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Figure 1: Schematic of the ``biological pump'' which transports CO from the atmosphere to the deep ocean. CO is assimilated by phytoplankton through photosynthesis in the sunlit surface waters and converted to organic carbon. This phytoplankton biomass fuels a complex foodweb through which nutrients and CO are recycled many times in the surface waters. The large phytoplankton are eaten by zooplankton and the small phytoplankton and bacteria are eaten by microzooplankton, which are in turn eaten by the zooplankton. Phytoplankton photosynthesize and excrete dissolved organic carbon which fuels the bacteria. In the

 
Figure 1. continued:

open ocean, most of the CO that is used in phytoplankton photosynthesis is ultimately released back into the atmosphere through their respiration, and that of the organisms who have eaten them. Some of the carbon finds its way to the deep ocean as zooplankton feces or pieces of dead organisms, where it is remineralized by bacteria back to CO. The net result is concentration of CO (and nutrients) in the deep ocean, and deposition of very small amounts of carbon on the sea floor. Since most of the organic carbon generated through photosynthesis is converted to CO with each trophic transfer (represented by arrows), the structure of the food web---its design and the relative abundance of its components---is a critical determinant of the efficiency of the biological pump.

 
Figure 2: Depth profiles of (A) inorganic nutrients and (B) dissolved inorganic carbon in the Atlantic and Pacific Oceans. Nutrients are essentially depleted in the surface waters where they limit phytoplankton production. Largely through the mechanism of the biological pump, both nutrients and CO are concentrated in the deep sea. (A: Adapted from Richards, 1968; B: Adapted from Broecker, 1974).

 
Figure 3: ``Typical'' results from iron-enrichment bottle incubation experiments---in this case from the subarctic Pacific (after Martin & Fitzwater, 1988). Chlorophyll (A), and nitrate (B) concentrations in bottles incubated with (open circles) and without (closed circles) added iron (5 nmol kg) for 6 days. The iron-enriched bottles show dramatically increased chlorophyll concentrations and decreased nitrate and phosphate relative to the control bottles. Examination of the phytoplankton community in the bottles revealed substantial growth of pennate diatoms in the iron enriched bottles, which were a minor component of the phytoplankton community initially.

 
Figure 4: The ``life history'' of an iron-enriched patch in the surface waters of the equatorial Pacific (adapted from Fitzwater and Hunter, 1994). The iron was distributed on Julian day 297 (Oct. 24, 1993) and followed as it drifted in the surface currents with the aid of satellite monitored drifting buoys. A low-salinity front intruded on the patch on Julian day 302 and pushed it down to 20-40 m. Although it could still be tracked, the experiment was disrupted because of the reduced light levels at these depths.

 
Figure 5: Comparison of phytoplankton activity inside and outside the patch during the iron fertilization experiment. (A) Average chlorophyll concentrations (a measure of photosynthetic biomass) and (B) average primary production (the rate of photosynthesis) inside and outside the patch between Julian day 299 and 302. The mixed layer, and hence the iron enriched waters, extended down to about 40 m. (Data courtesy of R. Bidigare and R. Barber; after Martin et al., 1994)

 
Figure 6: Response of various size fractions of the phytoplankton community to the addition of iron compared with those outside the patch, two days following the beginning of iron enrichment. (A) Increases in F/F, which is a measure of the quantum efficiency of part of the photosynthetic apparatus in phytoplankton and reaches a maximum of 0.65 under optimal growth conditions (Kolber et al., 1988), and (B) Chlorophyll concentrations in different size fractions of the phytoplankton community in and out of the patch. (After Kolber et al., 1994.)

 
Figure 7: Comparison of the response of the phytoplankton in the iron-enriched patch with those in bottle incubations done in parallel on board ship. The bottles were filled with water from 25m, enriched with 5 nM Fe (as a sulfate), and incubated on deck at simulated in situ light intensities. Closed symbols are the Fe-enriched waters in all panels. Note the difference in scale in (C). Note also that day-0 values in (A) were calculated from ``out-of-patch'' stations. (A) After Martin et. al. 1994. (B,C) Data courtesy of K. Coale, R. Bidigare.

 
Figure 8: Iron and CO levels in an ice core from the Antarctic, showing an inverse correlation through two glacial/interglacial transitions. Martin hypothesized that the elevated iron concentrations stimulated primary productivity in the waters around the Antarctic, thus removing significant amounts of CO from the atmosphere (adapted from Martin, 1992). Ice core ages are in years ``before present.''

Table 1. Relative molar concentrations of trace elements in today's oceans (normalized to P), and in the anoxic ocean characteristic of early evolutionary times (molar concentrations), compared with the molar ratios of these elements in today's phytoplankton (modified from Brand, 1991; Martin et al 1993). Comparing the availability of the various elements in the water today with that in the phytoplankton reveals that iron is in short supply relative to the requirements of the cells. In the ancient ocean when biochemical pathways were evolving, iron was readily available relative to other trace elements.

---------------------------------------------------------------------------------------------------------- P Fe Zn Mn Cu ---------------------------------------------------------------------------------------------------------- Average Plankton Composition 1000 10 2 0.4 0.6 (molar ratios)

Average Seawater Atlantic 1000 0.45 1.7 0.58 1.7 Composition (molar ratios) Pacific 1000 0.2 3.0 0.075 1.5

Ancient (anoxic) Oceans (molar concentrations) 10 10 10 10 ----------------------------------------------------------------------------------------------------------