Nitrogen

NITROGEN (N) Sources of N:  precipitation  N fixation in the water & sediments  input from surface & groundwater drainage Losses of N:  effluent outflow from the basin  reduction of NO3- to N2 by bacterial denitrification (N2) return to the atmosphere  permanent loss of inorganic & organic Ncontaining compounds to the sediments N occurs in freshwaters in numerous forms:  dissolved molecular N  organic compounds: amino acids, amines, proteins, humic compounds  ammonia (NH4+)  nitrite (NO2-)  nitrate (NO3-)

i. Nitrate & nitrite •

Plant cells use reduced N:

NO3

NO2

Nitrate reductase

NO2

N2

Nitrite reductase

• the quantity of enzymes differ between algae • drinking water: < 10mg/l NO3-N • NO2 can form methemoglobin (reduce the oxygencarrying capacity of the blood

ii. Ammonia (NH3) & ammonium ion (NH4+)  ammonia occur in aquatic system as ammonium ion (NH4+)  ammonia occur in balance b/ween animal excretory; plant uptake & bacterial oxidation  high toxicity of NH3: NH3 + H2O

NH4OH

NH4 + OH-

 NH4OH is toxic. Toxicity of NH4OH is depend on: - pH - temperature - DO - hardness - species & age of organisms  NH3 in freshwater: < 0.1 mg/l NH4-N  sewage outfall: 10 – 30 mg/l NH4-N

iii. Dissolved organic N (DON)  more abundant in eutrophic systems  eg. Urea: - animal excretory product - broken down to form NH3 by bacteria or enzyme urease  amino sugars: - excreted by plants - utilized for energy & N sources  a chemical modifier that alters the ionic state of many metals iv. N2 gas  max concentration in winter  under stratified conditions: supersaturated at metalimnion & hypolimnion

N TRANSFORMATION 1. Biological N fixation • • • • •

Transformation of N gas to ammonia by an enzyme N2 NH3 nitrogenase (repressed by ammonia)

in lakes: carried out by blue-green algae can accelerate lake eutrophication

•

photosynthetic blue-green algae that generate/dominate N fixation in lakes: - Aphanizomenon - Anabaena - Gleotrichia - Nodularia

•

In streams: Nostoc

•

In estuary: Nodularia, Aphanizomenon

• Photosynthetic N2-fixing bacteria: - important under the anoxic conditions in hypolimnion •

enzyme responsible: nitrogenase

 nitrogenase activity is repressed by the presence of ammonia  further enzyme synthesis is repressed by the presence of nitrate  relationship of N fixation with depth: light dependent  inverse relationship b/ween the rate of N fixation by blue-green algae & the concentration of inorganic N  heterocyst formation & N fixation are suppressed in the presence of nitrate or ammonia  N fixation positively correlated with [DON]  diurnal & seasonal rates of N fixation: - the littoral benthic contribution is larger than the phytoplankton - N fixation needs Fe & Mo

1. i. N fixation by bacteria • Heterotrophic N2 fixing bacteria: • Azotobacter & Clostridium pasteurianum • Growing epiphytically with aquatic plants • Occur in the sediments b) Photosynthetic bacteria • Facultative aerobes & anaerobes • Widely distributed in lake regions • Occurs in the light • Anaerobic condition: green & purple bacteria

2. Nitrification  biological conversion of organic & inorganic nitrogenous compounds from a reduced state to a more oxidized state NH4+ + 11/2 O2

2H+ + NO2- + H2O

(Δ Fº = -66.0 kcal) NH4+

H2N2O2

NH2OH

HNO2

NO2-

 oxidation of NH4+

(bacteria Nitrosomonas – mesophilic)  further oxidation of nitrite to nitrate: NO2- + 1/2O2

NO3-

(bacteria Nitrobacter)

(Δ Fº = -66.0 kcal)

Overall nitrification reactions: NH4+ + 2O2

NO3- + H2O + 2H+

 sediments do not contribute nitrate by nitrification, except at the littoral & during circulation 3. Nitrate reduction & denitrification  nitrate assimilated by algae & larger hydrophytes – reduced to ammonia  molybdenum is required  occur in trophogenic zone (epilimnion)  denitrification by bacterial metabolisme: - the biochemical reduction of oxidized N anions in the oxidation of organic matter: NO3-

NO2-

N2O

N2

• anaerobic bacteria; - Pseudomonas - Achromobacter - Escherichia - Bacillus - Micrococcus • enzyme nitrogen reductase & cofactors iron & Mo • operate similarly under aerobic & anaerobic conditions • eg. Reaction of the oxidation of glucose & reduction of nitrate: C6H12O6 + 12NO3-

12NO2- + 6CO2 + 6H2O

The reduction of nitrite to molecular N: C6H12O6 + 8NO2-

4N2 + 2CO2 + 4CO32- + 6H2O

Oxidation of S:  denitrifying S bacteria, Thiobacillus denitrificans  i. utilization of Sº: 5S + 6KNO3 + 2H2O

3N2 + K2SO4 + 4KHSO4

 ii. Reduction of S compound, eg. thiosulfate: 5Na2S2O3 + 8KNO3 + 2NaHCO3

6Na2SO4 + 4K2SO4 + 2CO2 + H2O + 4N2

 occur chemosynthetically under dark, anaerobic conditions  denitrification decrease in acidic waters & at low T  nitrification & denitrification can occur simultaneously

Oxidation state -3

+5

0

Ammonification

Amino acid synthesis

-(NH2)

Nitrate assimilation Nitrification

NH4+

NH2OH

N2 fixation

H2N2O2

NO2-

NO3-

Denitrification

N2 N2O

Biochemical reactions that influence the distribution of N compounds in water

Seasonal cycle of nitrate:  in winter: - nitrate inflow > uptake by algae - more nitrate is mixed up from the hypolimnion  in summer: - nitrate uptake by plants > inflow - input from the hypolimnion is limited by the thermocline  in the anoxic hypolimnia: - nitrate is denitrified or converted to ammonia  in the hypolimnia of deep oligotrophic lakes: - nitrate persists unchanged or even increased - organic N is converted to nitrate & diffuse from the sediments

0

N03-N mg liter-1 NH4-N mg liter-1 NO2-N mg liter-1

0.2

Depth, m

0 N03 10

Eutrophic

Metalimnion

N02

Anoxic hypolimnion

NH4 30

Oxygenated epilimnion

Sediments (a)

Depth, m

0

NH4-N mg liter-1 N03-N mg liter-1

0.01

0 Oligotrophic 10

30

Oxygenated epilimnion Metalimnion

(in photic zone) NH4

N03 (dark) (nitrite is Undetectable) Sediments (b)

Oxygenated hypolimnion

STREAMS, RIVERS, RAINFALL,SEAWAGE >90% NO3 or NH4 N2

N2

ix

N2O

NO2

–f

n io t a ic ic) f i r nt nox e D (a

at io

n

(a

no xi

c)

Bacteria + growth

PHYTOPLANKTON

ifi

NO2

Zooplankton

gro w

N itr

th

NO3

ca

n

io et

r

tio

n

(o

xi

c)

NH4

N2O anoxia SEDIMENTS

c ex

Fish

DON

Min e

ral

iza

tion

ORGANIC DETRITUS

Nitrate in the photic zone (approximate levels), mg liter-1

0.5

Shallow eutrophic (e.g., Clear Lake, California) Mesotrophic (e.g., Windermere, England) Oligotrophic (e.g., Lake Superior)

Tropical eotrophic, (e.g., Lake George, Uganda)

Oligotrophic (e.g., lake Tahoe, California)

0 Winter

Spring

Summer

Fall

Season

Seasonal changes in nitrate available for plant growth

Water NH4

NO3

N2

Sediment Diffuses up

Diffuses up

Oxic Organic-N + NH4

NO3

Diffuses down

Denitrification Anoxic zone NO3

Diffuses up

N2

Nitrification & denitrification in lake sediments