河口生態(tài)學(xué)Lecture6NutrientCycles

1、Lecture 6 (mostly from Capone 2000) N is a key constituent of life on Earth Occurs in a complex array of different chemical pools and states in the biosphere All living organisms require N for growth (N - ammonium, amino acids, proteins, organisms) N is thought to limit primary production in much of

2、 the worlds oceans and most estuaries Recent increases in N inputs by man (fertilizer use, emissions) has impact on global carbon cycle (more CO2 fixation & primary production) Exists as a gas, solid, liquid - most of the atmosphere is N2 gasN2 = 78%O2 = 21%CO2 = 0.03%Sources and Inputs Rivers -

3、 deliver organic and inorganic N, in both particulate and dissolved forms Agricultural and urban watersheds are major sources of N Atmospheric Deposition - Wet and Dry; acid rain What are the sources for atmospheric N? Groundwater - septic systems are a major source of N - recent findings for the ou

4、ter continental shelf indicate significant direct inputs Relative Importance of different sources depends on location and climate Nitrogen is removed from marine waters by denitrification and sedimentation (as particulates) to the benthos Most of the N in marine waters is in the form of N2 (dissolve

5、d dinitrogen gas) at a concentration of 1 mM Concentrations relatively uniform in the open ocean - vary as a function of temperature-salinity-pressure dependent solubility Nitrous Oxide (N2O) and nitric oxide (NO) are trace constituents in seawater - due to microbial activity Nitrate (NO3-), Nitrite

6、 (NO2-), and ammonium (NH4+) are the most abundant (non-gaseous) chemical speciesNitrate is mobile, usually the dominant form of N in runoff, riverine input, groundwater discharge & atmospheric deposition Concentrations can vary widely, Large reservoir below the permanent thermocline Nitrite is

7、usually a very minor component of the total N sometimes higher at redox interfaces due to microbial activity Ammonium concentrations vary widely, depending on location Usually at trace concentrations in the open ocean Usually high in hypoxic/anoxic environments - near sewage/wastewater inputs, agric

8、ultural runoff, areas of high benthic biomass (e.g., oyster reefs, clam beds) Concentrations of DIN usually range from 0 to 100 M In Sediments - Most of the DIN is Ammonium - at very high concentrations in estuariesHow is NH4+ different from NH3? A major N pool in marine systems - composed of a vari

9、ety of compounds Amino acids, nucleic acids, urea - much of the DON remains uncharacterized Most DON seems to be composed of refractory biopolymers生物聚合物 (peptidoglycans from bacterial cell walls) Define Labile vs. RefractorySource: Alongi 1998Nitrogen Fixation - conversion of N2 gas to organic N and

10、 ammonium Enzyme is nitrogenase cleaves N2 molecule Prokaryotes only Inhibited by high NH4+ and O2 cyanobacteria heterocysts spatio-temporal separation of nitrogen fixation cyanobacterial mats & marine snow Trichodesmium束毛藻（Trichodesmium）為絲狀的固氮藍(lán)綠細(xì)菌Source: Alongi 1998/crswr/het

11、/crswr/heterocyst.html /./P/./Procaryotes.htm /.//./hetspacing-phenom.html Cyanobacterial Heterocysts Sites of nitrogen fixationhttp:/www.ird.nc/

12、images/trichodesmium_28_03_03.jpghttp:/www.sciam.ru/2005/9/img/cianobacterii.jpghttp:/www.veda.cz/dwn/5430/11369B_Trichodesmium.jpg/science/B/people/ewebb/trichocolonies.jpgTrichodesmiumAn important nitrogen fixing cyanobacteria in oligotrophic surface watersBenthic Cyanobacteria C

13、yanobacterialBenthic Cyanobacteria Cyanobacterial Mats MatsMeasuring rates of Nitrogen Fixation (Nfix) Nitrogenase enzyme cleaves triple bond N2 molecule Acetylene乙炔also contains a triple bond which is cleaved by nitrogenase and forms ethylene Ethylene乙烯concentrations can be measured by gas chromato

14、graphy Method actually measures nitrogenase activity Nitrogenase activity rate slightly higher than N-fix rateN NC CHHC CHHHHAcetylene(ethyne)Ethylene(ethene)Nitrification - oxidation of NH4+ to NO2+ to NO3-Requires O2Denitrification - reduction of NO3- to N2Inhibited by O2Limited by NO3-Is an N exp

15、ort mechanismMay be very important in some estuariesCompetition with phototrophsNitrification-Denitrification arecoupled processesBOTH are microbial processesSource: Alongi 1998Denitrification and N regenerationrates in coastal watersSource: Valiela 1995Source: Valiela 1995Ammonification - conversio

16、n of NO3- to NH4+NO3 and NO2 must be converted to NH3 beforeit can be used by cellsSource: Alongi 1998/terry/229sp02/lectures/Ncycleanim.htmlCopyright 1998, Thomas M. Terry, The University of Conn Nitrate reductase catalyzes NH4 to glutamate谷氨酸谷氨酸 glutamate is a biologically us

17、eable form of NSource: Capone 2000谷氨酰胺Glutamine Conversions between organic and inorganic forms of N are biologically- mediated (mostly by prokaryotes)DIN and DON (?) is used by photoautotrophs and heterotrophic bacteria Note the Redox state of different N formsSource: Capone 2000New vs. Regenerated

18、 NSource: Capone 2000Source: Valiela 1995Source: Valiela 1995General Concepts Bottom water anoxia inhibits nitrification - leads to high NH4+ in overlying waters The Cyanobacterial Paradox The Nutricline Coupling of oxic/anoxic processes Benthic-Pelagic Exchanges Many nitrogen budgets do not include

19、 export due to harvesting and migration(mostly from Valiela 1995) Phosphorus is also an important chemical for life on earth (ATP and nucleic acids!) Is a major element in organic matter Occurs in a molar ratio of 106 C : 16 N : 1 P in particulate organic matter in ocean waters (Redfield Ratio) In n

20、ature, does NOT have a gas state - only exists in particulate or dissolved forms This property has implications for transport mechanismsRemember that N compounds can occur as a solid, liquid, or gas In aerobic environments, phosphorus occurs almost exclusively as orthophosphates Orthophosphate正磷酸鹽is

21、 any salt of H3PO4 (phosphoric acid) Dissociation products are H2PO4-, HPO42-, and PO43-HPO42- is the major ion in seawater Removal of H2O results in condensed forms termed Polyphosphates Polyphosphates common in cells, rare in seawater (may indicate pollution) Organophosphates (low concentrations)

22、are phosphate esters derived from living cells Phosphate (as orthophosphate) is usually the most abundant form of P in estuarine & coastal watersConcentrations of P in coastal waters off Nova ScotiaArrows indicate exchange rates in M P day-1Source: Valiela 1995Two properties of phosphate that de

23、termine concentrations in seawater are. Phosphates readily adsorb to particulates Under aerobic conditions, phosphate adsorbs onto oxyhydrides, calcium carbonate, clay mineral particles. Phosphates are substituted for silicates in the lattice structure of clays Phosphates tend to form insoluble comp

24、ounds with certain metals Readily precipitate with cations such as Ca2+, Al3+, Fe3+ (francolite, hydroxyapatite) Phosphate concentrations usually very low in carbonate environments (coral reefs, carbonate sands)In Anaerobic Environments The combination of bacteria and H2S results in the reduction of

25、 ferric iron (Fe3+) to ferrous iron (Fe2+) This results in greater availability of dissolved phosphate Ferric oxyhydrides dissolve and solubilize phosphate (organic matter coats the clay particles) Thus interstitial porewaters usually have high dissolved phosphate concentrations Some of the dissolve

26、d phosphate may reprecipitate at the oxic/anoxic interface If the water column is anoxic, sediments will be a source of phosphate Phosphate is Regenerated at the sediment-water interfaceThis is a very important process in estuarine environments! Phosphorus occurs as dissolved inorganic phosphorus (D

27、IP), dissolved organic phosphorus (DOP), or as particulate phosphorus (PP) The concentrations of PP DOP DIP (but depends on biology) Phosphate is regenerated mostly by decay of organic phosphorus and animal excreta Major source of “New” P is from terrestrial environments - In estuaries, rivers are t

28、he main input source Concentrations much reduced following the phosphate detergent ban (1970s) River inputs and sediment efflux vary seasonally Benthos is a major reservoir for P in estuaries Source: Valiela 1995 (mostly from Valiela 1995) Sulfur is found in some amino acids (and therefore proteins)

29、 Disulfide bonds are important for complex (secondary & tertiary) protein structure Many microbes use sulfur compounds for energy Sulfur dynamics in seawater are complex! Exists in solid, liquid, gas phases Sulfate (SO42-) is most abundant form in aerobic environments (lots in seawater, 7.7% by

30、weight, only Na and Cl are higher!) Amino acids (methionene -S-), cystine (-S-S-), and cysteine (S-H) are reduced organic sulfur compounds Sulfolipids and sulfate esters are also common (but not very abundant) formsSource: Valiela 1995/terry/229sp02/lectures/Scycleanim.htmlCopy

31、right 1998, Thomas M. Terry, The University of Conn Dissimilative sulfate reduction is the principal source of sulfide (H2S) in anoxic environments High concentrations of sulfide are toxic to aerobes (divers beware!) In oxic environments, sulfide can be oxidized to elemental sulfur, thiosulfate硫代硫酸鹽

32、, sulfite亞硫酸鹽, sulfate硫酸鹽 Some of the sulfide in anaerobic sediments reacts with dissolved iron (Fe) to form FeS and FeS2 (pyrite) Some bacteria may use sulfate as an electron acceptor for respiration Thiosulfate is an important “intermediate” formSource: Valiela 1995 Most of the sulfur transformati

33、ons occur in sedimentsOxidized sediments are orange/brown in colorReduced sediments are black and smelly Sulfate Reducers (Desulfovibrio) and sulfide oxidizers (Beggiatoa) occur close together In a typical vertical profile in estuarine sediments.0 -10 cm - sulfate concentration similar to overlying

34、waters (bioturbation, irrigation)10 - 15 cm - peak in sulfate reduction - formation of FeS15 cm - most of the S is bound as pyrite (FeS2) Sulfate reduction is usually highest in areas with high sedimentation rates (and carbon inputs)Source: Valiela 1995 (mostly from Dawes 1981) Silicon (Si) is an important element for cell wall formation in diatoms and silicoflagellates,