Nutrient Limitation of Biological Productivity in the Ocean during the LGM

1. Nutrient Limitation of Biological Productivity in the Ocean during the LGM Dana Ionita James Holland Meryl Mims Falkowski (1997) and Ganeshram (2002)

2. 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.

3. Evidence from GEOSECS (Tyrrell, Nature,1999) • GEOSECS: water samples from all over the world Strong correlation of N to P as predicted by Redfield ratio

4. Nitrogen Cycle N2 denitrifiers Nitrogen Fixation diazotrophs NO3- NH4+ Nitrification bacteria Denitrification

5. 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)

6. Nitrogen Fixation • Highly conserved DNA for nitrogenase • Believed to have ancient common ancestor *Both indicate strong evolutionary selection for nitrogen fixation

7. 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)

8. Denitrification Denitrification Nitrogen Fixation N2 NO3  N2 Denitrification 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

9. Evolution of the nitrogen cycle and its influence on the biological sequestration of CO2 in the ocean by Paul G. Falkowski (1997)

10. 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

11. 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

12. 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.

13. 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

14. 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). Trichodesmium courtesy www.cyanocite.bio.perdue.org Diazotrophs (N-fixers) need 100 times more Fe than organisms that use fixed nitrogen.

15. 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

16. 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

17. 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 ) Aeolian Fe Flux: Ice core records: Atmospheric CO2 declined from ~290 to 190 during a period of ~10,000 years over the last interglacial-glacial maximum. Three-box model suggests 800 Pg C would have been sequestered if biological pump accounted for this change. “In theory” - total increased sequestration of ~740 Pg C Pg = 1015g HNLC – High-Nitrate, Low Chlorophyll LNLC – Low-Nutrient, Low Chlorophyll

18. 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.

19. 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)

20. 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.

21. 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)

22. 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 • d15N correlates with denitrification rates (how?) • Sedimentary organic carbon inc. with depth (laminated at depth of min O) • Interstitial dissolved phosphate conc. exceeding 40mM  supersaturation, P precip. Phosphogenesis occurs almost exclusively in organic-rich, suboxic upwelling margin sediments that underlie oxygen-deficient/ denitrifying bottom waters

23. 10-m-long piston core (NH15P; 425 m water depth from the upper slope of Mexican margin) • d18O 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

24. 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

25. Slowly  glacial N contents increased until N/P ratio was high enough to affect N fixation Rapidly  Redfield ratio restored rapidly As a result, rising N/P shifted ecological advantage from nitrogen-fixers (diazotrophs) to non-nitrogen-fixing algae  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

26. 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 d18O • 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?

27. 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?