The hidden seascape

When standing at the shoreline there is much to contemplate.

At five foot ten inches tall, standing on the beach and gazing to sea on a calm day, it is possible to see three miles to the horizon. As I look I imagine, if the sea were removed, one would be at the top of a hill with a downward gentle slope across Hebridean plains to the edge of the Atlantic Rockall Trough some 80 miles away – easily visible due to the air’s clarity and absence of water vapour.

Standing on the top of Mull’s highest point on Benmore on the same day, there would be a view across the same Hebridean plain and beyond into to the Rockall Trough 12,800 feet below. To make a comparison, this would be a drop in height roughly equal to that from Everest North Base Camp, across Tibet, across Nepal and all the way to the Indian coast at the Bay of Bengal in two-tenths the distance. The drama would be unparalleled.

However, sea covers this slope with its diverse myriad of life flying high above its bed in the water column. In the world’s seas there are, on average, more than 2 miles of water for every surface square foot resulting in a hard to imagine dark and unseen underwater seascape.


Although oceans cover 71% of earth, due to their immense depth, they provide 99% of the world’s living space. The most inaccessible parts of the ocean floor are 1.3 miles deeper than Everest is high and here the water pressure is the same as a person holding 50 jumbo jets on a single fingertip.

So when standing at the shoreline there is much to contemplate.

With a wetsuit and fins, the warm waters of a Hebridean September and a good lung full of air it is possible for the average person to descend to 40 feet without too much difficulty. With years of training the world’s best free divers manage 300 feet (about 100m). This is .01% to the sea bottom.


An 80 year old, a 13 year old and a double amputee have all successfully reached the summit of Everest highlighting the difficulties at depth human bodies must surmount –nitrogen narcosis, oxygen toxicity, decompression sickness, paralysis, hypothermia, shrinking lungs and more.

Humans are not adapted to depths at sea as well as other mammals – seals for example empty their lungs before each dive eliminating the problems of water pressure squeezing gases into blood vessels causing the bends.

Deep dive

Using this technique seals stay under for half an hour reaching depths of 600 feet using oxygen already in their blood when they slow their heart rate from 100 beats per minute to 10 beats so as to descend.

Looking straight out to sea, there is more life in the visible three miles of water column before me (numerically and by mass) than if it were a 360º view across land stretching for 1,000 miles in every direction. The sea is a microbial soup.


Microbes have explored more ways and places to live than any other creature on this planet and millions live in the water column’s every drop – they live at hydro-thermal vents above boiling point in total darkness, they live on your fingertips as you scroll this page, they are more numerous inside each human than the cells that make up the body itself. Microbes are everywhere and, as well as being right under your nose, they are indeed currently lurking inside it.

Clear mystery

In Darwin’s day microscopes were plentiful and so were the amoebae, protozoa and other single cell organisms living in single drops of water under his new scrutiny. Microbes however, much smaller and invisible even under these new lenses, remained as yet undiscovered. Looking closely at ever-busy paddling protozoa, a puzzled Darwin wondered “but on what, in the clear blue water, do they subsist?”

Technological progress of the 1970s made it possible to see further into the water world and, with immensely powerful microscopes, scientists were for the first time able to see millions of microbes in the very seawaters Darwin thought clear and blue.

The numbers of microbes in single droplets of seawater were extraordinary and, when extrapolated to make projections for the world’s oceanic microbe population, it was estimated that 1029 organisms live in our oceans – that is a number greater than there are stars in the universe. In plainer English it is 10,000,000,000,000,000,000,000,000,000 living things.

On a smaller scale and perhaps more easy to comprehend, a litre of seawater contains more microbes than the current human population of India.

Helvetica 9 pt stop

Microbes are so small that, if a thousand were lined up end to end, the combined length would fit well within the full stop at the end on this sentence. Indeed, if all the sea’s microbes were to join an orderly queue (the English among them being more easily persuaded) the line would stretch around the Milky Way galaxy thirty times.

The Palumbi brothers, in their book The Extreme Life of the Sea are happy to substantiate the enormity of this extraordinary notion as follows: ‘The oceans’ 1029 microbes, each one millionth of a metre long, would stretch out to 1020 kilometres or about 10,000,000 light years. The Milky Way’s circumference measures roughly 300,000 light-years.’

One drop

These discoveries changed the way the ocean was perceived overnight. Microbes are now understood to comprise 95% of the world’s 11 billion tons of biomass – that’s more than the krill, fish and whales together, by a long shot. One drop of seawater contains approximately 10 million viruses and one million microbial bacteria and that’s not to mention the 1,000 protazoas grazing on both whilst patrolling every drop.

It is the microbes which start the food chain for they have no smaller organisms to feed on. To feed they absorb both sunlight and liquid dissolved carbon turning them both into sugars. Predator protazoas graze on the microbes and pass this energy up the food chain. Without microbes the sea would not sustain life and would turn into an acidic, carbon rich miasma.

Power of the future

Able to absorb carbon and sunlight to create food for the chain above them, it is thanks to these little critters that we are able exist. It is estimated a ‘billion microbes weigh 0.1 milligram, but they can produce an outsized amount of energy if they are photosynthetic. Such is their metabolic rate that a colony of photosynthesising bacteria the mass of 100 human beings could produce as much energy as a nuclear power plant.*

Worth pondering whilst staring to sea.

*Stephen R Palumbi, Professor of Biology and Director of the Hopkins Marine Station at Stanford University. ‘The Extreme Life of the Sea’, 2014. Princeton University Press

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