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Scientists discover source of essential nutrients for open-ocean algae
Algae obtain nitrate from deep waters 250 m beneath the surface
For almost three decades, oceanographers have been puzzled by the ability of microscopic algae (‘micro algae’) to grow in open-ocean areas where there is very little nitrate, an essential nutrient for the algae.
In last week’s (16 June 2010) issue of the journal Nature, Monterey Bay Aquarium Research Institute (MBARI) chemical oceanographer Ken Johnson, along with co-authors Stephen Riser at the University of Washington and David Karl at the University of Hawaii, show that open-ocean micro algae obtain nitrate from deep waters as much as 250mbelow the surface.
The finding will help scientists predict how open-ocean ecosystems could respond to global warming.
The sea around Hawaii may be clear and blue, but beneath the surface has been an enduring oceanographic mystery. Surface waters in this and other open-ocean areas contain almost no nitrate or other plant nutrients. Yet each year, microscopic algae flourish in these vast regions. In the process, the algae consume about one-fifth of all the carbon dioxide taken up by plants worldwide.
To solve the mystery, Johnson and colleagues used a robotic drifter called an Apex float, which automatically moves from the sea’s surface down to 1000 m and back again, profiling the water column as it goes.
Researchers at the University of Washington outfitted the drifter with an oxygen sensor and a custom version of Johnson’s In-situ Ultraviolet Spectrometer (ISUS), which measures nitrate concentrations in seawater.
Monterey Bay’s kelp forests and many other ocean ecosystems are sustained by micro algae. (Image: NOAA)
The design and deployment of this custom drifter was funded by the National Science Foundation (NSF).
Study designed to answer long-standing question
‘This elegant study very nicely answers a long-standing question,’ says Don Rice, director of NSF’s chemical oceanography programme, ‘not just for the Pacific Ocean, but for much of the rest of the world oceans as well: how does deep-water feed life at the surface? We now have some answers.’
In December 2007, researchers from the University of Hawaii placed the drifter in the ocean northeast of Oahu, where it collected ocean profiles once every five days for almost two years.
From January through October of each year, the instruments on the drifter showed a gradual increase in oxygen concentrations in the upper 100m of the ocean.
At the same time, the float detected a gradual decrease in concentrations of nitrate in deeper waters 100 to 250m below the surface.
Johnson and co-authors found that the amount of oxygen produced near the surface through photosynthesis was directly proportional to the amount of nitrate consumed in deeper water.
Based on the decline in nitrate concentrations at depth, the researchers estimated how much algal growth could have taken place during the year. Their estimates of algal growth were very similar to algal growth rates measured during the University of Hawaii’s oceanographic cruises in that part of the Pacific Ocean.
Do the answers lie in eddies?
Because there is not enough sunlight for algae to grow below 100m, the researchers conclude that algae growing near the surface somehow obtain nitrate from deeper waters, and use this nitrate to grow and reproduce. Exactly how the algae obtain these deep nutrients is still unclear. One possible mechanism is ocean eddies.
Satellite and drifter data suggest that slow, swirling eddies occasionally form hundreds of metres below the surface of the Pacific. ISUS data demonstrate that some of these eddies can carry nitrate upward to about 70m below the ocean surface.
Yet these pulses of nitrate do not appear to reach the upper 50m of the water column, where most of the algae grow. The scientists speculate that dormant micro algae may inhabit the waters below 100m.
Open-ocean eddies may occasionally carry these algae upward, to depths of perhaps 70m. At this point, the algae may consume any available nitrate and then migrate farther up into surface waters.
Johnson suggests that testing this hypothesis will provide an interesting challenge for marine biologists.
Scientists already know that some algae can swim, using tiny, whip-like flagellae. Other algae can actively change their buoyancy, just like the Apex float, and either sink down or float upward.
Over the next year or two, the oceanographers will outfit several groups of drifters with nitrate and oxygen sensors.
Tiny organisms – but doing great work
Some of these drifters will be deployed near Hawaii. Others will be deployed near Bermuda in the mid-Atlantic. Still other drifters will be placed in the far North Pacific and in the Southern Ocean, where nitrate supplies and algal growth are typically high.
Such studies in the open ocean may seem remote from human activities on land. Yet the oxygen produced by open-ocean algae is essential for the survival of life on Earth.
When these algae die and sink to the seafloor, they remove huge amounts of carbon dioxide from Earth’s atmosphere.
By moving massive amounts of carbon from the atmosphere into the ocean, the algae help control Earth’s climate.
‘The life you can’t see without a microscope,’ says Johnson, ‘is doing all the work.’
The research was also funded by the Office of Naval Research, the National Oceanic and Atmospheric Administration, the Gordon and Betty Moore Foundation, and the David and Lucile Packard Foundation.
Source: National Science Foundation
Thumbnail caption: Microscopic algae in surface waters depend on nitrate, only available in deep waters. (Image: Kim Fulton-Bennett, MBARI)
