MIT 12 097 - Nutrients and Biological Productivity

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Nutrients andBiological ProductivityWhat is a “nutrient”?What concentrations are required?How do organisms acquire nutrients?N and PThe Marine P Cycle: Depth ProfileThe Marine P Cycle: Surface VariabilityThe Marine P CycleThe Marine N Cycle: Depth ProfileThe Marine N Cycle: Surface VariabilityThe Marine N CycleThe Marine N CycleMicrobial N CycleN transformations in presence of O2Anoxic N transformationsNitrogen fixationN contamination in coastal env’sWhat does it all mean for the productivity of the ocean?Productivity: DefinitionsLightNutrients andBiological Productivity12.097 LectureJanuary 17, 2006What is a “nutrient”?• An element or compound that is beneficial or required for biological growth• “Essential” nutrients are those that are required for growth – i.e., everything dies if the concentration drops below a threshold• Major: nitrogen (N), phosphorus (P) – silica (Si), sulfur (S) [not for all organisms] • Minor: trace metals (e.g., Fe, Co, Mn, Zn), vitamins (e.g., B12)What concentrations are required?• Stoichiometry of life: Redfield ratio– Empirical ratio of phytoplankton collected from various ocean and lake environments– C:N:P = 106:16:1– The “limiting” nutrient is the one which is in lowest concentration relative to the other two– In freshwater systems, P is limiting nutrient– In marine systems, N is limiting nutrient (though this varies widely in coastal vs. open-ocean systems)How do organisms acquire nutrients?Uptake rate of nutrients is described by Michaelis-Menten kinetics. V = velocity of uptake rateVmax= maximum velocityS = substrate concentrationKS= substrate concentration at V = ½ VmaxNearly linear at small [S]Nearly flat at high [S]SKSVVS+=maxImage removed due to copyright considerations. Please see:http://www.steve.gb.com/science/enzymes.htmlN and P • Different “sizes”: dissolved and particulate– Dissolved = in aqueous solution– Particulate = associated with particles (surface-adsorbed, imbedded within, etc.)• Different “forms”: inorganic and organic–N: NH4+, NO2-, NO3-, urea, proteins, nucleic acids–P: PO4-3(+ HPO4-2, H2PO4-, H3PO4), phospholipids, nucleic acids• Oxidation states– N: N(V) in NO3-, N(0) in N2, N(-3) in NH4+– P: P(V) in PO4-3, other redox states insignificantThe Marine P Cycle: Depth Profile• Depleted at surface (biological uptake)• Remineralized during heterotrophic activity (~1000m and sediments)• Remineralization is Temp-dependent (max in spring in coastal areas)Images from eWOCE removed due to copyright considerations.The Marine P Cycle: Surface VariabilityPlease see: Levitus 1994. (See readings.)Image removed due to copyright considerations.The Marine P Cycle• Abiotic reactions:– Adsorption to positive-charged minerals (e.g., clays)– Complexation with metals (e.g., Fe+3, Al+3, Ca+2)• Biotic reactions:– Uptake by phytoplankton & bacteria•Sources:– Continental weathering of P-containing minerals– Anthropogenic activity (soap)Î River runoff (~90%)• Sinks:– Burial of particulate material (cells, minerals) in sediments– Sea bird guano• τresof diss. PO4-3: 1-2 minPlease see: Valiela, 1995 (Marine Ecological Processes) (See readings.)Image removed due to copyright considerations.The Marine N Cycle: Depth ProfileVery similar behavior to PO4-3Depleted at surface; remineralizedat depthDeep water is a source of NO3-to surface ocean (via upwelling)NH4+and NO2-follow similar depthprofiles (though much lower conc’s)[DON] >> [DIN]Image removed due to copyright considerations.The Marine N Cycle: Surface VariabilityPlease see: Levitus 1994. (See readings.)Image removed due to copyright considerations.The Marine N Cycle• Many more reactions in N cycle than in P cycle– Due to possible changes in redoxstate, presence of gaseous component (N2)• No abiotic reactions• Biotic reactions:–NO3-reduction:• Assimilatory (N used in biosynthesis)• Dissimilatory (N not used in biosynthesis) = Denitrification–NH4+oxidation• Sources:–N2fixation (anthro & natural)– River runoff–Sewage• Sinks:– Biological uptake by cells– Burial in sedimentsPlease see: Valiela, 1995 (Marine Ecological Processes) (See readings.)Image removed due to copyright considerations.The Marine N CycleSurface oceanDeep oceanParticulateNDissolved N2NO3-NO3-NH4+NitrificationVertical mixing/upwellingNH4+DONAssimilationNitrogen fixationWater columndenitrificationSedimentary denitrificationBurial of organic matterCourtesy of Karen Casciotti. Used with permission.Microbial N CycleNH3 NO2-NON2ON2NO3-NH2OHAmmonia oxidationNitrite oxidationNitrificationDenitrificationNitrogen fixationAssimilationAnammox(-III) (-I) (0) (I) (II) (III) (V)N redoxState:Org.NCourtesy of Karen Casciotti. Used with permission.N transformations in presence of O2• Assimilatory NO3-reduction:NO3-Æ NH4+•NH4+is most bioavailable form due to biosynthesis needs (e.g., proteins & nucleic acids)• Organisms prefer NH4+uptake rather than NO3-due to E requirement of NO3-reduction•NH4+is primary remineralization product of zooplankton and other heterotrophs Æ this can be largest source of N for phytoplankton and bacteria in open ocean (low nutrient environments)• Nitrification (oxidation of N):–NH3+ 3/2 O2Æ NO2-+ H2O + 2H+–NO2-+ ½ O2Æ NO3-– Steps 1 and 2 performed by different organisms (e.g., Nitrosomonas and Nitrospira)– Require conservative enzymes such as ammonia monooxygenase (AMO)– Requires O2but can occur at low [O2]– Reduces CO2to organic C– Maximum rate at ~150m in open oceanAnoxic N transformations• Denitrification (reduction of N):• Organic C is oxidized to CO2• Requires 0 or low [O2]• Important organisms: heterotrophic bacteria• Enzymes are highly conserved and membrane-bound (e.g., nitrate reductase)• Largest sink of N in estuaries (15-71%) = 4-5X N2-fixation• Anammox (oxidation andreduction of N):–NH4++ NO2-Æ N2+ 2 H2O• Newly discovered process occurring in bacterial order Planctomycetales• Potential cellular system: Anammoxosome• Seen in anoxic zones of Black Sea, wastewater treatment plants• Rates and prevalence unknownNitrogen fixation•N2-fixation (reduction of N):–N2Æ NH4+• Requires high [Fe+2] and 0 or low [O2] – cells often have anoxic microzones to reduce local [O2]• Inhibited by high [NH4+]• Important organisms:Trichodesmium, OscillatoriaAnthropogenic impact on N2fixation:Currently = natural fixationImage removed due to


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MIT 12 097 - Nutrients and Biological Productivity

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