Nutrient Limitation of Biological Productivity in the Ocean

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

Redfield Ratio of Carbon, Nitrogen, and Phosphorous. C: N: P = 106: 1 In the open ocean: C: N = ~1000: 15 N: P = ~15: 1 Much debate surrounding N vs. P for the limiting factor.

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

Nitrogen Cycle N 2 Nitrogen Fixation denitrifiers diazotrophs Nitrification NH 4 NO 3 - + bacteria Denitrification

Nitrogen Fixation N 2 (atm) reduction (fixation) NH 4 via diazotrophs (cyanobacteria), specifically using nitrogenase (enzyme) What does this mean? • Large reservoir of N 2 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 O 2 in the oceans by oxygenic photoautotrophs. • Provided nitrate, eventual source of: – N for photoautotrophs – Electron acceptor for anerobic bacteria (denitrifiers)

Denitrification N 2 NO 3 N 2 Nitrogen Fixation Denitrification • Occurs in three major regions: -areas of low circulation (fiords) -continental margin sediments -oxygen minima zones • Completes N cycle by returning N 2 to the atmosphere N 2

Evolution of the nitrogen cycle and its influence on the biological sequestration of CO 2 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 • N 2 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. ØDevelopment of the Nitrogen Cycle

Since: Sinking flux of particulate organic N and particulate P Then: > 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 • CO 2 exchange is dependent on limiting nutrient • Dissolved inorganic nitrogen limits productivity Ratio of nitrogen fixation to denitrification is crucial to CO 2 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). Trichodesmium courtesy www. cyanocite. bio. perdue. org Diazotrophs (N-fixers) need 100 times more Fe than organisms that use fixed nitrogen.

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 CO 2 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 N 2 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 ) “In theory” - total increased sequestration of ~740 Pg C Pg = 1015 g HNLC – High-Nitrate, Low Chlorophyll LNLC – Low-Nutrient, Low Chlorophyll Ice core records: Atmospheric CO 2 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.

Summarizing Falkowski • The biological CO 2 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 (NH 15 P; 425 m water depth from the upper slope of Mexican margin)

Box cores: • Near-zero O conc. btwn 150 – 800 m depth • Denitrification = respiration process in low. O waters (prefers 14 N) • Nitrate deficit correlates with zero O • d 15 N correlates with denitrification rates (how? ) • Sedimentary organic carbon inc. with depth (laminated at depth of min O) • Interstitial dissolved phosphate conc. exceeding 40 m. M supersaturation, P precip. Phosphogenesis occurs almost exclusively in organic-rich, suboxic upwelling margin sediments that underlie oxygen-deficient/ denitrifying bottom waters

10 -m-long piston core (NH 15 P; 425 m water depth from the upper slope of Mexican margin) • d 18 O 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

• 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 organicrich, suboxic upwelling margin sediments that underlie oxygen-deficient/ denitrifying bottom waters • P burial compensated in glacial sediments in nonphosphogenic 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-nitrogenfixing 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 (nutrientpoor) 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 d 18 O • 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 CO 2 uptake by the ocean during glacial time?

References Falkowski, PG. 1997. Evolution of the nitrogen cycle and its influence on the biological sequestration of CO 2 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/tricho 4. jpg http: //www. uyseg. org/catalysis/ammonia/amm 7. htm Any Questions?