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
Steve Shaw shows Kate how to get wet behind the ears and how to align her neck
healthSteven Shaw - the 'Buddha of Breaststroke' - applies Alexander Technique to the watery sport
Arts and Entertainment
The sight of a bucking bronco in the shape of a pink penis was too much for Hollywood actor and gay rights supporter Martin Sheen, prompting him to boycott a scene in the TV series Grace and Frankie
footballShirt then goes on sale on Gumtree
Terry Sue-Patt as Benny in the BBC children’s soap ‘Grange Hill’
voicesGrace Dent on Grange Hill and Terry Sue-Patt
Arts and Entertainment
Performers drink tea at the Glastonbury festival in 2010
Arts and Entertainment
Twin Peaks stars Joan Chen, Michael Ontkean, Kyle Maclachlan and Piper Laurie
tvName confirmed for third series
Cameron Jerome
footballCanaries beat Boro to gain promotion to the Premier League
Arts and Entertainment
ebookA unique anthology of reporting and analysis of a crucial period of history
Latest stories from i100
Have you tried new the Independent Digital Edition apps?
Independent Dating

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

iJobs Job Widget
iJobs General

Guru Careers: Software Developer / C# Developer

£40-50K: Guru Careers: We are seeking an experienced Software / C# Developer w...

Guru Careers: Software Developer

£35 - 40k + Benefits: Guru Careers: We are seeking a Software Developer (JavaS...

SThree: Trainee Recruitment Consultant / Resourcer

£18000 - £23000 per annum + Commission: SThree: As a Trainee Recruitment Consu...

Ashdown Group: UI Developer - (UI, HTML, CSS, JavaScript, AngularJS)

£25000 - £40000 per annum: Ashdown Group: UI Developer - (UI, JavaScript, HTML...

Day In a Page

Abuse - and the hell that came afterwards

Abuse - and the hell that follows

James Rhodes on the extraordinary legal battle to publish his memoir
Why we need a 'tranquility map' of England, according to campaigners

It's oh so quiet!

The case for a 'tranquility map' of England
'Timeless fashion': It may be a paradox, but the industry loves it

'Timeless fashion'

It may be a paradox, but the industry loves it
If the West needs a bridge to the 'moderates' inside Isis, maybe we could have done with Osama bin Laden staying alive after all

Could have done with Osama bin Laden staying alive?

Robert Fisk on the Fountainheads of World Evil in 2011 - and 2015
New exhibition celebrates the evolution of swimwear

Evolution of swimwear

From bathing dresses in the twenties to modern bikinis
Sun, sex and an anthropological study: One British academic's summer of hell in Magaluf

Sun, sex and an anthropological study

One academic’s summer of hell in Magaluf
From Shakespeare to Rising Damp... to Vicious

Frances de la Tour's 50-year triumph

'Rising Damp' brought De la Tour such recognition that she could be forgiven if she'd never been able to move on. But at 70, she continues to flourish - and to beguile
'That Whitsun, I was late getting away...'

Ian McMillan on the Whitsun Weddings

This weekend is Whitsun, and while the festival may no longer resonate, Larkin's best-loved poem, lives on - along with the train journey at the heart of it
Kathryn Williams explores the works and influences of Sylvia Plath in a new light

Songs from the bell jar

Kathryn Williams explores the works and influences of Sylvia Plath
How one man's day in high heels showed him that Cannes must change its 'no flats' policy

One man's day in high heels

...showed him that Cannes must change its 'flats' policy
Is a quiet crusade to reform executive pay bearing fruit?

Is a quiet crusade to reform executive pay bearing fruit?

Dominic Rossi of Fidelity says his pressure on business to control rewards is working. But why aren’t other fund managers helping?
The King David Hotel gives precious work to Palestinians - unless peace talks are on

King David Hotel: Palestinians not included

The King David is special to Jerusalem. Nick Kochan checked in and discovered it has some special arrangements, too
More people moving from Australia to New Zealand than in the other direction for first time in 24 years

End of the Aussie brain drain

More people moving from Australia to New Zealand than in the other direction for first time in 24 years
Meditation is touted as a cure for mental instability but can it actually be bad for you?

Can meditation be bad for you?

Researching a mass murder, Dr Miguel Farias discovered that, far from bringing inner peace, meditation can leave devotees in pieces
Eurovision 2015: Australians will be cheering on their first-ever entrant this Saturday

Australia's first-ever Eurovision entrant

Australia, a nation of kitsch-worshippers, has always loved the Eurovision Song Contest. Maggie Alderson says it'll fit in fine