Nutrient Limitation of Biological Productivity in the

Ocean during the LGM

Dana Ionita James Holland Meryl Mims

Falkowski (1997) and Ganeshram (2002)

Redfield Ratio Ratio of Carbon, Nitrogen, and Phosphorous.

C:N:P = 106:16:1

In the open ocean:

C:N = ~1000:15

N:P = ~15:1

Much debate surrounding N vs. P for the limiting factor.

•GEOSECS: water samples from all over the world

Evidence from GEOSECS (Tyrrell, Nature,1999)

Strong correlation of N to P as predicted by Redfield

ratio

Nitrogen Cycle

OHCONHNOOCH 22232 752445

HOHNOONH 22 2324

Nitrification

Denitrification

223

422 225 OOHNHOHN

Nitrogen FixationN2

NH4+

NO3-

diazotrophs

bacteria

denitrifiers

Nitrogen Fixation

N2 (atm) reduction (fixation) NH4

via diazotrophs (cyanobacteria), specifically using nitrogenase (enzyme)

What does this mean?

•Large reservoir of N2 in atmosphere; no large P reservoir… could that cause P limitation?

•What role does the evolution of bigeochemical cycles play in the most important limiting nutrient? Nitrogenase (courtesy www.uyseg.org)

Nitrogen Fixation

• Highly conserved DNA for nitrogenase• Believed to have ancient common

ancestor

*Both indicate strong evolutionary selection for nitrogen fixation

Nitrification Ammonium Nitrite Nitrate

• Progression from ammonium to nitrate as a result of two groups of aerobic bacteria.

• Evolved after formation of free O2 in the oceans by oxygenic photoautotrophs.

• Provided nitrate, eventual source of:– N for photoautotrophs– Electron acceptor for anerobic bacteria (denitrifiers)

DenitrificationDenitrification

NO3 N2

Nitrogen Fixation

Denitrification N2

N2

•Occurs in three major regions:-areas of low circulation (fiords)-continental margin sediments-oxygen minima zones

•Completes N cycle by returning N2 to the atmosphere

Evolution of the nitrogen cycle and its influence on the biological

sequestration of CO2 in the ocean

by Paul G. Falkowski (1997)

Falkowski’s Position

• Fixed nitrogen limits the accumulation of oxygen in, and drawdown of carbon dioxide from, the Earth’s atmosphere.

• The ratio of nitrogen fixation to denitrification is determined primarily by the supply of trace elements in the ocean, especially Fe

Archaean Atmosphere & Ocean

• N2 is stable and abundant in atm.

• Fixed inorganic nitrogen was scarce before the evolution of diazotrophic organisms.

• Soluble phosphorus was abundant.

• Very low reactive N:P ratio existed in the dissolved inorganic phase.

Development of the Nitrogen Cycle

Since: Then:

Sinking flux of particulate organic N and particulate P

>

N:P ratio of dissolved pool of inorganic nutrients

Rising flux of inorganic nutrients is enriched in P relative to N

This causes inorganic fixed N to limit primary production in the world oceans.

Redfield Ratio of 16:1 for particulate organic matter is an upper bound for N:P in the dissolved inorganic phase.

Today’s ocean has an average dissolved inorganic N:P ratio of ~14.7:1

Implies an imbalance between nitrogen fixation and denitrification

•CO2 exchange is dependent on limiting nutrient

•Dissolved inorganic nitrogen limits productivity

Ratio of nitrogen fixation to denitrification is crucial to CO2 exchange

The role of Iron

• Trichodesmium: major diazotrophs in the open ocean, need nitrogenase for energy

• Nitrogenase requires Fe (limiting element) to facilitate electron transfer reactions (produce energy).

Diazotrophs (N-fixers) need 100 times more Fe than organisms that use fixed nitrogen.

Trichodesmium courtesy www.cyanocite.bio.perdue.org

Aeolian Iron Flux

Most significant source of iron in central ocean basins

Trichodesmium population size proportional to aeolian flux of Fe

Spatial distribution of Trichodesmium suggests nitrogen fixation limited by Fe

Aeolian Iron Flux and the LGM

• Decreased rates of denitrification relative to nitrogen fixation (higher dissolved inorganic N)

• Could have allowed ratio of dissolved inorganic N to P to reach that of the sinking flux and to enhance the biological CO2 pump

Aeolian Fe Flux:

•Facilitates the biological utilization of preformed nutrients in HNLC regions

Fertilizing HNLC regions leads to sequestering of 140 Pg C

•Stimulates N2 fixation in LNLC regions

Restoring the 16:1 ratio in glacial periods would have sequestered additional 600 Pg C (C:N ~6.6 & N deficit of 2.7 ) kg

mol

“In theory” - total increased sequestration of ~740 Pg C

Pg = 1015g

HNLC – High-Nitrate, Low Chlorophyll

LNLC – Low-Nutrient, Low Chlorophyll

Ice core records:

Atmospheric CO2 declined from ~290 to 190 during a period of ~10,000 years over the last interglacial-glacial maximum.

mol

mol

Three-box model suggests 800 Pg C would have been sequestered if biological pump accounted for this change.

Summarizing Falkowski

• The biological CO2 pump and biogeochemical productivity is largely based upon the ratio of nitrogen fixation to denitrification.

• When large quantities of Fe are added to the ocean, nitrogen fixation increases and the N:P ratio in the internal ocean approaches the Redfield ratio resulting in large quantities of C being sequestered from the atmosphere, as in the LGM.

Reduced nitrogen fixation in the glacial ocean inferred from

changes in marine nitrogen and phosphorus inventories

by R.S. Ganeshram, T.F. Pedersen, S.E. Calvert, R

Francois (2002)

Compared to Falkowski…

• Ganeshram et al. find evidence for reduced biological activity and…

• Reduced nitrogen fixation during the LGM

• …based on a sediment core off the NW continental margin of Mexico.

Locations of phosphogenesis (particulate phosphorus deposition) & water column denitrification are: off Mexico’s NW coast in the Pacific, off Peru’s coast in the Pacific, and in the Arabian Sea (also off Namibia and W. Australia in smaller amounts)

10-m-long piston core (NH15P; 425 m water depth from the upper slope of Mexican margin)

Box cores:• Near-zero O conc. btwn

150 – 800m depth• Denitrification =

respiration process in low-O waters (prefers 14N)

• Nitrate deficit correlates with zero O

• 15N correlates with denitrification rates (how?)

• Sedimentary organic carbon inc. with depth (laminated at depth of min O)

• Interstitial dissolved phosphate conc. exceeding 40M supersaturation, P precip.

Phosphogenesis occurs almost exclusively in organic-rich, suboxic upwelling margin sediments that underlie oxygen-deficient/ denitrifying bottom waters

• 18O is influenced by what?• Correlation btwn organic matter min, phosphogenesis

min, denitrification min during LGM biological productivity and upwelling rates diminished over the margin during glacial periods

10-m-long piston core (NH15P; 425 m water depth from the upper slope of Mexican margin)

• Concomitant declines in phosphogenesis and denitrification during glacial times much larger effect on the oceanic inventory of N than of P

• Phosphogenesis occurs almost exclusively in organic-rich, suboxic upwelling margin sediments that underlie oxygen-deficient/ denitrifying bottom waters

• P burial compensated in glacial sediments in non-phosphogenic areas

• N oceanic residence time = 3 kyr• P oceanic residence time = 20-30 kyr N concentrations increased much faster than P

concentrations in response to changes P conc. could not have increased more than 10% of

today, but glacial increase in fixed N was about 50% of today

As a result, rising N/P shifted ecological advantage from nitrogen-fixers (diazotrophs) to non-nitrogen-

fixing algae

• Slowly glacial N contents

increased until N/P ratio was high enough to affect N fixation

• Rapidly Redfield ratio

restored rapidly

N fixation was lower, due to limitations from higher N/P ratio ice-age flux of aerosol iron to oligotrophic (nutrient-poor) regions of the ocean is unlikely to have been as important as Falkowski says

Conclusions:• We see decreased biological activity during glacial based

on this core• N fixation limited by N/P ratios rather than Fe fluxes• What limits N fixation can change, since conditions are

highly variable over the globe• Core not C- dated, besides identifying the glacial periods

with 18O• Drawing conclusions on a global level from one core

might not be very accurate• Lower sea level?• Tectonic uplift?• Change of min-O layer depth?• Different ocean circulation? – El Nino vs La Nina –like• If N-fixation not Fe limited, then what could have caused

increased CO2 uptake by the ocean during glacial time?

References

Falkowski, PG. 1997. Evolution of the nitrogen cycle and its influence on the biological sequestration of CO2 in the ocean. Nature 387:272-275.

Ganeshram, RS; Pedersen, TF; Calvert, SE; Francois, R. 2002. Reduced nitrogen fixation in the glacial ocean inferred from changes in marine nitrogen and phosphorus inventories. Nature 415:156-159.

Redfield, AC. 1958. The biological control of chemical factors in the environment. Am. Sci. 46: 205-221.

Tyrrell, T. 1999. The relative influences of nitrogen and phosphorus on oceanic primary production. Nature 400:525-531.

http://www-cyanosite.bio.purdue.edu/images/lgimages/tricho4.jpg

http://www.uyseg.org/catalysis/ammonia/amm7.htm

Any Questions?