Aquatic Ecology

Back to ChemistryWhat are the forms of N?Where do we naturally find N (forms of N)?

Forms of NitrogenUrea CO(NH2)2

Ammonia NH3 (gaseous)Ammonium NH4+

Nitrate NO3-

Nitrite NO2-

Atmospheric Dinitrogen N2

Organic N

N Cycle in Tanks

Reservoirs of NitrogenRocks - weatheringAtmosphere – 78% of atmosphereOceans – Soluble in waterFreshwater – Headwater Streams = sinksPrimary Producers – Use ammonium &

nitrates to make proteinsConsumers – digest proteins into AA

Nitrogen History• More money and effort are spent on themanagement of N and S than any othermineral nutrient:– Deficiencies are world wide, in cultivatedand natural environments– Excesses cause a degradation of thequality of life, N pollution

Nitrogen History• Adding N to soils is one of the mostcostly parts of agriculture

Significance of N• Nitrogen (N) is an essential component ofDNA, RNA and proteins, the building blocksof life• All organisms require nitrogen to live andgrow• Although the majority of the air we breathe isN2, most of the nitrogen in the atmosphere isunavailable for use by organisms• This is because the strong triple bondbetween the N atoms in N2 molecules makesit relatively inert

Significance of NNitrogen is an incredibly versatileelement, existing in both inorganic andorganic forms as well as many differentoxidation states• The movement of nitrogen between theatmosphere, biosphere and geospherein different forms is described by thenitrogen cycle

Significance of NN2 gas must first be converted to more achemically available form such as ammonium(NH4+), nitrate (NO3-), or organic nitrogen(e.g. urea - (NH3)2CO)• The inert nature of N2 means that

biologicallyavailable nitrogen is often in short supply innatural ecosystems, limiting plant growth andbiomass accumulation

Chemistry of N• The valence range which N undergoesin its biogeochemical cycling is full– Going from loss of all five of its outer shellelectrons (+5) to other elements– To the gain of three electrons from otherelements (-3) to complete all of the orbitalsof its outer electron shell

Chemistry of NOn the right-hand side of the depicted Ncycle, the N atom can eventually lose allfive of its outer shell electrons to O• With this, N can eventually become fullyoxidized as nitrate (NO3-)

Chemistry of N• On the left-hand of the depicted N cycle, Ncan eventually add three electrons to fill all ofits outer shell electron orbitals from elementssuch as hydrogen (H) and carbon (C)• With such gain of electrons, N can be fullyreduced to ammonia (NH3) − which mostcommonly exists in its ionic form, ammonium(NH4+)• Or N can be fully-to-partially reduced inorganic compounds

Nitrogen Cycle - Animation

6 Important Processes1. Nitrogen fixation 2. Nitrogen uptake/Assimilation 3. Nitrogen mineralization4. Nitrification5. Denitrification6. Volatilization

Nitrogen Fixation

R-NH2

NH4 NO2

NO3

NO2

NO

N2O

N2

Nitrogen fixationN2 → NH4+

• N2 is converted to ammonium

• Essential because it is the only way that organisms can attain nitrogen directly from the atmosphere

Energy intensive process: N2 + 8H+ + 8e- + 16

ATP = 2NH3 + H2 + 16ADP + 16 Pi

Nitrogen fixationCertain bacteria, Rhizobium, are the onlyorganisms that fix nitrogen through metabolicprocesses• N fixing bacteria often form symbioticrelationships with host plants (e.g. beans,peas, and clover)• N fixing bacteria inhabit legume root nodulesand receive carbohydrates and a favorableenvironment from their host plant in exchangefor some of the nitrogen they fix

N fixation w/ Blue-Green AlgaeIn aquatic

environments, blue-green algae (really a bacteria called cyanobacteria) is an important free-living nitrogen fixer

Plates 19 & 20 Anacystis bloom; Ford Lake August 2002.

N Fixation Cont’d½ can be

contributed by N-fixing org.

The rest comes from atmospheric deposition (lightning) or runoff.

SalmonAlders

Nitrification

R-NH2

NH4 NO2

NO3NO2

NO

N2O

N2

Nitrification• NH4+ → NO3- or NO2

• Some of the ammonium produced bydecomposition is converted to nitrate via aprocess called nitrification– Nitrosomonas and

Nitrobacter• The bacteria that carry out this reaction gainenergy• Requires the presence of oxygen– circulating or flowing waters and the very

surfacelayers of soils and sediments

Drinking WaterThe U.S. Environmental Protection Agency

has established a standard for nitrogen in drinking water of 10 mg per liter nitrate-N

Unfortunately, many systems (particularly in agricultural areas) already exceed this level

By comparison, nitrate levels in waters that have not been altered by human activity are rarely greater than 1 mg/L

Where would there be higher levels of N in drinking water?

MethemoglobinemiaNitrate is one of the most common

groundwater contaminants in rural areas.It is regulated in drinking water primarily

because excess levels can cause methemoglobinemia, or "blue baby" disease.

Affects nursing infants b/c gut is too acidic (pH 2) for denitrifying bacteria to reduce nitrate to nitrite.

Nitrite combines w/ hemoglobin to produce methemoglobin, does not break down easily or carry Oxygen.

MethemoglobinemiaNitrate in groundwater originates primarily from fertilizers,

septic systems, & manure storage or spreading operations. Fertilizer nitrogen that is not taken up by plants, volatilized, or

carried away by surface runoff leaches to the groundwater in the form of nitrate (nitrification NH4 → NO3).

This not only makes the nitrogen unavailable to crops, but also can elevate the concentration in groundwater above the levels acceptable for drinking water quality.

Nitrogen from manure similarly can be lost from fields, barnyards, or storage locations.

Septic systems also can elevate groundwater nitrate concentrations because they remove only half of the nitrogen in wastewater, leaving the remaining half to percolate to groundwater.

Denitrification

R-NH2

NH4 NO2

NO3

NO2

NO

N2O

N2

DenitrificationNO3- → N2+ → N2O

• Oxidized forms of nitrogen such as nitrate and nitrite (NO2-) are converted to dinitrogen (N2) and, to a lesser extent, nitrous oxide gas

• An anaerobic process that is carried out by denitrifying bacteria, which convert nitrate to dinitrogen in the following sequence:

• NO3- → NO2- → NO → N2O → N2.

DenitrificationEffluent of sewage treatment plants.Denitrification by bacteria converts nitrogen-

oxygen compounds into nitrogen gas.N leaves treatment plant as a gas to reduce

the amount of DIN in effluent.

N Uptake/AssimilationNH4 + → Organic N• The ammonia produced by nitrogen fixingbacteria is usually quickly incorporated intoprotein and other organic nitrogencompounds, either by a host plant, thebacteria itself, or another soil organism• When organisms nearer the top of the foodchain eat, they are using nitrogen that hasbeen fixed initially by nitrogen fixing bacteria

R-NH2

NH4 NO2

NO3

NO2

NO

N2O

N2

Ammonification/MineralizationOrganic N → NH4+• After nitrogen is incorporated into organicmatter, it is often converted back intoinorganic nitrogen• During this process, usually called decay, asignificant amount of the nitrogen containedwithin the dead organisms is converted toammonium• Once in the form of ammonium, nitrogen isavailable for use by plants

Ammonia Volatilization• The process of nitrogen loss as ammonia gas from urea forms under alkaline conditions• During this process, ammonium is converted into NH3 gas which is then lost to the air• In cooler conditions the enzyme breaks down urea much slower• Thus, little ammonia gas is lost when urea is applied to cool soils• Urea may originate from animal manure, urea fertilizers and, to a lesser degree, the decay of plant materials.

ExcretionN compounds are metabolized by animals for

energy & NH3 is a waster product.

If O2 is present = oxidized to NO3 or NO4

If O2 is absent = NH3 will accumulate.Aquatic Snails – N is excreted by diffusion of

(highly toxic) ammonia NH3 into the water.Terrestrial Snails – excrete N as cyclic C-N

compounds (uric acid) b/c NH3 cannot be easily washed away. NH3 would poison their lungs.

Human InfluenceEarly in the 20th century, a German

scientist named Fritz Haber figured out how to fix nitrogen chemically at high temperatures and pressures, creating fertilizers that could be added directly to soil

This technology has spread rapidly over the past century, and, along with the advent of new crop varieties, the use of synthetic nitrogen fertilizers has led to an enormous boom in agricultural productivity

Surface ContaminationAdded nitrogen can lead to nutrient

overenrichment, particularly in coastal waters receiving the inflow from polluted rivers

This nutrient over-enrichment, also called eutrophication, has been blamed for

Increased frequencies of coastal fish-kill events, increased frequencies of harmful algal blooms, and species shifts within coastal ecosystems

Acid RainReactive nitrogen (like NO3-

and NH4+) present in surface waters and soils, can also enter the atmosphere as the smog component nitric oxide (NO) an nitrous oxide (N2O)

Eventually, this atmospheric nitrogen can be blown into nitrogen-sensitive terrestrial environments, causing long-term changes

Acid rain from nitrogen oxides has been blamed for forest death and decline in parts of Europe and the Northeast United States

Acid Rain and Species Shifts

Increases in atmospheric nitrogen deposition have also been blamed for more subtle shifts in dominant species and ecosystems

On nitrogen-poor serpentine soils of northern Californian grasslands, plant assemblages have historically been limited to native species that can survive without a lot of nitrogen

There is now some evidence that elevated levels of atmospheric N input from nearby industrial and agricultural development have paved the way for invasion by non-native plants