the ocean's invisible forest

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 earth’s climate.

Could they also be used to combat global warming?

BY PAUL G. FALKOWSKI

ForestOcean’s

54 S C I E N T I F I C A M E R I C A N A U G U S T 2 0 0 2

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COPYRIGHT 2002 SCIENTIFIC AMERICAN, INC.

contains thousands of free-floating, mi-croscopic flora called phytoplankton.These single-celled organisms—includingdiatoms and other algae—inhabit threequarters of the earth’s 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 doesn’t stop this virtuallyinvisible forest from making a bold markon the planet’s 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 populations—by addingnutrients to the oceans—in 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 alterphytoplankton’s impact on the planet’scarbon 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 world’s most abundant typeof phytoplankton. Phytoplankton and allland-dwelling plants—which evolvedfrom phytoplankton about 500 millionyears ago—use 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 planet’s 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 matter—the 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 NASA

launched the Sea Wide Field Sensor (Sea-WiFS), the first satellite capable of ob-serving the entire planet’s phytoplankton


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(pre

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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 world’s marine phytoplankton were to die today, the concentration of CO2 in the atmosphere would rise by 200 parts per million—or 35 percent—in 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-phyll—and, 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 theircells—nearly 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 phytoplankton’s ultimate fate, whichstrongly modifies the planet’s 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-vide—every six days on average—half 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 temperature—

and higher density—of 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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NEWFOUNDLANDBloom

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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.


THE EARTH’S CARBON CYCLE can dramatically influence globalclimate, depending on the relative amounts of heat-trappingcarbon dioxide (CO2) that move into (yellow arrows) and out of(green arrows) the atmosphere and upper ocean, which exchangegases every six years or so. Plantlike organisms called phyto-plankton play four critical roles in this cycle. These microscopicocean dwellers annually incorporate about 50 billion metric tons ofcarbon into their cells during photosynthesis, which is oftenstimulated by iron via windblown dust (1). Phytoplankton alsotemporarily store CO2 in the deep ocean via the biological pump:about 15 percent of the carbon they assimilate settles into thedeep sea, where it is released as CO2 as the dead cells decay (2).Over hundreds of years, upwelling currents transport the dissolvedgas and other nutrients back to sunlit surface waters.

A tiny fraction of the dead cells avoids being recycled bybecoming part of petroleum deposits or sedimentary rocks in theseafloor. Some of the rock-bound carbon escapes as CO2 gas andreenters the atmosphere during volcanic eruptions after millions ofyears of subduction and metamorphism in the planet’s interior (3).

Burning of fossil fuels, in contrast, returns CO2 to the atmosphereabout a million times faster (4). Marine phytoplankton andterrestrial forests cannot naturally incorporate CO2 quickly enoughto mitigate this increase; as a consequence, the global carboncycle has fallen out of balance, warming the planet. Some peoplehave considered correcting this disparity by fertilizing the oceanswith dilute iron solutions to artificially enhance phytoplanktonphotosynthesis and the biological pump. —P.G.F.

Phytoplankton’s Influence on the Global Carbon Cycle

RESPIRATION AND DECAY

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where they stimulate additional phyto-plankton growth. This cycle keeps the bi-ological pump at a natural equilibrium inwhich the concentration of CO2 in the at-mosphere is about 200 parts per millionlower than it would be otherwise—a sig-nificant factor considering that today’sCO2 concentration is about 365 parts permillion.

Over millions of years, however, thebiological pump leaks slowly. About onehalf of 1 percent of the dead phytoplank-ton cells and fecal matter settles intoseafloor sediments before it can be recy-cled in the upper ocean. Some of this car-bon becomes incorporated into sedimen-tary rocks such as black shales, the largestreservoir of organic matter on earth. Aneven smaller fraction forms deposits ofpetroleum and natural gas. Indeed, theseprimary fuels of the industrial world arenothing more than the fossilized remainsof phytoplankton.

The carbon in shales and other rocksreturns to the atmosphere as CO2 only af-ter the host rocks plunge deep into theearth’s interior when tectonic plates collideat subduction zones. There extreme heatand pressure melt the rocks and thus forceout some of the CO2 gas, which is eventu-ally released by way of volcanic eruptions.

By burning fossil fuels, people arebringing buried carbon back into circula-tion about a million times faster than vol-canoes do. Forests and phytoplanktoncannot absorb CO2 fast enough to keeppace with these increases, and atmo-spheric concentrations of this greenhousegas have risen rapidly, thereby almost cer-tainly contributing significantly to theglobal warming trend of the past 50 years.

As policymakers began looking in theearly 1990s for ways to make up for thisshortfall, they turned to the oceans, whichhave the potential to hold all the CO2

emitted by the burning of fossil fuels. Sev-eral researchers and private corporationsproposed that artificially accelerating the

biological pump could take advantage ofthis extra storage capacity. Hypothetical-ly, this enhancement could be achieved intwo ways: add extra nutrients to the up-per ocean or ensure that nutrients not ful-ly consumed are used more efficiently. Ei-ther way, many speculated, more phyto-plankton would grow and more deadcells would be available to carry carboninto the deep ocean.

Fixes and LimitsUNTIL MAJOR DISCOVERIES over thepast 10 years clarified the natural distri-bution of nutrients in the oceans, scien-tists knew little about which ocean fertil-izers would work for phytoplankton. Ofthe two primary nutrients that all phyto-plankton need—nitrogen and phospho-rus—phosphorus was long thought to bethe harder to come by. Essential for syn-thesis of nucleic acids, phosphorus occursexclusively in phosphate minerals within

continental rocks and thus enters theoceans only via freshwater runoff such asrivers. Nitrogen (N2) is the most abun-dant gas in the earth’s atmosphere anddissolves freely in seawater.

By the early 1980s, however, biologi-cal oceanographers had begun to realizethat they were overestimating the rate atwhich nitrogen becomes available for useby living organisms. The vast majority ofphytoplankton can use nitrogen to buildproteins only after it is fixed—in otherwords, combined with hydrogen or oxy-gen atoms to form ammonium (NH4

+),nitrite (NO2

–) or nitrate (NO3–). The vast

majority of nitrogen is fixed by small sub-

sets of bacteria and cyanobacteria thatconvert N2 to ammonium, which is re-leased into seawater as the organisms dieand decay.

Within the details of that chemicaltransformation lie the reasons why phy-toplankton growth is almost always lim-ited by the availability of nitrogen. To cat-alyze the reaction, both bacteria and cy-anobacteria use nitrogenase, an enzymethat relies on another element, iron, totransfer electrons. In cyanobacteria, theprimary energy source for nitrogen fixa-tion is another process that requires alarge investment of iron—the productionof adenosine triphosphate (ATP). Forthese reasons, many oceanographers thinkthat iron controls how much nitrogenthese special organisms can fix.

In the mid-1980s the late John Martin,a chemist at the Moss Landing MarineLaboratories in California, hypothesizedthat the availability of iron is low enough

in many ocean realms that phytoplanktonproduction is severely restricted. Using ex-tremely sensitive methods to measure themetal, he discovered that its concentrationin the equatorial Pacific, the northeasternPacific and the Southern Ocean is so lowthat phosphorus and nitrogen in these sur-face waters are never used up.

Martin’s iron hypothesis was initiallycontroversial, in part because previousocean measurements, which turned out tobe contaminated, had suggested that ironwas plentiful. But Martin and his co-workers pointed out that practically theonly way iron reaches the surface watersof the open ocean is via windblown dust.


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PAUL G. FALKOWSKI is professor in the Institute of Marine and Coastal Sciences and the de-partment of geology at Rutgers University. Born and raised in New York City, Falkowskiearned his Ph.D. from the University of British Columbia in 1975. After completing a post-doctoral fellowship at the University of Rhode Island, he joined Brookhaven National Labo-ratory in 1976 as a scientist in the newly formed oceanographic sciences division. In 1997he and Zbigniew Kolber of Rutgers University co-invented a specialized fluorometer that canmeasure phytoplankton productivity in real time. The next year Falkowski joined the facultyat Rutgers, where his research focuses on the coevolution of biological and physical systems.

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Phytoplankton draw nearly as much CO2 out of theoceans and atmosphere as all land plants do.


Consequently, in vast areas of the openocean, far removed from land, the con-centration of this critical element seldomexceeds 0.2 part per billion—a fiftieth toa hundredth the concentrations of phos-phate or fixed inorganic nitrogen.

Historical evidence buried in layers ofice from Antarctica also supported Mar-tin’s hypothesis. The Vostok ice core, arecord of the past 420,000 years of theearth’s history, implied that during iceages the amount of iron was much high-er and the average size of the dust parti-cles was significantly larger than duringwarmer times. These findings suggestthat the continents were dry and windspeeds were high during glacial periods,thereby injecting more iron and dust intothe atmosphere than during wetter inter-glacial times.

Martin and other investigators alsonoted that when dust was high, CO2 waslow, and vice versa. This correlation im-plied that increased delivery of iron to the

oceans during peak glacial times stimu-lated both nitrogen fixation and phyto-plankton’s use of nutrients. The resultingrise in phytoplankton productivity couldhave enhanced the biological pump,thereby pulling more CO2 out of the atmosphere.

The dramatic response of phyto-plankton to changing glacial conditionstook place over thousands of years, butMartin wanted to know whether smallerchanges could make a difference in a mat-ter of days. In 1993 Martin’s colleaguesconducted the world’s first open-oceanmanipulation experiment by adding irondirectly to the equatorial Pacific. Their re-search ship carried tanks containing a fewhundred kilograms of iron dissolved in di-lute sulfuric acid and slowly released thesolution as it traversed a 50-square-kilo-meter patch of ocean like a lawn mower.The outcome of this first experiment waspromising but inconclusive, in part be-cause the seafaring scientists were able to

schedule only about a week to watch thephytoplankton react. When the samegroup repeated the experiment for fourweeks in 1995, the results were clear: theadditional iron dramatically increasedphytoplankton photosynthesis, leading toa bloom of organisms that colored thewaters green.

Since then, three independent groups,from New Zealand, Germany and theU.S., have demonstrated unequivocallythat adding small amounts of iron to theSouthern Ocean greatly stimulates phy-toplankton productivity. The most ex-tensive fertilization experiment to datetook place during January and Februaryof this year. The project, called the South-ern Ocean Iron Experiment (SOFeX) andled by the Monterey Bay Aquarium Re-search Institute and the Moss LandingMarine Laboratories, involved three shipsand 76 scientists, including four of mycolleagues from Rutgers. Preliminary re-sults indicate that one ton of iron solution


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IRON FERTILIZER FOR PHYTOPLANKTON ADDING THE IRONThe Southern Ocean Iron Experiment, conducted earlythis year, confirmed that trace amounts of ironstimulates the growth of single-celled organismscalled phytoplankton.

The first ship released about oneton of dissolved iron over some 300square kilometers south of NewZealand. Researchers mapped thefertilized patches using SeaSoar,which pumped seawater to theship. They also deployed buoys andtraps to mark the patches and tointercept sinking phytoplankton.

Preliminary results indicatethat the fertilizer—powdered

iron (shown at right) dissolvedin dilute sulfuric acid and

seawater—increasedphytoplankton growth 10-fold.

TRACKING THE BLOOMA second ship sailedthree weeks later toinspect the phyto-plankton’s response.Scientists retrieved theparticle traps and usedother equipment totrack changes inplankton and nutrients.

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released over about 300 square kilome-ters resulted in a 10-fold increase in pri-mary productivity in eight weeks’ time.

These results have convinced most bi-ologists that iron indeed stimulates phy-toplankton growth at high latitudes, butit is important to note that no one has yetproved whether this increased produc-tivity enhanced the biological pump orincreased CO2 storage in the deep sea.The most up-to-date mathematical pre-dictions suggest that even if phytoplank-ton incorporated all the unused nitrogenand phosphorus in the surface waters ofthe Southern Ocean over the next 100years, at most 15 percent of the CO2 re-

leased during fossil-fuel combustion couldbe sequestered.

Fertilizing the OceanDESPITE THE MYRIAD uncertaintiesabout purposefully fertilizing the oceans,some groups from both the private andpublic sectors have taken steps towarddoing so on much larger scales. Onecompany has proposed a scheme inwhich commercial ships that routinelytraverse the southern Pacific would re-lease small amounts of a fertilizer mix.Other groups have discussed the possi-bility of piping nutrients, including ironand ammonia, directly into coastal wa-ters to trigger phytoplankton blooms.Three American entrepreneurs have evenconvinced the U.S. Patent and Trade-mark Office to issue seven patents forcommercial ocean-fertilization technolo-gies, and yet another is pending.

It is still unclear whether such ocean-fertilization strategies will ever be techni-cally feasible. To be effective, fertilizationwould have to be conducted year in andyear out for decades. Because ocean cir-culation will eventually expose all deepwaters to the atmosphere, all the extraCO2 stored by the enhanced biologicalpump would return to the atmospherewithin a few hundreds years of the last

fertilizer treatment. Moreover, the reachof such efforts is not easily controlled.Farmers cannot keep nutrients containedto a plot of land; fertilizing a patch of tur-bulent ocean water is even less manage-able. For this reason, many ocean expertsargue that that once initiated, large-scalefertilization could produce long-termdamage that would be difficult, if not im-possible, to fix.

Major disruptions to the marine foodweb are a foremost concern. Computersimulations and studies of natural phy-toplankton blooms indicate that enhanc-ing primary productivity could lead to lo-cal problems of severe oxygen depletion.

The microbes that consume dead phyto-plankton cells as they sink toward the sea-floor sometimes consume oxygen fasterthan ocean circulation can replenish it.Creatures that cannot escape to moreoxygen-rich waters will suffocate.

Such conditions also encourage thegrowth of microbes that produce methaneand nitrous oxide, two greenhouse gaseswith even greater heat-trapping capacitythan CO2. According to the NationalOceanic and Atmospheric Administration,severe oxygen depletion and other prob-lems triggered by nutrient runoff have al-ready degraded more than half the coastalwaters in the U.S., such as the infamous“dead zone” in the northern Gulf of Mex-ico. Dozens of other regions around theworld are battling similar difficulties.

Even if the possible unintended con-sequences of fertilization were deemedtolerable, any such efforts must also com-

pensate for the way plants and oceanswould respond to a warmer world. Com-paring satellite observations of phy-toplankton abundance from the early1980s with those from the 1990s suggeststhat the ocean is getting a little bit green-er, but several investigators have notedthat higher productivity does not guaran-tee that more carbon will be stored in thedeep ocean. Indeed, the opposite may betrue. Computer simulations of the oceansand the atmosphere have shown that ad-ditional warming will increase stratifica-tion of the ocean as freshwater from melt-ing glaciers and sea ice floats above denser,salty seawater. Such stratification would

actually slow the biological pump’s abili-ty to transport carbon from the sea sur-face to the deep ocean.

New satellite sensors are now watch-ing phytoplankton populations on a dai-ly basis, and future small-scale fertilizationexperiments will be critical to better un-derstanding phytoplankton behavior. Theidea of designing large, commercial ocean-fertilization projects to alter climate, how-ever, is still under serious debate amongthe scientific community and policymak-ers alike. In the minds of many scientists,the potential temporary human benefit ofcommercial fertilization projects is notworth the inevitable but unpredictableconsequences of altering natural marineecosystems. In any case, it seems ironicthat society would call on modern phy-toplankton to help solve a problem cre-ated in part by the burning of their fos-silized ancestors.


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Aquatic Photosynthesis. Paul G. Falkowski and John A. Raven. Blackwell Scientific, 1997.

The Global Carbon Cycle: A Test of Our Knowledge of the Earth as a System. Paul G. Falkowski et al. in Science, Vol. 290, pages 291–294; October 13, 2000.

The Changing Ocean Carbon Cycle: A Midterm Synthesis of the Joint Global Ocean Flux Study.Edited by Roger B. Hanson, Hugh W. Ducklow and John G. Field. Cambridge University Press, 2000.

Ocean primary productivity distribution maps and links to satellite imagery are located athttp://marine.rutgers.edu/opp/index.htmlDetails about the Southern Ocean Iron Experiment are located atwww.mbari.org/education/cruises/SOFeX2002/index.htm

M O R E T O E X P L O R E

Adding iron to the Southern Ocean greatlystimulates phytoplankton productivity.


the ocean's invisible forest

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