Iron Enhancement of Marine Phytoplankton
As atmospheric carbon dioxide levels increase, efforts have been made to help restore these levels to their natural state. An intriguing hypothesis has been put forward in an attempt to reduce these levels. By seeding regions of the oceans, known as high-nitrate low chlorophyll areas, with iron, it is believed that phytoplankton populations will grow. This in turn, through the process of photosynthesis, will lower atmospheric carbon dioxide levels. Two major experiments that have been conducted in the last decade, examine the reality of such an undertaking. The potential for this method to be used on a global scale appear to be limited at best.
Ever since the Industrial Revolution began in the late 1700 s, the amount of carbon dioxide released into the atmosphere has increased at dramatic rate. The majority of CO2 released into the atmosphere comes from the burning of fossil fuels and from land use such as deforestation, biomass burning and agricultural expansion, which limits the environments ability to recycle CO2 emissions (Walker and Kasting 1992). The concentration of CO2 is expected to double by the middle of the next century, causing global warming to increase by 1.5.C to 4.5.C. Rising global temperatures are expected to raise sea levels, as well as change precipitation levels and other local climate conditions. Changing regional climate could alter forests, crop yields, and water supplies (Rao and Chakravarty 1992). Also, numerous animal and plant species could be in danger of extinction due to an altering of their ecosystems, such as an expansion of deserts into existing range lands (Knox 1999). Due to the wide ranging effects of increased CO2 levels in the atmosphere, many scientists are trying to find solutions that will slow the rate of increasing CO2 levels. One solution that has been suggested is the iron enhancement of productivity in marine phytoplankton as a potential method for the reduction of atmospheric carbon dioxide.
The rich plant life that can be found in our oceans is the major user of CO2 from the atmosphere. It is now believed that the oceans absorb between 30% and 50% of the CO2 released into the atmosphere from the burning of fossil fuels (King et al. 1992). CO2 levels in the atmosphere and dissolved in the ocean s surface layer determine the ocean-water absorption and emission of gas. The amount of CO2 dissolved in water is in turn influenced by the amount of phytoplankton, which consumes CO2 during photosynthesis (LeBorgne and Rodier 1997). Phytoplankters, which are composed of algae and cyanobacteria, are minute single-celled ocean plants that are responsible for approximately 40% of the planet s total annual photosynthetic production and help to reduce atmospheric CO2 levels (Raven 1994). The phytoplankton reduce atmospheric levels by carrying the CO2 they absorb during photosynthesis and transporting them deep into the ocean by the way of dead plants, body parts, and feces (LeBorgne and Rodier 1997).
It might seem that a simple way to reduce atmospheric CO2 levels, would be to increase phytoplankton populations in the oceans of the world. But, there has been some debate as to why phytoplankton populations are not naturally higher than what they currently are. Areas known as high-nitrate, low-chlorophyll (HNLC) regions, are oceanic systems that have low phytoplankton standing stocks, despite high levels of macronutrients (Boyd et al. 1996). These areas include the equatorial Pacific, Southern Ocean and the Subarctic Pacific (Cullen 1991). Some recent theories have suggested the low phytoplankton stocks are the result of grazing by animals further up the food chain, ammonium inhibition of nitrate uptake, mixed layers in excess of the critical depth or, the most popular theory of late, the lack of an essential micronutrient such as iron (Chisholm and Morel 1991).
Due to the extreme insolubility of iron in oxygenated seawater (Moffett and Zika 1987) the potential role of iron as a limiting factor in phytoplankton productivity was appreciated as early as the 1930 s (Harvey 1938). But it wasn t until the clean techniques developed in the 1980 s, was it possible to determine that open-ocean iron concentrations were indeed below the requirement of phytoplankton (Fitzwater et al. 1996).
The next step was to determine if increasing the concentration of iron would indeed increase phytoplankton growth. To test this, a study was conducted using bottles filled with surface waters from the HNLC regions and iron was added to half the bottles and the other half were left alone as controls (Martin et al. 1991). Phytoplankton abundance was monitored in the bottles by various means to see if the addition of iron allowed the phytoplankton to assimilate additional nutrients. The general results were always the same, the total chlorophyll in the iron-enriched bottles were higher than in the control bottles at the end of the experiment, and nitrates were more depleted in the iron-enriched bottles relative to the control bottles (Martin et al. 1991).
The final obstacle in determining the feasibility of using iron enhancement to increase marine phytoplankton productivity to reduce atmospheric CO2 was to conduct an experiment outside of the laboratory, and in the ocean itself. Two large scale experiments have been conducted in an attempt to ascertain the answer to the previously mentioned question concerning the role phytoplankton could play in reducing atmospheric CO2 levels.
A team of researchers led by Kenneth H. Coale organized an experiment named IronEx in 1993, to exam the effects of fertilizing an HNLC zone with iron (Coale et al. 1998). The addition of the iron caused an initial doubling of the amount of phytoplankton, and the rate of growth quadrupled. However, after only one day, the phytoplankton activity leveled off. It was also relatively ineffective in reducing the amount of CO2 in the air above the waters where the iron sulfate was added. The result was that only ten percent of the CO2 was removed as compared to the expected amount for how much iron sulfate was added (Coale et al. 1998).
Ironex-I: The Results
Despite the fact that IronEx did not reduce atmospheric CO2 levels, it did provide definitive proof that iron does indeed limit phytoplankton growth and biomass in the HNLC waters of the equatorial Pacific (Coale et al. 1998). Also gained from this experiment, was a better understanding of how light limitation effects phytoplankton growth. As the iron patch began to sink to a depth below 30m, there was an overall decrease in phytoplankton growth (Lindley and Barber 1998). Because of the rapid loss of iron from the system, phytoplankton growth occurred over only a few cell divisions (Coale et al. 1998). This meant that in order to maintain a viable iron enriched patch on the ocean surface, the scientists had to find a way to prevent the iron patch from sinking too rapidly.
A second experiment, conducted in 1995, named IronEx-II, was also performed in the equatorial waters of the Pacific about 800 miles west of the Galapagos Islands. IronEx-II was different from IronEx-I, because instead of dumping in all of the iron at once, the iron was added continuously over a one week period (Cavender-Bars et al. 1999). This eliminated the problem of the iron immediately sinking to the ocean floors. Close to 1000 pounds of iron was added over a one-week period. This increased the surface water iron concentrations by 100 parts per trillion. Within a week, two million pounds of additional phytoplankton had grown.
Ironex-II: The Results
Concentrations of chlorophyll were increased by a factor between 30 and 40. The increase was so great, it caused the waters to turn green. This fertilization caused a growth in phytoplankton, which in turn caused an additional 2.3 million kg of CO2 to be absorbed by the phytoplankton. After ten days, the concentration of CO2 had gone down by 20% (Coale et al. 1996).
Overall, the IronEx-II experiment produced an increase in phytoplankton growth rate and abundance (Coale et al. 1996). Indicators of phytoplankton photosynthetic capacity showed an immediate and sustained increase (Behrenfeld et al. 1996). Also, as the bloom developed, the partial pressure of CO2 in the center of the patch decreased rapidly, reducing the ocean to atmosphere CO2 flux by 60% (Cooper et al. 1996).
While all of this sounds encouraging, most biologists believe that even if the Antarctic Ocean was fertilized with iron for the next hundred years, it would only create an estimated 10% reduction in atmospheric CO2 levels (Peng and Broeker 1991). One of the main concerns, is that iron is not the only factor limiting phytoplankton production. In areas like the Antarctic, factors such as winter darkness and extreme turbulence play an important role in phytoplankton populations (Peng and Broeker 1991).
Another important component of the iron fertilization concept, is the potential change in composition of the phytoplankton community (Landry et al. 1997). Since different phytoplankton groups require iron at varying levels (Cavender-Bares et al. 1999), a complete alteration of the phytoplankton diversity could ensue. This could lead to an upset in the natural balance of the oceans.
While the idea of seeding the oceans with iron to reduce atmospheric CO2 levels may sound appealing, it is apparent that there still remain too many unknown consequences of such an endeavor. A more feasible solution to lowering the increased levels of CO2 in the atmosphere is to attack the problem at its source, the burning of fossil fuels. If more effort were made to reduce the anthropogenic sources of CO2 being released into the atmosphere, the need for such elaborate solutions would not be needed.
Behrenfeld, M., A. Bale, Z. Kolber, J. Alken, and P.
Falkowski. 1996. Confirmation of iron limitation of
phytoplankton photosynthesis in the equatorial Pacific
Ocean. Nature. 383:508-511.
Boyd, P., D. Muggli, D. Varela, R. Gordblatt, R. Chretien,
K. Orians, and P. Harrison. 1996. In vitro iron enrichment experiments in the NE subarctic Pacific. Mar. Ecol. Prog. Ser. 136:179-193.
Cavender-Bares, K., S. Frankel, and S. Chisholm. 1998. A
dual sheath flow cytometer for shipboard analyses of
phytoplankton communities from the oligotrophic ocean.
Limnol. Oceanogr. 43:1383-1388.
Cavender-Bares, K., E. Mann, S. Chisholm, M. Ondrusek, and
R. Bidigare. 1999. Differential response of equatorial
Pacific phytoplankton to iron fertilization. Limnol. Oceanogr. 44:237-246
Chisholm, S., and F. Morel. 1991. What controls
phytoplankton production in nutrient-rich areas of the
open sea? Limnol. Oceanogr. 36:1507-1970.
Coale, K., S. Fitzwater, R. Gordon, K. Johnson, and E.
Barber. 1996. Control of community growth and export
production by upwelled iron in the equatorial Pacific
Ocean. Nature 379:621-624.
Coale, K., K. Johnson, S.Fitzwater, S. Blain, T. Stanton
And T. Coley. 1998. IronEx-I, an in situ iron-
enrichment experiment: Experimental design, implementation and results. Deep-Sea Res. 45:919-945.
Cooper, D., A. Watson, and P. Nightingale. 1996. IronExII-
The principal results. Nature. 383:505-511.
Cullen, J. 1991. Hypothesis to explain high-nutrient
conditions in the open sea. Limnol. Oceanogr. 36:1579-1599.
Fitzwater, S., K. Coale, R. Gordon, K. Johnson, and M.
Ondrusek. 1996. Iron deficiency and phytoplankton
growth in the equatorial Pacific. Deep-Sea Res.II
Harvey, H. 1938. The supply of iron to diatoms. J. Mar.
King, A., W. Emanuel, and W. Post. 1992. Projecting future
concentrations of atmospheric CO2 with global carbon
cycle models- the importance of simulating historical
changes. Environ. Manage. 16:91-108.
Knox, R. 1999. Physical aspects of the greenhouse effect
and global warming. Amer. Jour. Phys. 67:1227-1238.
Landry, M., R. Barber, R. Bidigare, F. Chai, K. Coale, H.
Dam, M. Lewis, S. Lindley, J. McCarthy, M. Roman, D. Stoecker, P. Verity, and J. White. 1997. Iron and grazing constraints on primary production in the central equatorial Pacific: An EqPac synthesis. Limnol. Oceanogr. 42:405-418.
LeBrogne R., and M. Rodier. 1997. Net zooplankton and the
biological pump: a comparison between the oligotrophic
and mesotrophic equatorial Pacific. Deep-Sea Res. II.
Lindley, S., and R. Barber. 1998. Phytoplankton response to
natural and experimental iron addition. Deep-Sea Res.
Martin, J., R. Gordon, and S. Fitzwater. 1991. The case for
iron. Limnol. Oceanogr. 36:1793-1803.
Moffett, J., and R. Zika. 1987. Reaction kinetics of
hydrogen peroxide with copper and iron in seawater.
Environ. Sci. Technol. 21:804-810.
Peng, T., and W. Broeker. 1991. Factors limiting the
reduction of atmospheric CO2 by iron fertilization.
Limnol. Oceanogr. 36:1919-1927.
Rao U., and S. Chakravarty. 1992. An evaluation of global
warming and its impact. Curr. Sci. 62:469-478.
Raven J. 1994. Carbon fixation and carbon availability in
marine phytoplankton. Photosynth. Res. 39:259-273.
Walker J., and J. Kasting. 1992. Effects of fuel and forest
conservation on future levels of atmospheric carbon
dioxide. Glob. Plan. Change. 97:151-189.