Why do plants, algae etc 'choose' to be green... is it the most
efficient part of the spectrum (if you know what I mean)? Why have
these organisms not evolved to be many different colours to take
advantage of the whole spectrum? Or can I just not remember my school
biology lessons correctly? (Which is almost certainly the case!)
Thanks.

This article may interest you:

http://www.abc.net.au/science/k2/trek/4wd/may99_1.htm

Thanks Pink... you're a star!

Let me start by saying that modern science may not have a correct
answer to your question. Plant leaves are green because of chlorophil.
Chlorophil is green because it absorbs most other wave lengths of
visable light except green light which it reflects. Perhaps it is more
correct to say chlorophil emits green light. That apparently means
chlorophil can not use green light to make sugar and oxygen from
water, sunlight and carbon dioxide. Plant leaves would appear dull
black if they used the entire visable light spectrum. I wonder if an
experiment has been done suppling plants only with moncromatic green
light? Now that I have commented, I will read the link supplied by
pink. At very least I will find out how to spell chlorophil.   Neil

The website posted by pinkfreud is interesting, although it is
ridiculously false in some parts. Email me if you want me to point out
the details of why. But, to answer your question, if you look at a
spectrum of the wavelengths of light that reach the earth from the
sun, there is a peak at 400-700 nm. Now, there are two types of
chlorophyll, a and b. Both of these absorb everything from 500-600 nm,
but do not absorb at 450 an 630 for chlorophyll a, and not 400 and 650
for b. (Green is around 450 nm, purple is around 400 nm.) (All these
numbers are only approximate.) Whether the plant appears blue or green
or purple only depends on the ratio of chlorophylls a and b. So
actually, the current mechanism of light absorbtion is one of the most
efficient possible. In fact, plants almost always have to slow down
carbon fixation because they don't have enough water, not because they
don't have enough light. Let's imagine that you were to invent a new
mechanism that were more efficient-- you would totally have to
redesign the porphyrin ring and the magnesium ion in the center,
because each molecule has certain wavelengths of light that it absorbs
and reflects. I'm guessing that nature found a mechanism that works
and stuck with it.

P.S.-- there have been experiments where they shined green light onto
plants-- it won't grow!

Also, I forgot to add that other pigments like carotenoids
(beta-carotene and lutein) also can help light absorbtion, thus
allowing some organisms to absorb light that might be absorbed by
organisms above them, etc., and also changing the color of the
organism. All this info can be found in Lehninger's Principles of
Biochemistry. Later!

In addition to what Purkinje-Ja posted, it is important to note that
plants such as seaweed are red in colour, and are saturated with
carotenoid accessory pigment (as opposed to chlorophyll) because they
float beneath the sea, at a depth where green light does not penetrate
easily.

This question is largely concerned with an area of photosynthesis
known as 'Absorption Spectra'. Page 177 in Advanced Biology by Jones &
Jones (Cambridge University Press, ISBN #: 0-521-48473-1) is very
useful.  I will try and find some other references for you.  Best
Wishes  M

i have little to add, but important:

1) the limiting factor for most plant's growth rate is water and
minerals, not sun: so it can chose which color to absorb and which to
reflect.

if it absorved all the sunlight available, it wouldn't have minerals
and water to do anything. like when you go to the supermarket, the
limiting factor is your money, not the food types. so why do you buy
coke or beer instead of the best champagne?


2) clorophile is fragile, so it rejects the most powerfull part of the spectrum: 

because it is an organic compound, it breaks easily. plants regenerate
it, but slowly. reflecting the most intense part of the spectrum (aka
green and yellow light) is like you using sunglasses to protect your
eyes.
the solar spectrum power density is like a bell, with the peak at
yellow - green and tails in blue and red.