botanist, I don't know if this is true.

So you gave me a good question, but I don't have a particularly good answer for you, just more questions.

As far as I've understood, you are totally correct.Water absorbs a lot of the red light, so only blue and green light are able to reach the plants deep underwater. Therefore, plants had to evolve a different strategy for collecting the most energy out of the light spectrum that they are given. Their photoreceptors are more sensitive to the blue side of the spectrum, and the red light is often reflected, which is why a lot of underwater plants are reddish looking. Many different colors can be found in different plants, typically dependent on the quality and spectrum of light that they receive.

Great observation. Light penetration in water (usually ocean water although this also applies to fresh water) alters which color wavelengths occur at different depths. Within the first 30 feet reds, oranges, yellows and purples fall off of the available spectrum leaving blues and greens penetrating into the deeper waters. Since green plants use reds and oranges for photosynthesis, green plants can no longer function at any depth greater than 10 feet. Therefore for a plant to undergo photosynthesis it must be of a color that absorbs blues or green. The complement color to green is red and for blue is orange, so that is why brown and red algae is found at depth and green algae is found at the ocean surface or in tide pools.

As for the why, that is a lesson in evolution. Many times several organisms enter the same area or niche, each with a different strategy for survival.The organism either survives (thus their strategy works) or doesn't. Those that survive continue to live and reproduce until the conditions change. At that time, the organism again either has or doesn't have the strategies to survive. For each change in environment (short term or over millions of years) an organism survives only because of pre-existing abilities. An example: if the climate changed tomorrow to being below freezing everywhere, only those people already owning down jackets, insulated jackets and snow boots would survive.

If you have any further questions, specifically about plants or evolution don't hesitate in asking. I love answering these types of questions.

Good question!

There are three major lineages of multicellular photosynthetic organisms, the green algae, the red algae, and the brown algae. These three types have their unicellular equivalents as well. Each of these have different pigments they use for photosynthesis, which give them their respective colors. Land plants evolved from the green algae, which is the immediate reason why land plants are green.

So why is it that only the green algae were able to successfully invade the land? There are several possibilities, one of them being simply that the greens got there first and were already adapted to the land environment before the browns or reds got the chance. It's also possible that the green pigments are better at getting energy at high light levels such as happen on land, as opposed to the lower (and more heavily composed of blue light) light levels that exist a few meters below the surface of the ocean, which may give the greens a competitive edge. A third possibility is that the green algae live primarily in fresh water, and it may be that evolving from fresh water to life on land may be physiologically easier than going directly from the ocean to land (note that fish evolved in fresh water, so vertebrate life on land came from fresh water as well - although I'm not sure the same is true of insects, who may have come straight from the ocean).

There is a fossil that lived between about 440 million years ago to maybe 350 million years ago that was a large, tree-sized organism, may even have been the first "tree", and some people have thought it might have been a brown alga. Recent work however has indicated that this bizarre fossil was more likely a kind of "woody" fungus related to mushrooms and not an attempt by a non-green alga to colonize the land but failed.