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
News
Young Winstone: His ‘tough-guy’ image is a misconception
people
Sport
Adnan Januzaj and Gareth Bale
footballManchester United set to loan out Januzaj to make room for Bale - if a move for the Welshman firms up
Arts and Entertainment
Ellie Levenson’s The Election book demystifies politics for children
bookNew children's book primes the next generation for politics
News
Outspoken: Alexander Fury, John Rentoul, Ellen E Jones and Katy Guest
newsFrom the Scottish referendum to the Ice Bucket Challenge, our writers voice their opinions
PROMOTED VIDEO
News
i100
Sport
Yaya Sanogo, Mats Hummels, Troy Deeney and Adnan Januzaj
footballMost Premier League sides are after a striker, but here's a full run down of the ins and outs that could happen over the next month
Arts and Entertainment
L to R: Hawkeye (Jeremy Renner), Captain America (Chris Evans) & Black Widow (Scarlett Johansson) in Avengers Assemble
film
News
Nigel Farage celebrates with a pint after early local election results in the Hoy and Helmet pub in South Benfleet in Essex
peopleHe has shaped British politics 'for good or ill'
Arts and Entertainment
Pharrell Williams' “Happy” was the most searched-for song lyric of 2014
musicThe power of song never greater, according to our internet searches
Sport
Tim Sherwood raises his hand after the 1-0 victory over Stoke
footballFormer Tottenham boss leads list of candidates to replace Neil Warnock
ebooks
ebooksA year of political gossip, levity and intrigue from the sharpest pen in Westminster
Arts and Entertainment
Sink the Pink's 2013 New Year's Eve party
musicFour of Britain's top DJs give their verdict on how to party into 2015
Voices
Strictly Come Dancing was watched by 6.9m viewers
voicesIt has been hard to form generally accepted cultural standards since the middle of the 19th century – and the disintegration is only going to accelerate, says DJ Taylor
Arts and Entertainment
Roffey says: 'All of us carry shame and taboo around about our sexuality. But I was determined not to let shame stop me writing my memoir.'
books
News
i100
News
Caplan says of Jacobs: 'She is a very collaborative director, and gives actors a lot of freedom. She makes things happen.'
people
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

Selby Jennings: VP/SVP Credit Quant- NY- Investment Bank

Not specified: Selby Jennings: VP/SVP Credit Quant Top tier investment bank i...

Ashdown Group: Senior Marketing Executive- City of London, Old Street

£40000 - £43000 per annum + benefits: Ashdown Group: Senior Marketing Executiv...

Ashdown Group: Marketing Manager

£40000 - £43000 per annum + benefits: Ashdown Group: An international organisa...

Ashdown Group: Internal Recruiter -Rugby, Warwickshire

£25000 - £30000 per annum: Ashdown Group: Internal Recruiter -Rugby, Warwicksh...

Day In a Page

War with Isis: The West needs more than a White Knight

The West needs more than a White Knight

Despite billions spent on weapons, the US has not been able to counter Isis's gruesome tactics, says Patrick Cockburn
Return to Helmand: Private Davey Graham recalls the day he was shot by the Taliban

'The day I was shot by the Taliban'

Private Davey Graham was shot five times during an ambush in 2007 - it was the first, controversial photograph to show the dangers our soldiers faced in Helmand province
Revealed: the best and worst airlines for delays

Revealed: the best and worst airlines for delays

Many flyers are failing to claim compensation to which they are entitled, a new survey has found
The stories that defined 2014: From the Scottish independence referendum to the Ice Bucket Challenge, our writers voice their opinions

The stories that defined 2014

From the Scottish independence referendum to the Ice Bucket Challenge, our writers voice their opinions
Stoke-on-Trent becomes first British city to be classified as 'disaster resilient' by the United Nations

Disaster looming? Now you know where to head...

Which British city has become the first to be awarded special 'resilience' status by the UN?
Finally, a diet that works: Californian pastor's wildly popular Daniel Plan has seen his congregation greatly reduced

Finally, a diet that works

Californian pastor's wildly popular Daniel Plan has seen his congregation greatly reduced
Say it with... lyrics: The power of song was never greater, according to our internet searches

Say it with... lyrics

The power of song was never greater, according to our internet searches
Professor Danielle George: On a mission to bring back the art of 'thinkering'

The joys of 'thinkering'

Professor Danielle George on why we have to nurture tomorrow's scientists today
Monique Roffey: The author on father figures, the nation's narcissism and New Year reflections

Monique Roffey interview

The author on father figures, the nation's narcissism and New Year reflections
Introducing my anti-heroes of 2014

Introducing my anti-heroes of 2014

Their outrageousness and originality makes the world a bit more interesting, says Ellen E Jones
DJ Taylor: Good taste? It's all a matter of timing...

Good taste? It's all a matter of timing...

It has been hard to form generally accepted cultural standards since the middle of the 19th century – and the disintegration is only going to accelerate, says DJ Taylor
Olivia Jacobs & Ben Caplan: 'Ben thought the play was called 'Christian Love'. It was 'Christie in Love' - about a necrophiliac serial killer'

How we met

Olivia Jacobs and Ben Caplan
Bill Granger recipes: Our chef's breakfasts will revitalise you in time for the New Year

Bill Granger's healthy breakfasts

Our chef's healthy recipes are perfect if you've overindulged during the festive season
Transfer guide: From Arsenal to West Ham - what does your club need in the January transfer window?

Who does your club need in the transfer window?

Most Premier League sides are after a striker, but here's a full run down of the ins and outs that could happen over the next month
The Last Word: From aliens at FA to yak’s milk in the Tour, here’s to 2015

Michael Calvin's Last Word

From aliens at FA to yak’s milk in the Tour, here’s to 2015