Presentation on theme: "Nitrogenous Compounds in Water. Evolution of the Nitrogen Cycle Unlike carbon or oxygen, nitrogen is not very available to life. It’s conversion requires."— Presentation transcript:
Evolution of the Nitrogen Cycle Unlike carbon or oxygen, nitrogen is not very available to life. It’s conversion requires biological activity nitrogen cycle is required by life, but also driven by it. Cycle is rather complex and has evolved as the atmosphere became oxygenated. As we know, Earth’s original atm was oxygen-poor.
Evolution of the Nitrogen Cycle Earliest forms of nitrogen-reducing bacteria had to have been anaerobic. other option: NH 4 + already existed in some form. Today, these ancient N-fixers either only exist in anaerobic environments or the N-fixing apparati are carefully guarded from intracellular oxygen.
Evolution of the Nitrogen Cycle As Earth’s atmosphere became more O 2 -rich, more NO 3 became available. This created niches occupied by organisms that could reduce NO 3 to NH 3 (many higher plants can do this). Converting NO 3 back to N 2 (denitrification) is an arduous process and has evolved more recently.
Composition of Atmosphere The atmosphere is primarily composed of nitrogen (N 2, 78%), oxygen (O 2, 21%), and argon (Ar, 1%). A number of other very influential components are also present: the water (H 2 O, 0 to 7%), "greenhouse" gases or ozone (O, 0 to 0.01%), carbon dioxide (CO 2, 0.01to 0.1%). A number of other very influential components are also present: the water (H 2 O, 0 to 7%), "greenhouse" gases or ozone (O, 0 to 0.01%), carbon dioxide (CO 2, 0.01to 0.1%).
Gaseous Nitrogen Nitrogen is the major gas in the atmosphere. After oxygen, second limiting factor. Constitutes 78.1% of total gases in air. Solubility in water is largely dependent upon two physio-chemical factors: temperature and salinity. At saturation/equilibrium it has higher values than oxygen or CO 2 .
Generalized Nitrogen Cycle Nitrogen dynamics in the environment involves some fairly complex cycling. N is relatively unreactive as an element. cyclic conversions from one form to another are mainly mediated by bacteria. Cycle occurs in both aerobic and anaerobic environments. nitrogen cycle
Cycling of Nitrogen Four processes participate in the cycling of nitrogen through the biosphere: 1) Nitrogen fixation 2) Decay 3) Nitrification 4) Denitrification Microorganisms play major roles in these processes
Simplified diagram of the nitrogen cycle that is established in a saltwater aquaria
Process 1: Nitrogen Fixation Three “Fixation” processes are responsible for most of the nitrogen fixation in the biosphere are … Three “Fixation” processes are responsible for most of the nitrogen fixation in the biosphere are … Atmospheric fixation by lightning. Atmospheric fixation by lightning. Biological fixation by certain microbes - alone or in a symbiotic relationship with plants. Biological fixation by certain microbes - alone or in a symbiotic relationship with plants. Industrial fixation. Industrial fixation.
Process 1: Fixation Nitrogen fixation refers to the conversion of N 2 to either NO 3 or NH 4 by bacteria. Terrestrial systems: soil bacteria in root nodules of legumes. Aquatic systems: blue green algae. Biological, meteorological, industrial transformations also occur.
Process 2: Decay The proteins made by plants enter and pass through food webs just as carbohydrates do. At each trophic level, their metabolism produces organic nitrogen compounds that return to the environment, chiefly in excretions. The proteins made by plants enter and pass through food webs just as carbohydrates do. At each trophic level, their metabolism produces organic nitrogen compounds that return to the environment, chiefly in excretions. The final beneficiaries of these materials are microorganisms of decay. The final beneficiaries of these materials are microorganisms of decay. They breakdown the molecules in excretions and dead organisms into ammonia (NH 3 ). They breakdown the molecules in excretions and dead organisms into ammonia (NH 3 ).
Nitrogen Fixation Type of FixationN 2 fixed (10 12 g per year) Non-biological industrialAbout 50 combustionAbout 20 lightningAbout 10 TotalAbout 80 Biological Agricultural landAbout 90 Forest + nonag landAbout 50 SeaAbout 35 TotalAbout 175
Process 3: Nitrification The term nitrification refers to the conversion of ammonium to nitrate (pathway 3-4 opposite). Responsible: nitrifying bacteria known as chemoautotrophs. These bacteria gain their energy by oxidizing NH 3, while using CO 2 as a source of carbon to synthesize organic compounds. The nitrogen cycle, once more!
Process 4: Denitrification By this process, NO 3 in soil or water is converted into atm N 2, nitric oxide or nitrous oxide. This must occur under anaerobic conditions (anaerobic respiration). Presence of O 2 can reverse the reaction. Again, mediated by bacteria (Pseudomonas sp., Alkaligenes sp. and Bacillus sp.). Denitrification = step 5, above
Aquatic Nitrogen Cycling For aquaculturists, cycling transforms usually begin with the decomposition of organic matter from either plant or animal sources. major source in aquaculture: feeds Ultimately excreted as amine groups on amino acids or excreted in soluble form primarily as NH 3 /NH 4 +, other compounds. amino acid
Release of NH 3 NH 3 separated from organic protein via microbial activity. Process referred to as deaminification or ammonification. NH 3 is released to water column (mineralization) and assimilated into primary productivity (NH 3 + H + --> NH 4 + ). ammonification is heterotrophic, under aerobic or anaerobic conditions. ammonification
Aquatic Nitrogen Cycling NH 3 and NH 4 + are both either assimilated by aquatic plants for growth or nitrified (oxidized) to NO 3 - (nitrate). Nitrate can also be used as a growth substrate (Guillard’s F medium). Two step process: NH 4 + + 1.5O 2 NO 2 - + 2H + + H 2 O NO 2 - + 0.5O 2 NO 3 - Note: these are oxygen-driven reactions.
Aquatic Nitrogen Cycling Conversion of ammonia (NH 3 ) to nitrate (NO 3 - ) is via chemoautotrophic bacteria. First step by Nitrosomonas sp. second step by Nitrobacter sp. Both steps/reactions use NH 4 + and NO 2 - as an energy source, CO 2 as a carbon source. This is a non-photosynthetic type of growth.
Aquatic Nitrogen Cycling Reaction runs best at pH 7-8 and 25-30 o C. however; under low DO, it runs in reverse. NO 3 - is converted to NO 2 = and other forms. Can go all the way backwards to NH 3. Occurs in the hypolimnion under eutrophic (stagnant) conditions. REM: nitrogen also fixed by leguminous plants, free living bacteria, blue-green algae. Magnitude of this transform not well studied.
Nitrogen: aqueous forms Gaseous form of nitrogen (N 2 ) is most prevalent. Followed by: nitrite, nitrate, ammonia or ammonium. Nitrite is seldom a problem unless DO levels are low (to be discussed later). Ratio of NH 3 :NH 4 + rises with pH. Unfertilized ponds: TAN (NH 3 +NH 4 + ) = 0.05- 0.075 mg/L. Fertilized ponds: TAN = 0.5 mg/L, 0.075 mg NO 3 -
Nitrogen Amendments Nitrogen added as fertilizer to ponds: urea Immediately upon addition, it starts to decline. Only small portion detectable from metabolic processes. Plants typically take it up, die, mud deposit. Inorganic nitrogen typically denitrified in the hypolimnion. High afternoon pH = increased volatization. urea
Nitrogen Equilibria: NH 3 /NH 4 + Ammonia (NH 3 ) is toxic to fish/inverts. pH affects proportion of NH 3 /NH 4 +. As pH increases, NH 3 increases. Calculation example TAN = 1.5 mg/L, 26 o C, pH = 8.6 Answer: 0.35 mg NH 3 /L Affect of pH/temp on NH 3 /NH 4 + equilibria
More on Ammonia As mentioned, initial source: feed, direct source: excretion. NPU Can calculate daily dosage/loading if you know: NPU and % protein in feed. NPU is 0.4 (approx.) for most aquaculture feeds Equ.: (1.0 - NPU)(pro/6.25)(1000) = g NH 3 /kg feed. For 1.0 ha pond receiving 50 kg of 28% protein feed/day, loading is 1,345 g NH 3. Dilution in 10 x 10 6 L is 0.135 mg NH 3 /L. If NPU stays constant, NH 3 production increases with increased feeding.
Ammonia Toxicity Both NH 3 and NH 4 + are toxic to fish/inverts: As medium NH 3 increases, ability to excrete internal NH 3 decreases (fighting gradient). Blood/tissue NH 3 increases causes increase in blood pH. Result: imbalance in enzyme activity, reduced membrane stability. Increased O 2 consumption by tissues, gill damage, reduced O 2 transport (Root/Bohr, but other direction). Reduced growth, histological changes in gills/other organs.
Ammonia Toxicity Short term exposure toxic at 0.7-2.4 mg/L. 96 hr LC 50 varies from 0.5-3.8 mg/L for most fish. Toxicity tolerance varies due to biological variability of different strains of species. Eggs are most tolerant (fish). Larvae least tolerant, older = more tolerant. Same probably holds true for inverts.
Ammonia Toxicity in Ponds NH 3 is more toxic when DO levels are low. However, toxic effect is probably nullified by resultant increase in CO 2. Thus, increased CO 2 = decreased NH 3. Increased CO 2 = decreased pH. In some cases, fish have been shown to acclimate to increases in NH 3.
Nitrite (NO 2 - ) Toxicity Nitrite reacts with hemoglobin to form methemoglobin. In process, iron converted from ferrous (Fe 2+ ) to ferric (Fe 3+ ) form. Ferric form of iron cannot bind with oxygen Blood changes from red to brown, appears anemic. Those fish having methemoglobin reductase enzyme can convert iron moeity back to ferrous Maybe same for crustaceans?
Nitrite (NO 2 - ) Toxicity Recovery from nitrite toxicity usually occurs when fish are transferred to better water. Complete recovery can occur in 24 h. How does it get into system in first place? Nitrite is quickly transported across gill membrane by lamellar chloride cells. Cells can’t distinguish between NO 2 - and Cl - Thus: nitrite absorption regulated by nitrite:chloride ratio in medium.
Nitrite (NO 2 - ) Toxicity Nitrite is about 55 times more toxic in freshwater vs. 16 ppt seawater. Question: Can you add NaCl to water to reverse nitrite toxicity? 24 hr LC 50 values vary tremendously in fish. Safe bet: authors say 4.5 mg/L
Nitrate (NO 3 - ) Toxicity Nitrate builds up in ponds, like nitrite, when ponds are cooler. Nitrobacter does not function well under cool or cold water conditions. however, nitrates are least toxic form of soluble nitrogen. Effects are similar to nitrite toxicity, but values of levels are much higher.