Dumb waiters that square the ocean's food circle: Gabrielle Walker explains how algae make meals for all the fish in the sea

ALMOST all inhabitants of the oceans, from the haddock to the great white shark, depend ultimately upon sea- dwelling algae - the floating vegetation of the oceans. As on the land, the fate of the plant is to be eaten by a herbivore, which, in turn, becomes the prey of a carnivore (or in the marine case, a piscivore).

But how do the algae obtain the nutrients they need as plants, such as nitrogen, while getting the sunlight necessary for photosynthesis, which they also need? It is a tricky problem, because the nutrients tend to be deep in the sea out of the reach of sunlight. According to the science magazine Nature, the algae form tiny submersible 'mats'.

These mats seem to play an astonishing role in the transport of foodstuffs. Dr Tracy Villareal, from the Massachusets Institute of Technology, and his colleagues found that tangled Rhizosolenia mats only a few square centimetres in size appear able to travel vertically for hundreds of metres to carry nutrients to the surface waters. And this discovery may help to explain the dynamics of the upper ocean.

The tiny sea-dwelling plants need to spend time close to the water surface because, like land plants, they depend on the Sun's energy for growth, and sunlight does not penetrate much below the ocean surface.

Like land plants, they also need nutrients such as nitrate and phosphate. But the upper ocean is typically poor in nutrients, which tend to reside hundreds of metres below the surface. Also, although the nutrients in the upper ocean are continually recycled, particles of organic matter can fall into the deep ocean, taking their nutrients with them, and this has to be balanced by fresh inputs from below. How then do the nutrients make it to the surface to supply the growing plant matter?

One theory is that they simply diffuse upwards with time. The problem with this is that existing measurements of the rate of diffusion suggest that it is much too slow. Nutrients, it seems, are more likely to be carried in some way.

With this in mind, Dr Villareal's team set out to investigate. Divers collected samples of the algal mats close to the surface of the Pacific Ocean. They then left the mats standing in jars of seawater for several minutes. To their surprise, the investigators found that some of the mats rose to the top of the jar while others fell to the bottom. Even more surprisingly, the mats travelling upwards turned out to be transporting much larger pools of nitrate than those travelling downwards.

Dr Villareal and his colleagues decided to try a more sophisticated analysis of the nutrients carried by the mats. Nitrogen has two isotopes - one slightly heavier than the other - and the ratio of these give any material containing the element a characteristic signature.

In fact, the nitrate that resides several hundred metres below the ocean surface has a different signature from the nitrogen-containing species recycled in the upper regions; the researchers reasoned that this could be an indicator of the source of the nitrate in the Rhizosolenia mats; and, sure enough, the mats turned out to have a signature that was almost identical to that of the deep nitrate pool.

Thus it seems that the tiny mats can penetrate right down to the rich nitrate pool and transport nitrate back to the surface. They appear to rise with their load, bask in the sunlight while using up the nutrients, then fall back to the deep water to replenish their stocks.

It is not clear whether the mats supply nutrients to other inhabitants of the upper waters, rather than simply transporting material for their own private use. But even if the latter is true, herbivorous species eat the growing mats, so that the fresh nutrient input will inevitably pass through the food chain. In any case, it seems clear that the mats must play an important role in the nutrient transport.

Dr Villareal estimates that the Rhizosolenia transport mechanism could supply up to half the required nitrogen to the upper waters of the North Pacific. And the mats are widespread in the North Atlantic and Indian oceans.

The investigators are still puzzled about how the mats change their buoyancy to suit their convenience. Dr Villareal suggests that the algae that make up the mats are like balloons - where the dense cell walls are the ballast and the light cell fluids provide the lift.

According to Dr Villareal, it is possible that when the mats are in sunlight, they manufacture starch and store it as grains inside the cell. These grains could act as 'sandbags', weighing the cells down until the mats become too heavy to float and drop out of the sunlit region. The mats would then need to eat the starch, and would be unable to make more; thus they would become lighter and rise back to the surface.

The research team has managed to produce cultures of some of the mats in the laboratory for more specific experiments. Dr Villareal said: 'The field experiments have told us what the mats do. Now all we have to do is figure out how on earth they're doing it.'

The author is an assistant editor of 'Nature'.

Suggested Topics
Start your day with The Independent, sign up for daily news emails
Life and Style
Google celebrates the 126th anniversary of the Eiffel Tower opening its doors to the public for the first time
techGoogle celebrates Paris's iconic landmark, which opened to the public 126 years ago today
News
Cleopatra the tortoise suffers from a painful disease that causes her shell to disintegrate; her new prosthetic one has been custom-made for her using 3D printing technology
newsCleopatra had been suffering from 'pyramiding'
Life and Style
Baroness Lane-Fox warned that large companies such as have become so powerful that governments and regulators are left behind
techTech giants have left governments and regulators behind
News
people
Arts and Entertainment
Coachella and Lollapalooza festivals have both listed the selfie stick devices as “prohibited items”
music
Sport
Nigel Owens was targeted on Twitter because of his sexuality during the Six Nations finale between England and France earlier this month
rugbyReferee Nigel Owens on coming out, and homophobic Twitter abuse
Arts and Entertainment
Tracey Emin visits her 1990s work ‘My Bed’ at Tate Britain in London, where it is back on display from today
artsBut how does the iconic work stand up, 16 years on?
ebooks
ebooksA special investigation by Andy McSmith
  • Get to the point
Latest stories from i100
Have you tried new the Independent Digital Edition apps?
Independent Dating
and  

By clicking 'Search' you
are agreeing to our
Terms of Use.

iJobs Job Widget
iJobs General

Recruitment Genius: Senior Web Designer / Front End Developer

£28000 - £32000 per annum: Recruitment Genius: This fast expanding web managem...

Ashdown Group: Trainee Consultant - Surrey / South West London

£22000 per annum + pension,bonus,career progression: Ashdown Group: An establi...

Ashdown Group: Trainee Consultant - Surrey/ South West London

£22000 per annum + pension,bonus,career progression: Ashdown Group: An establi...

Ashdown Group: Recruitment Consultant / Account Manager - Surrey / SW London

£40000 per annum + realistic targets: Ashdown Group: A thriving recruitment co...

Day In a Page

No postcode? No vote

Floating voters

How living on a houseboat meant I didn't officially 'exist'
Louis Theroux's affable Englishman routine begins to wear thin

By Reason of Insanity

Louis Theroux's affable Englishman routine begins to wear thin
Power dressing is back – but no shoulderpads!

Power dressing is back

But banish all thoughts of Eighties shoulderpads
Spanish stone-age cave paintings 'under threat' after being re-opened to the public

Spanish stone-age cave paintings in Altamira 'under threat'

Caves were re-opened to the public
'I was the bookies’ favourite to be first to leave the Cabinet'

Vince Cable interview

'I was the bookies’ favourite to be first to leave the Cabinet'
Election 2015: How many of the Government's coalition agreement promises have been kept?

Promises, promises

But how many coalition agreement pledges have been kept?
The Gaza fisherman who built his own reef - and was shot dead there by an Israeli gunboat

The death of a Gaza fisherman

He built his own reef, and was fatally shot there by an Israeli gunboat
Saudi Arabia's airstrikes in Yemen are fuelling the Gulf's fire

Saudi airstrikes are fuelling the Gulf's fire

Arab intervention in Yemen risks entrenching Sunni-Shia divide and handing a victory to Isis, says Patrick Cockburn
Zayn Malik's departure from One Direction shows the perils of fame in the age of social media

The only direction Zayn could go

We wince at the anguish of One Direction's fans, but Malik's departure shows the perils of fame in the age of social media
Young Magician of the Year 2015: Meet the schoolgirl from Newcastle who has her heart set on being the competition's first female winner

Spells like teen spirit

A 16-year-old from Newcastle has set her heart on being the first female to win Young Magician of the Year. Jonathan Owen meets her
Jonathan Anderson: If fashion is a cycle, this young man knows just how to ride it

If fashion is a cycle, this young man knows just how to ride it

British designer Jonathan Anderson is putting his stamp on venerable house Loewe
Number plates scheme could provide a licence to offend in the land of the free

Licence to offend in the land of the free

Cash-strapped states have hit on a way of making money out of drivers that may be in collision with the First Amendment, says Rupert Cornwell
From farm to fork: Meet the Cornish fishermen, vegetable-growers and butchers causing a stir in London's top restaurants

From farm to fork in Cornwall

One man is bringing together Cornwall's most accomplished growers, fishermen and butchers with London's best chefs to put the finest, freshest produce on the plates of some of the country’s best restaurants
Robert Parker interview: The world's top wine critic on tasting 10,000 bottles a year, absurd drinking notes and New World wannabes

Robert Parker interview

The world's top wine critic on tasting 10,000 bottles a year, absurd drinking notes and New World wannabes
Don't believe the stereotype - or should you?

Don't believe the stereotype - or should you?

We exaggerate regional traits and turn them into jokes - and those on the receiving end are in on it too, says DJ Taylor