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'.