The Ocean's Invisible Forest

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  • DIATOMS ARE THE GIANTS of the phytoplankton world. The species pictured here, Actinocyclus sp., can measure up to a millimeter in diameter.

    InvisibleMarine phytoplankton play

    a critical role in regulating the earths climate.

    Could they also be used to combat global warming?



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  • contains thousands of free-floating, mi-croscopic flora called phytoplankton.These single-celled organismsincludingdiatoms and other algaeinhabit threequarters of the earths surface, and yetthey account for less than 1 percent of the600 billion metric tons of carbon con-tained within its photosynthetic biomass.But being small doesnt stop this virtuallyinvisible forest from making a bold markon the planets most critical natural cycles.

    Arguably one of the most consequen-tial activities of marine phytoplankton istheir influence on climate. Until recently,however, few researchers appreciated thedegree to which these diminutive oceandwellers can draw the greenhouse gas car-bon dioxide (CO2) out of the atmosphereand store it in the deep sea. New satelliteobservations and extensive oceanograph-ic research projects are finally revealinghow sensitive these organisms are tochanges in global temperatures, oceancirculation and nutrient availability.

    With this knowledge has come a temp-tation among certain researchers, entre-preneurs and policymakers to manipulatephytoplankton populationsby addingnutrients to the oceansin an effort tomitigate global warming. A two-monthexperiment conducted early this year inthe Southern Ocean confirmed that in-jecting surface waters with trace amountsof iron stimulates phytoplankton growth;however, the efficacy and prudence ofwidespread, commercial ocean-fertiliza-tion schemes are still hotly debated. Ex-ploring how human activities can alterphytoplanktons impact on the planetscarbon cycle is crucial for predicting thelong-term ecological side effects of suchactions.

    Seeing GreenOVER TIME SPANS of decades to cen-turies, plants play a major role in pullingCO2 out of the atmosphere. Such hasbeen the case since about three billion

    years ago, when oxygenic, or oxygen-pro-ducing, photosynthesis evolved in cyano-bacteria, the worlds most abundant typeof phytoplankton. Phytoplankton and allland-dwelling plantswhich evolvedfrom phytoplankton about 500 millionyears agouse the energy in sunlight tosplit water molecules into atoms of hy-drogen and oxygen. The oxygen is liber-ated as a waste product and makes pos-sible all animal life on earth, including ourown. The planets cycle of carbon (and, toa large extent, its climate) depends onphotosynthetic organisms using the hy-drogen to help convert the inorganic car-bon in CO2 into organic matterthe sug-ars, amino acids and other biological mol-ecules that make up their cells.

    This conversion of CO2 into organicmatter, also known as primary produc-tion, has not always been easy to mea-sure. Until about five years ago, most bi-ologists were greatly underestimating thecontribution of phytoplankton relative tothat of land-dwelling plants. In the sec-ond half of the 20th century, biologicaloceanographers made thousands of indi-vidual measurements of phytoplanktonproductivity. But these data points werescattered so unevenly around the worldthat the coverage in any given month oryear remained extremely small. Evenwith the help of mathematical models tofill in the gaps, estimates of total globalproductivity were unreliable.

    That changed in 1997, when NASAlaunched the Sea Wide Field Sensor (Sea-WiFS), the first satellite capable of ob-serving the entire planets phytoplankton

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    ge) Rapid life cycles of marine phytoplankton transfer heat-trapping carbon dioxide

    (CO2) from the atmosphere and upper ocean to the deep sea, where the gasremains sequestered until currents return it to the surface hundreds of years later.

    If all of the worlds marine phytoplankton were to die today, the concentration of CO2 in the atmosphere would rise by 200 parts per millionor 35 percentin amatter of centuries.

    Adding certain nutrients to the ocean surface can dramatically enhance the growthof phytoplankton and thus their uptake of CO2 via photosynthesis, but whetherintentional fertilization increases CO2 storage in the deep sea is still uncertain.

    Artificially enhancing phytoplankton growth will have inevitable but unpredictableconsequences on natural marine ecosystems.

    Overview/Climate Regulators

    Every drop of waterin the top 100 meters of the ocean


  • populations every single week. The abili-ty of satellites to see these organisms ex-ploits the fact that oxygenic photosyn-thesis works only in the presence ofchlorophyll a. This and other pigmentsabsorb the blue and green wavelengths ofsunlight, whereas water molecules scatterthem. The more phytoplankton soakingup sunlight in a given area, the darker thatpart of the ocean looks to an observer inspace. A simple satellite measurement ofthe ratio of blue-green light leaving theoceans is thus a way to quantify chloro-phylland, by association, phytoplank-ton abundance.

    The satellite images of chlorophyll,coupled with the thousands of productiv-ity measurements, dramatically improvedmathematical estimates of overall prima-ry productivity in the oceans. Althoughvarious research groups differed in theiranalytical approaches, by 1998 they hadall arrived at the same startling conclusion:every year phytoplankton incorporate ap-proximately 45 billion to 50 billion met-ric tons of inorganic carbon into theircellsnearly double the amount cited inthe most liberal of previous estimates.

    That same year my colleagues Chris-topher B. Field and James T. Randersonof the Carnegie Institution of Washingtonand Michael J. Behrenfeld of Rutgers Uni-versity and I decided to put this figure intoa worldwide context by constructing thefirst satellite-based maps that comparedprimary production in the oceans withthat on land. Earlier investigations hadsuggested that land plants assimilate asmuch as 100 billion metric tons of inor-ganic carbon a year. To the surprise ofmany ecologists, our satellite analysis re-vealed that they assimilate only about 52billion metric tons. In other words, phy-toplankton draw nearly as much CO2 outof the atmosphere and oceans throughphotosynthesis as do trees, grasses and allother land plants combined.

    Sinking out of SightLEARNING THAT phytoplankton weretwice as productive as previously thoughtmeant that biologists had to reconsiderdead phytoplanktons ultimate fate, whichstrongly modifies the planets cycle of car-bon and CO2 gas. Because phytoplank-

    ton direct virtually all the energy they har-vest from the sun toward photosynthesisand reproduction, the entire marine pop-ulation can replace itself every week. Incontrast, land plants must invest copiousenergy to build wood, leaves and rootsand take an average of 20 years to replacethemselves. As phytoplankton cells di-videevery six days on averagehalf thedaughter cells die or are eaten by zoo-plankton, miniature animals that in turnprovide food for shrimp, fish and largercarnivores.

    The knowledge that the rapid life cy-cle of phytoplankton is the key to theirability to influence climate inspired an on-going international research programcalled the Joint Global Ocean Flux Study(JGOFS). Beginning in 1988, JGOFS in-vestigators began quantifying the oceaniccarbon cycle, in which the organic matterin the dead phytoplankton cells and ani-mals fecal material sinks and is con-sumed by microbes that convert it backinto inorganic nutrients, including CO2.Much of this recycling happens in thesunlit layer of the ocean, where the CO2 is

    instantly available to be photosynthesizedor absorbed back into the atmosphere.(The entire volume of gases in the atmo-sphere is exchanged with those dissolvedin the upper ocean every six years or so.)

    Most influential to climate is the or-ganic matter that sinks into the deep oceanbefore it decays. When released belowabout 200 meters, CO2 stays put for muchlonger because the colder temperatureand higher densityof this water pre-vents it from mixing with the warmer wa-ters above. Through this process, knownas the biological pump, phytoplanktonremove CO2 from the surface waters andatmosphere and store it in the deep ocean.Last year Edward A. Laws of the Univer-sity of Hawaii, three other JGOFS re-searchers and I reported that the materi-al pumped into the deep sea amounts tobetween seven billion and eight billionmetric tons, or 15 percent, of the carbonthat phytoplankton assimilate every year.

    Within a few hundred years almost allthe nutrients released in the deep sea findtheir way via upwelling and other oceancurrents back to sunlit surface waters,

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    BLOOM OF PHYTOPLANKTON called coccolithophores swirls across several hundred square kilometersof the deep-blue Atlantic just south of Newfoundland. Such natural blooms arise in late spring, whencurrents deliver nutrients from the deep ocean to sunlit surface waters.