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 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 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….


Dale Fort Blog Number 31

7 04 2014

The History of Dale Fort Part the Eighth

 This blog continues our historical series carrying on from where we left it in Blog 26 in the 1590s and whisking us through another 200 years  to 1797.

After Elizabeth the First

Britain was at war with Spain when James I died and his son Charles I ascended to the throne. In 1627 Charles also attacked France. Both of these conflicts are deemed disastrous by historians of today. The point of them as far as we are concerned is that they continued to reinforce invasion worries, especially in vulnerable places like Milford Haven.


The Privy Council in 1627 voted £3000 to be spent on the forts at Dale Point, Stack Rock and Rat Island.

Rat with caption 2

It is very difficult to compare monetary values of today with nearly 400 years ago but I’m going to do it anyway. The Measuring Worth website calculates that £3000 in 1627 was worth anything from £500,000 to £103,000,000. That’s not really very useful so I shall use the method I devised in Scattering Dreams: A History of Dale Fort (on which these historical articles are based, available from Dale Fort for a mere £5.00 (or you can keep reading this blog and eventually you’ll get most of it)). This uses the SI Unit of beer equivalence or Pobe ( Pint of beer equivalent). It works like this:

In 1627 a pint of ale cost 12d.


a) There are actually some readers of this blog and   b) Some of them are under 150 years old or so

Here is what that means:

Long ago in Britain the symbol for a penny was the letter d (from the Latin Denarius which was a small silver coin over 2000 years ago). Until February 1971 there were 240 of these pennies in a pound.

Therefore £3000 in 1627 was worth = £3000 x 240/12 = 60,000 Pobes

In 2014 at most of the pubs in Pembrokeshire a pint of beer costs an astonishing £3.50p or so a pint.

Therefore £3000 today = £3000/3.50 = about 850 Pobes

Therefore £3000 in 1627 was worth about £3000 X 850 = £2,550,000 today.

Whichever approach you take it’s clear that it was a lot of money and that there was real concern for the defence of The Haven.

 Pirates in The Irish Sea

By the 1630s local concern was more with the activities of pirates in the western approaches to Britain. In 1630 the Sheriff of Pembrokeshire complained about a 50-ton vessel that was blockading the entrance to Milford Haven. A plea was sent to the Privy Council in 1630 by the Justices of the Peace of the County petitioning them about the state of the Milford Haven defences. The Haven was described as a receptacle for all pirates and robbers. The Privy Council sent Captain Sir Richard Plumleigh in the Whelp to clean up the Irish Sea. Samuel Rawson Gardner in The Cambridge History of Britain reports that soon piracy in The Irish Sea was the exception rather than the rule.

The Civil War

The first Civil War began in 1642. Most of Wales supported the King (Charles I). South Pembrokeshire and Denbighshire were the only Welsh counties that supported Cromwell and Parliament. Tenby, Haverfordwest and Pembroke Castles were therefore held for Parliament. Some actions took place in the Haven, Parliamentary naval forces controlled the waters and Royalists held the land (including Dale Castle, see Blog Number 22).

By the outbreak of the second Civil War, the main local protagonists Powell, Laugharne and Poyer were unhappy with Cromwell’s treatment of them first time around (because he had not paid their expenses as he had promised) and this time supported the King. Eventually, Cromwell himself arrived in Milford Haven and took and destroyed Pembroke Castle, using special heavy cannon balls made in Carmarthen. The three men were sent to London where it was decided that one of them would be executed. They were asked to draw lots. They refused. A small boy was drafted in to do the job and Poyet lost. He was executed by firing squad at Covent Garden in 1648 and the world lost the man who was once described as the greatest swearer in Wales.

PoyerA watercolour of John Poyer

The Seven Years War

By 1755 the Seven Years War against France had begun and with it still more unfulfilled plans for coastal defences. Colonel Bastide surveyed Milford Haven and recommended the construction of six forts. These recommendations suggested the building of forts in almost all the places they finally did get built in the nineteenth century. By 1759 the danger had passed and three forts had been planned. The only place actually built on was Paterchurch. Lesser defences were begun at Llanion and Neyland of which nothing remains. The main reason for fortifying the Haven was to prevent enemies using it as a place of refuge or an invasion point. Having forts this far up the Cleddau effectively left most of the waterway defenceless.

By virtue of its location near the entrance to the Haven it seems that by 1780, Dale Point was a part of the limited defences of Milford Haven. John Barrett in his Plain Man’s Guide to the Dale Peninsula says there was an earth rampart protecting a battery of sixteen 24-pound guns, two 6-pounders and lodging for 50 men. The plan produced for Scots Magazine of 1785 describes a battery of 8 guns and 30 men.

Haven Forts 1785

The Last Invasion of Britain

The rise of Napoleon Bonaparte coincided with the development of a ship building industry in Milford Haven. At various times between 1793, when war broke out with Napoleon’s France and 1805 when Nelson defeated the Spanish and French fleets at Trafalgar there was a real threat of invasion.

It seemed that if Napoleon did succeed in making a landing he would try for a quick strike at London, so fortification was concentrated around the southeast of England.

In fact a successful landing was made by the French in 1797, not far from Milford Haven, at Carreg Wastag near Fishguard. An Irish-American mercenary named Tate had been put in charge of a force of around 1500 Frenchmen, mostly from French military prisons. They were dressed in uniforms captured from French Royalists, originally red but now dyed almost black, they were known as The Black Legion.

Tate’s mission formed part of a three pronged attack. His job was to sail up the Bristol Channel and destroy Bristol. While Tate’s force was anchored off Lundy (a small island in the Bristol Channel), contrary winds blew up and made it impossible to sail up to Bristol. Tate had been given a secondary objective; this was to sail to North Pembrokeshire and to land, probably at Cardigan. Here, he was to recruit from what was thought to be a large population of rebellious Welshmen for his cause against the English.

Tate had a man from Fishguard on his ship who had local knowledge of the coastline. Sea conditions were unusually calm. So Tate landed all his men and their equipment in a most unlikely place on the north coast: Carreg Wastag (=Flat Rock). The ships left next day, ending any prospect of retreat and Tate set up his headquarters at a nearby farm.

Tate and his men had landed with lots of weapons and ammunition but little food. The plan was to move fast and obtain supplies from the locality. As a result Frenchmen were soon to be found wandering around the Fishguard region commandeering food supplies and farm animals. This would not have endeared them to the local people.

A further problem for Tate was that just a few weeks before, a vessel carrying port had been wrecked on the coast, so all the houses were extremely well stocked with alcohol. Tate’s men seem to have indulged massively in port and in partially cooked chickens. As a result many of them became incapacitated.

jemima-nicholas with caption

Tate’s position was untenable and he surrendered to the Pembrokeshire Militia at Fishguard after a short occupation. The Royal Oak in Fishguard was the scene of this historic event, the surrender of the last force to invade mainland Britain.

Royal Oak with caption

 Don’t miss Blog 32,

so healthy it counts as TWO of your seven a day….