Nutrients, biology and elemental stoichiometry

Transcription

1 Nutrients, biology and elemental stoichiometry Subtropics and tropics: oligotrophic = low nutrient, low biomass. Equatorial upwelling regions: Elevated nutrients (1 10 MNO 3 ) and biomass (relative to surrounding waters). High latitude: High nutrients (10 30 MNO 3 ), elevated biomass. 1

2 Nutrients make things grow Where do ocean nutrients come from? Rivers Air Recycling Deep Sea 1) Deep ocean (physics) 2) Recycling (biology) 3) River runoff (mostly physics) 4) Air (both physics and biology) 2

3 Distributions of nitrate Nitrate ( mol N L -1 ) Depth (m) Physical supply of nutrients to the upper ocean Mixingi Upwelling 3

4 Biology establishes direct linkages between elemental cycles Gruber and Galloway (2008) Nature 451: Organic matter production 106 CO HNO 3 + H 3 PO H 2 O (CH 2 O) 106 (NH 3 ) 16 (H 3 PO 4 )+138O 2 Consumes CO 2 Consumes nutrients Produces oxygen Aerobic remineralization of organic matter: (CH 2 O) 106 (NH 3 )16H 3 PO O 2 106CO H 2 O +16HNO 3 + H 3 PO 4 Consumes O 2 Produces CO 2 Recycles nutrients 4

5 Redox states and chemical forms of nitrogen 5 stable oxidation states Oxidized N Transformations of N among these different compounds are controlled by microorganism growth. Reduced N (Sarmiento & Gruber, 2006) Pools and pathways of nitrogen in the sea Nitrogen as a nutrient (nitrogen assimilation): NO 3 NO 2 NH + 4 DON N 2 via N 2 fixation Nitrogen as an e donor (dissimilatory): NH 4+ : ammonium oxidation/annamox NO 2 : nitrite oxidation DON : heterotrophic catabolism Nitrogen as an e acceptor (dissimilatory): NO 3 : denitirification NO 2 : denitrification, annamox NO: denitrification 5

6 N 2 : biologically inert (with a few exceptions); concentrations ~600 mol L 11 NO 3 : concentrations range nanomolar (surface ocean) to micromolar (deep sea) DON : concentrations typically 4 6 mol L 1 in surface ocean, decreasing with depth NO 2 : concentrations typically subnanomolar N 2 O : concentrations typically nanomolar NH 4+ : concentrations typically nanomolar Nitrogen assimilation Nitrogen is an essential nutrient found in amino acids, protein, ti and nucleic acids. Nitrogen is assimilated by both autotrophic and heterotrophic plankton. Nitrogen in organic matter is reduced to the level of NH 4+ ( 3 valence state). Most fixed nitrogen in the ocean exists as nitrate (most oxidized form) so energy is required to assimilate into biomass. 6

7 Plankton production is supported by 2 types of nitrogen: 1) new production supported by external sources of N (e.g. NO 3 and N 2 ), 2) recycled or regenerated production, sustained by recycling of N. Why does this generalization apply to the open sea but not near shore environments? The f ratio Assumptions: 1) N 2 fixation is low 2) Steady state system 3) Euphotic zone nitrification is low f = NO 3 / ( NO 3 + N R ) Note N R includes regenerated forms of N uptake (historically mostly considered urea and NH 4+ ) Mathematical description linking new production and organic matter export. At steady state, nitrogen input is balanced by nitrogen export. N 2 regenerated NH + 4 NO 3 Biological production export Under steady state (i.e. nitrate input balanced by export/grazing loss), if export is less than input, biomass accumulates. This biomass must eventually be exported to keep the system in steady state. NO 3 NO 2 NH 4 + N export 7

8 Determining the f ratio Incubate seawater in the presence of trace 15 NO + 3, 15 NH 4+, and sometimes 15 N urea Calculate NO3, NH4+, and DON uptake What makes this difficult for the oligotrophic ocean? Not all new nutrients are introduced to the euphotic zone from below Duce et al. (2008) Science 320: Atmospheric deposition (both dry and wet) can form an important source of nutrients. Advection: lateral input of nutrients N 2 fixation 8

9 N 2 fixation is the primary mode of introducing fixed nitrogen to the biosphere. N 2 fixation converts N 2 to NH 3 ; process is exclusively mediated by prokaryotes Energy expensive to break triple bond in N 2 N 2 + 8H + + 8e + 16 ATP 2NH 3 + H ADP + 16 PO 4 3 Fritz Haber Carl Bosch Haber Bosch N 2 fixation: 3CH 4 + 6H 2 O 3CO H 2 4N H 2 8NH 3 Requires high temperatures and pressure Provides nitrogen for >30% of the world s food supply. 9

10 Global estimate of N 2 fixation based on N DIC drawdown in NO 3 depleted warm waters is equivalent to Pg C yr 1 10

11 Each annual cycle at HOT and BATS has significant Dissolved Inorganic Carbon (DIC) drawdown, but not enough nitrate is present in surface water to support growth. from Karl et al. in Fasham, Ocean Biogeochemistry Fasham et al. (2003) N 2 fixation may also play an important role in controlling nutrient stoichiometry in the sea 11

12 TDN = 14.57(TDP) NO NO2 - ( mol L ) Nitrogen ( mol L 1 ) TDN : TDP NO3 - : PO4 3- Nearly identical slopes, but different intercepts Phosphorus ( mol L 1 ) PO 4 3- ( mol L -1 ) NO 3 = (PO 4 3 ) 1.08 Let s look at dissimilatory nitrogen transformations Oxidized N Energy to be gained in oxidation Reduced N (Sarmiento & Gruber, 2006) 12

13 Dissimilatory nitrogen transformations NH 2 OH NO N 2 O Nitrification: NH 4 + NO 2 NO 3 Denitrification: NO 3 NO 2 NO N 2 O N 2 Anammox: NO 2 + NH 4 + N 2 + 2H 2 O O 2 concentration ( mol O 2 L -1 ) Depth (m) N+N O NO NO 2 - ( mol L -1 ) (CH 2 O) 106 (NH 3 )16H 3 PO O 2 106CO H 2 O +16HNO 3 + H 3 PO 4 13

14 Organic matter production 106 CO HNO 3 + H 3 PO H 2 O (CH 2 O) 106 (NH 3 ) 16 (H 3 PO 4 )+138O 2 Consumes CO 2 Consumes nutrients Produces oxygen Aerobic remineralization of organic matter: (CH 2 O) 106 (NH 3 )16H 3 PO O 2 106CO H 2 O +16HNO 3 + H 3 PO 4 Consumes O 2 Produces CO 2 Recycles nutrients 14

15 De epth (m) 2000 Pacific Atlantic NO NO 2 - ( mol L -1 ) Aerobic regeneration of nitrogen Complete decomposition of organic matter (CH 2 O) 106 (NH 3 )16H 3 PO O 2 106CO H 2 O +16HNO 3 + H 3 PO 4 Multi step process. First step is the breakdown of amino acids to NH 4+ ; this process is mediated by heterotrophic microorganisms 2NH O 2 2NO 2 + 4H + + 2H 2 O 2NO 2 + O 2 2NO 3 These reactions yield energy (but not much ) Nitrification: predominately mediated by chemoautotrophic microbes (best studied are Nitrosomonas and Nitrobacter) 15

16 Degradation of organic N to ammonium occurs during heterotrophic metabolism. Nitrification is a 2 step process that is mediated by different groups of microbes. The first step (termed ammonium oxidation) oxidizes NH 4+ to NO 2, and the second step converts NO 2 to NO 3. Nitrification Biological oxidation of NH 3 to NO 3 using oxygen as terminal electron acceptor. Two step process; ammonia oxidation followed by nitrite oxidation; both reactions yield energy. NO 2 serves as an important intermediate; incomplete nitrification also yields N 2 O. 16

17 Oxidized N Energy to be gained in oxidation Reduced N (Sarmiento & Gruber, 2006) Recent isolation and cultivation of an abundant archaeal ammonium oxidizer 17