Dale Fort Blog Number 32

8 04 2014

Photosynthetic Pigments in Seaweeds

This edition asks the question:  Can chromatic adaptation possibly be true?

You may not realize that you want to know this, but keep reading and I’m sure you’ll find it quite interesting (if you really can’t be bothered to read on, scroll down and watch the video).

In 1704 Newton described how a prism could be used to split light from the sun into red, orange, yellow, green, blue, indigo and violet coloured lights. These coloured lights could not be separated further and are known as the visible spectrum. You can see them in rainbows and sprays of water droplets. Older readers might remember the song that asserts that rainbows also contain pink and purple. This is not true and it has caused a lot of trouble in GCSE science classes.   It does however make the lyric rhyme. All the colours mixed together are called white light.

dark side of the moon

Terrestrial plants are mostly green. This is because they contain a lot of chlorophyll. Chlorophyll is the substance they use to harvest light energy from the sun to power photosynthesis.

green forest

Chlorophyll absorbs some parts of the spectrum more effectively than others. The reason it looks green is because it doesn’t absorb green light but reflects it, to be detected by our eyes. It does absorb blue light strongly and to a lesser extent red light.

The light available to seaweeds is much more variable in quality and quantity to that available to land plants. Seaweeds are sometimes underwater and sometimes not. The water is sometimes clear and sometimes murky. Sometimes the water is calm and sometimes it’s rough. All of these factors affect the amount and type of light that penetrates through the water.

It is a common observation that seaweeds (at least roughly speaking) tend to be green in the shallowest waters, brown a bit further down and red in the deepest parts.

Green seaweeds have chlorophyll as their photosynthetic pigment.

 Cladophora

 Cladophora rupestris  a green seaweed

DictyotaDictyota dichotoma a brown seaweed

Brown seaweeds and red seaweeds also have chlorophyll but they also contain some extra pigments. These extra pigments tend to mask the green colour of the chlorophyll and make brown seaweeds look brown and red seaweeds look red. The brown and red pigments are called accessory pigments.

Grateloupia

Grateloupia filicina var. luxurians a red seaweed

The different colours of light penetrate the sea differently. Red light is mostly absorbed in the surface layers, blue light penetrates a bit deeper and green light penetrates deepest. (Green and blue are reversed in clear oceanic water).

In 1883 Professor Engelmann suggested that the light harvesting pigments of the different kinds of algae reflected their abilities to absorb the particular quality of light that was available at the depths at which they commonly grew. This idea became known as The Theory of Chromatic Adaptation.

Engelmann with caption

 

 

The idea worked like this:

Green seaweeds absorbed blue and red light using chlorophyll.

Brown seaweeds living deeper down had blue and green light available. Their brown pigment fucoxanthin absorbs blue light effectively and this light energy is then passed on to chlorophyll.

Red seaweeds living still deeper had green light available to them. Their red pigment phycoerythrin absorbs the green light and passes the energy onto chlorophyll.

The distribution of seaweeds and the quality of light available fit together beautifully and The Theory of Chromatic Adaptation became accepted by many people.

The problem was it wasn’t correct.

Firstly, you can find examples of red, green and brown algae in all shore zones and below the lowest tides. (The record depth for a growing, attached seaweed is at 268m off The Bahamas, where the light was about the same brightness as a full moon (0.0005% of surface irradiance).

Secondly, the pigment composition of the same species can vary a lot depending on the light regime individuals experience.

Thirdly, if you look at the distribution of seaweeds in a submarine cave, you tend to get a similar distribution as you see on seashores. Greens near the entrance, browns a bit farther in and reds at the back.

This has led most phycologists (seaweed people) to conclude that at the very least chromatic adaptation can’t be the whole story. Total quantity of light energy seems to be more important.

In conclusion, (although there are exceptions):

Green seaweeds absorb most light energy, browns a bit less and reds still less. If you expose many red seaweeds (and to a lesser extent browns) to lots of bright light, their accessory pigments denature and they can’t photosynthesise and die.

This is a really good example of a theory that seems to fit the observed facts so well that almost as soon as it was proposed it was accepted and people stopped thinking about it. Then someone made the simple observation regarding seaweeds in submarine caves and realized it was not actually true. Human history is full things that we thought of as self-evident truths (the sun goes around the earth, humans can’t fly, time goes at the same speed everywhere, a particle can only be in one place at once, One Direction won’t make it in America). How many more of these will turn out to be total tosh?

More blogs soon, possibly less philosophical….

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