The Impact of Microbes on the Environment and Human Activities (page 1) 

(This chapter has 4 pages) 

© Kenneth Todar, PhD 

Beneficial Effects of Microorganisms

Microbes are everywhere in the biosphere, and their presence invariably affects the environment that they are growing in. The effects of microorganisms on their environment can be beneficial or harmful or inapparent with regard to human measure or observation. Since a good part of this text concerns harmful activities of microbes (i.e., agents of disease) this chapter counters with a discussion of the beneficial activities and exploitations of microorganisms as they relate to human culture. 

The beneficial effects of microbes derive from their metabolic activities in the environment, their associations with plants and animals, and from their use in food production and biotechnological processes.

Nutrient Cycling and the Cycles of Elements that Make Up Living Systems

At an elemental level, the substances that make up living material consist of carbon (C), hydrogen (H), oxygen (O), nitrogen (N), sulfur (S), phosphorus (P), potassium (K), iron (Fe), sodium (Na), calcium (Ca) and magnesium (Mg). The primary constituents of organic material are C, H, O, N, S, and P. An organic compound always contains C and H and is symbolized as CH2O (the empirical formula for glucose). Carbon dioxide (CO2) is considered an inorganic form of carbon. 

The most significant effect of the microorganisms on earth is their ability to recycle the primary elements that make up all living systems, especially carbon (C), oxygen (O) and nitrogen (N). These elements occur in different molecular forms that must be shared among all types of life. Different forms of carbon and nitrogen are needed as nutrients by different types of organisms. The diversity of metabolism that exists in the microbes ensures that these elements will be available in their proper form for every type of life. The most important aspects of microbial metabolism that are involved in the cycles of nutrients are discussed below.

Primary production involves photosynthetic organisms which take up CO2 in the atmosphere and convert it to organic (cellular) material. The process is also called CO2 fixation, and it accounts for a very large portion of organic carbon available for synthesis of cell material. Although terrestrial plants are obviously primary producers, planktonic algae and cyanobacteria account for nearly half of the primary production on the planet.  These unicellular organisms which float in the ocean are the "grass of the sea", and they are the source of carbon from which marine life is derived.

NASA receives data from the Terra and Aqua satellites which measures net primary productivity on Earth. These false-color maps represents the rate at which photosynthetic organisms absorb carbon out of the atmosphere. The yellow and red areas show the highest rates, ranging from 2 to 3 kilograms of carbon taken in per square meter per year. The green, blue, and purple shades show progressively lower productivity. Tropical rain forests are generally the most productive places on Earth.  However, primary productivity near the sea�s surface over such a widespread area of the Earth makes the ocean roughly as productive as the land.  http://earthobservatory.nasa.gov/Newsroom/NPP/npp.html

Decomposition or biodegradation results in the breakdown of complex organic materials to forms of carbon that can be used by other organisms. There is no naturally-occurring organic compound that cannot me degraded by some microbe, although some synthetic compounds such as teflon, styrofoam, plastics, insecticides and pesticides are broken down slowly or not at all. Through the metabolic processes of fermentation and respiration, organic molecules are eventually broken down to CO2 which is returned to the atmosphere. 

Waste management, whether in compost, landfills or sewage treatment facilities,  exploits activities of microbes in the carbon cycle.  Organic (solid) materials are digested by microbial enzymes into substrates that eventually are converted to a few organic acids and carbon dioxide.

Nitrogen fixation is a process found only in some bacteria which removes N2from the atmosphere and converts it to ammonia (NH3), for use by plants and animals. Nitrogen fixation also results in replenishment of soil nitrogen removed by agricultural processes. Some bacteria fix nitrogen in symbiotic associations in plants. Other Nitrogen-fixing bacteria are free-living in soil and aquatic habitats.

Some habitats like this cactus community in the Sonoran Desert, rely on nitrogen-fixing bacteria at the base of the food chain as the source of nitrogen for maintenance of cell material. Every plant in this scene depends ultimately on biological nitrogen fixation. http://helios.bto.ed.ac.uk/bto/microbes/nitrogen.htm

Oxygenic photosynthesis occurs in plants, algae and cyanobacteria. It is the type of photosynthesis that results in the production of O2 in the atmosphere. At least 50 percent of the O2 on earth is produced by photosynthetic microorganisms (algae and cyanobacteria), and for at least a billion years before plants evolved, microbes were the only organisms producing O2 on earth. O2 is required by many types of organisms, including animals, in their respiratory processes.

The cyanobacterium, Synechococcus, is a primary component of marine and freshwater plankton and microbial mats,  The unicellular procaryote is involved in primary production, nitrogen fixation and oxygenic photosynthesis and thereby participates in the cycles of carbon, nitrogen and oxygen.  Synechococcus is among the most important photosynthetic bacteria in marine environments, estimated to account for about 25 percent of the primary production that occurs in typical marine habitats. Thomas D. Brock.

The Nitrogen Cycle: Processes, Players, and Human ImpactBy: Anne Bernhard (Department of Biology, Connecticut College) © 2010 Nature Education 

Citation: Bernhard, A. (2010) The Nitrogen Cycle: Processes, Players, and Human Impact. Nature Education Knowledge 3(10):25

Nitrogen is one of the primary nutrients critical for the survival of all living organisms. Although nitrogen is very abundant in the atmosphere, it is largely inaccessible in this form to most organisms. This article explores how nitrogen becomes available to organisms and what changes in nitrogen levels as a result of human activity means to local and global ecosystems.Aa Aa Aa

IntroductionNitrogen is one of the primary nutrients critical for the survival of all living organisms. It is a necessary component of many biomolecules, including proteins, DNA, and chlorophyll. Although nitrogen is very abundant in the atmosphere as dinitrogen gas (N2), it is largely inaccessible in this form to most organisms, making nitrogen a scarce resource and often limiting primary productivity in many ecosystems. Only when nitrogen is converted from dinitrogen gas into ammonia (NH3) does it become available to primary producers, such as plants.In addition to N2 and NH3, nitrogen exists in many different forms, including both inorganic (e.g., ammonia, nitrate) and organic (e.g., amino and nucleic acids) forms. Thus, nitrogen undergoes many different transformations in the ecosystem, changing from one form to another as organisms use it for growth and, in some cases, energy. The major transformations of nitrogen are nitrogen fixation, nitrification, denitrification, anammox, and ammonification (Figure 1). The transformation of nitrogen into its many oxidation states is key to productivity in the biosphere and is highly dependent on the activities of a diverse assemblage of microorganisms, such as bacteria, archaea, and fungi.

Figure 1: Major transformations in the nitrogen cycle© 2010 Nature Education All rights reserved. 

Since the mid-1900s, humans have been exerting an ever-increasing impact on the global nitrogen cycle. Human activities, such as making fertilizers and burning fossil fuels, have significantly altered the amount of fixed nitrogen in the Earth's ecosystems. In fact, some predict that by 2030, the amount of nitrogen fixed by human activities will exceed that fixed by microbial processes (Vitousek 1997). Increases in available nitrogen can alter ecosystems by increasing primary productivity and impacting carbon storage (Galloway et al. 1994). Because of the importance of nitrogen in all ecosystems and the significant impact from human activities, nitrogen and its transformations have received a great deal of attention from ecologists.

Nitrogen FixationNitrogen gas (N2) makes up nearly 80% of the Earth's atmosphere, yet nitrogen is often the nutrient that limits primary production in many ecosystems. Why is this so? Because plants and animals are not able to use nitrogen gas in that form. For nitrogen to be available to make proteins, DNA, and other biologically important compounds, it must first be converted into a different chemical form. The process of converting N2 into biologically available nitrogen is called nitrogen fixation. N2 gas is a very stable compound due to the strength of the triple bond between the nitrogen atoms, and it requires a large amount of energy to break this bond. The whole process requires eight electrons and at least sixteen ATP molecules (Figure 2). As a result, only a select group of prokaryotes are able to carry out this energetically demanding process. Although most nitrogen fixation is carried out by prokaryotes, some nitrogen can be fixed abiotically by lightning or certain industrial processes, including the combustion of fossil fuels.

Figure 2: Chemical reaction of nitrogen fixation© 2010 Nature Education All rights reserved. 

Figure 3: Nitrogen-fixing nodules on a clover plant root© 2010 Nature Education All rights reserved. 

Some nitrogen-fixing organisms are free-living while others are symbiotic nitrogen-fixers, which require a close association with a host to carry out the process. Most of the symbiotic associations are very specific and have complex mechanisms that help to maintain the symbiosis. For example, root exudates from legume plants (e.g., peas, clover, soybeans) serve as a signal to certain species of Rhizobium, which are nitrogen-fixing bacteria. This signal attracts the bacteria to the roots, and a very complex series of events then occurs to initiate uptake of the bacteria into the root and trigger the process of nitrogen fixation in nodules that form on the roots (Figure 3). 

Some of these bacteria are aerobic, others are anaerobic; some are phototrophic, others are chemotrophic (i.e., they use chemicals as their energy source instead of light) (Table 1). Although there is great physiological and phylogenetic diversity among the organisms that carry out nitrogen fixation, they all have a similar enzyme complex called nitrogenase that catalyzes the reduction of N2 to NH3 (ammonia), which can be used as a genetic marker to identify the potential for nitrogen fixation. One of the characteristics of nitrogenase is that the enzyme complex is very sensitive to oxygen and is deactivated in its presence. This presents an interesting dilemma for aerobic nitrogen-fixers and particularly for aerobic nitrogen-fixers that are also photosynthetic since they actually produce oxygen. Over time, nitrogen-fixers have evolved different ways to protect their nitrogenase from oxygen. For example, some cyanobacteria have structures called heterocysts that provide a low-oxygen environment for the enzyme and serves as the site where all the nitrogen fixation occurs in these organisms. Other photosynthetic nitrogen-fixers fix nitrogen only at night when their photosystems are dormant and are not producing oxygen.

Genes for nitrogenase are globally distributed and have been found in many aerobic habitats (e.g., oceans, lakes, soils) and also in habitats that may be anaerobic or microaerophilic (e.g., termite guts, sediments, hypersaline lakes, microbial mats, planktonic crustaceans) (Zehr et al. 2003). The broad distribution of nitrogen-fixing genes suggests that nitrogen-fixing organisms display a very broad range of environmental conditions, as might be expected for a process that is critical to the survival of all life on Earth.

Table 1: Representative prokaryotes known to carry out nitrogen fixation© 2010 Nature Education.

NitrificationNitrification is the process that converts ammonia to nitrite and then to nitrate and is another important step in the global nitrogen cycle. Most nitrification occurs aerobically and is carried out exclusively by prokaryotes. There are two distinct steps of nitrification that are carried out by distinct types of microorganisms. The first step is the oxidation of ammonia to nitrite, which is carried out by microbes known as ammonia-oxidizers. Aerobic ammonia oxidizers convert ammonia to nitrite via the intermediate hydroxylamine, a process that requires two different enzymes, ammonia monooxygenase and hydroxylamine oxidoreductase (Figure 4). The process generates a very small amount of energy relative to many other types of metabolism; as a result, nitrosofiers are notoriously very slow growers. Additionally, aerobic ammonia oxidizers are also autotrophs, fixing carbon dioxide to produce organic carbon, much like photosynthetic organisms, but using ammonia as the energy source instead of light.

Figure 4: Chemical reactions of ammonia oxidation carried out by bacteria

Reaction 1 converts ammonia to the intermediate, hydroxylamine, and is catalyzed by the enzyme ammonia monooxygenase. Reaction 2 converts hydroxylamine to nitrite and is catalyzed by the enyzmer hydroxylamine oxidoreductase.© 2010 Nature Education All rights reserved. 

Unlike nitrogen fixation that is carried out by many different kinds of microbes, ammonia oxidation is less broadly distributed among prokaryotes. Until recently, it was thought that all ammonia oxidation was carried out by only a few types of bacteria in the genera Nitrosomonas, Nitrosospira, andNitrosococcus. However, in 2005 an archaeon was discovered that could also oxidize ammonia (Koenneke et al. 2005). Since their discovery, ammonia-oxidizing Archaea have often been found to outnumber the ammonia-oxidizing Bacteria in many habitats. In the past several years, ammonia-oxidizing Archaea have been found to be abundant in oceans, soils, and salt marshes, suggesting an important role in the nitrogen cycle for these newly-discovered organisms. Currently, only one ammonia-oxidizing archaeon has been grown in pure culture, Nitrosopumilusmaritimus, so our understanding of their physiological diversity is limited.The second step in nitrification is the oxidation of nitrite (NO2

-) to nitrate (NO3-) (Figure 5). This step is carried out by a

completely separate group of prokaryotes, known as nitrite-oxidizing Bacteria. Some of the genera involved in nitrite oxidation include Nitrospira, Nitrobacter, Nitrococcus, andNitrospina. Similar to ammonia oxidizers, the energy generated from the oxidation of nitrite to nitrate is very small, and thus growth yields are very low. In fact, ammonia- and nitrite-oxidizers must oxidize many molecules of ammonia or nitrite in order to fix a single molecule of CO2. For complete nitrification, both ammonia oxidation and nitrite oxidation must occur.

Figure 5: Chemical reaction of nitrite oxidation© 2011 Nature Education All rights reserved. 

Ammonia-oxidizers and nitrite-oxidizers are ubiquitous in aerobic environments. They have been extensively studied in natural environments such as soils, estuaries, lakes, and open-ocean environments. However, ammonia- and nitrite-oxidizers also play a very important role in wastewater treatment facilities by removing potentially harmful levels of ammonium that could lead to the pollution of the receiving waters. Much research has focused on how to maintain stable populations of these important microbes in wastewater treatment plants. Additionally, ammonia- and nitrite-oxidizers help to maintain healthy aquaria by facilitating the removal of potentially toxic ammonium excreted in fish urine.

AnammoxTraditionally, all nitrification was thought to be carried out under aerobic conditions, but recently a new type of ammonia oxidation occurring under anoxic conditions was discovered (Strous et al. 1999). Anammox (anaerobic ammonia oxidation) is carried out by prokaryotes belonging to the Planctomycetes phylum of Bacteria. The first described anammox bacterium was Brocadiaanammoxidans. Anammox bacteria oxidize ammonia by using nitrite as the electron acceptor to produce gaseous nitrogen (Figure 6). Anammox bacteria were first discovered in anoxic bioreactors of wasterwater treatment plants but have since been found in a variety of aquatic systems, including low-oxygen zones of the ocean, coastal and estuarine sediments, mangroves, and freshwater lakes. In some areas of the

ocean, the anammox process is considered to be responsible for a significant loss of nitrogen (Kuypers et al. 2005). However, Ward et al. (2009) argue that denitrification rather than anammox is responsible for most nitrogen loss in other areas. Whether anammox or denitrification is responsible for most nitrogen loss in the ocean, it is clear that anammox represents an important process in the global nitrogen cycle.

Figure 6: Chemical reaction of anaerobic ammonia oxidation (anammox)© 2010 Nature Education All rights reserved. 

DenitrificationDenitrification is the process that converts nitrate to nitrogen gas, thus removing bioavailable nitrogen and returning it to the atmosphere. Dinitrogen gas (N2) is the ultimate end product of denitrification, but other intermediate gaseous forms of nitrogen exist (Figure 7). Some of these gases, such as nitrous oxide (N2O), are considered greenhouse gasses, reacting with ozone and contributing to air pollution.

Figure 7: Reactions involved in denitrificationReaction 1 represents the steps of reducing nitrate to dinitrogen gas. Reaction 2 represents the complete redox reaction of denitrification.© 2010 Nature Education All rights reserved. 

Unlike nitrification, denitrification is an anaerobic process, occurring mostly in soils and sediments and anoxic zones in lakes and oceans. Similar to nitrogen fixation, denitrification is carried out by a diverse group of prokaryotes, and there is recent evidence that some eukaryotes are also capable of denitrification (Risgaard-Petersen et al. 2006). Some denitrifying bacteria include species in the genera Bacillus, Paracoccus, and Pseudomonas. Denitrifiers are chemoorganotrophs and thus must also be supplied with some form of organic carbon.Denitrification is important in that it removes fixed nitrogen (i.e., nitrate) from the ecosystem and returns it to the atmosphere in a biologically inert form (N2). This is particularly important in agriculture where the loss of nitrates in fertilizer is detrimental and costly. However, denitrification in wastewater treatment plays a very beneficial role by removing unwanted nitrates from the wastewater effluent, thereby reducing the chances that the water discharged from the treatment plants will cause undesirable consequences (e.g., algal blooms).

Ammonification

When an organism excretes waste or dies, the nitrogen in its tissues is in the form of organic nitrogen (e.g. amino acids, DNA). Various fungi and prokaryotes then decompose the tissue and release inorganic nitrogen back into the ecosystem as ammonia in the process known as ammonification. The ammonia then becomes available for uptake by plants and other microorganisms for growth.

Ecological Implications of Human Alterations to the Nitrogen CycleMany human activities have a significant impact on the nitrogen cycle. Burning fossil fuels, application of nitrogen-based fertilizers, and other activities can dramatically increase the amount of biologically available nitrogen in an ecosystem. And because nitrogen availability often limits the primary productivity of many ecosystems, large changes in the availability of nitrogen can lead to severe alterations of the nitrogen cycle in both aquatic and terrestrial ecosystems. Industrial nitrogen fixation has increased exponentially since the 1940s, and human activity has doubled the amount of global nitrogen fixation (Vitousek et al. 1997).In terrestrial ecosystems, the addition of nitrogen can lead to nutrient imbalance in trees, changes in forest health, and declines in biodiversity. With increased nitrogen availability there is often a change in carbon storage, thus impacting more processes than just the nitrogen cycle. In agricultural systems, fertilizers are used extensively to increase plant production, but unused nitrogen, usually in the form of nitrate, can leach out of the soil, enter streams and rivers, and ultimately make its way into our drinking water. The process of making synthetic fertilizers for use in agriculture by causing N2 to react with H2, known as the Haber-Bosch process, has increased significantly over the past several decades. In fact, today, nearly 80% of the nitrogen found in human tissues originated from the Haber-Bosch process (Howarth 2008).Much of the nitrogen applied to agricultural and urban areas ultimately enters rivers and nearshore coastal systems. In nearshore marine systems, increases in nitrogen can often lead to anoxia (no oxygen) or hypoxia (low oxygen), altered biodiversity, changes in food-web structure, and general habitat degradation. One common consequence of increased nitrogen is an increase in harmful algal blooms (Howarth 2008). Toxic blooms of certain types of dinoflagellates have been associated with high fish and shellfish mortality in some areas. Even without such economically catastrophic effects, the addition of nitrogen can lead to changes in biodiversity and species composition that may lead to changes in overall ecosystem function. Some have even suggested that alterations to the nitrogen cycle may lead to an increased risk of parasitic and infectious diseases among humans and wildlife (Johnson et al. 2010). Additionally, increases in nitrogen in aquatic systems can lead to increased acidification in freshwater ecosystems.

SummaryNitrogen is arguably the most important nutrient in regulating primary productivity and species diversity in both aquatic and terrestrial ecosystems (Vitousek et al. 2002). Microbially-driven processes such as nitrogen fixation, nitrification, and denitrification, constitute the bulk of nitrogen transformations, and play a critical role in the fate of nitrogen in the Earth's ecosystems. However, as human populations continue to increase, the consequences of human activities continue to threaten our resources and have already significantly altered the global nitrogen cycle.

References and Recommended Reading

Galloway, J. N. et al. Year 2020: Consequences of population growth and development on deposition of oxidized nitrogen. Ambio 23, 120–123 (1994).Howarth, R. W. Coastal nitrogen pollution: a review of sources and trends globally and regionally. Harmful Algae 8, 14–20. (2008).Johnson, P. T. J. et al. Linking environmental nutrient enrichment and disease emergence in humans and wildlife. Ecological Applications 20, 16–29 (2010).Koenneke, M. et al. Isolation of an autotrophic ammonia-oxidizing marine archaeon. Nature 437, 543–546 (2005).

Kuypers, M. M. M. et al. Massive nitrogen loss from the Benguela upwelling system through anaerobic ammonium oxidation. Proceedings of the National Academy of Sciences of the United States of America 102, 6478–6483 (2005).Risgaard-Petersen, N. et al. Evidence for complete denitrification in a benthic foraminifer. Nature 443, 93–96 (2006).Strous, M. et al. Missing lithotroph identified as new planctomycete. Nature 400, 446–449 (1999).Vitousek, P. M. et al. Human alteration of the global nitrogen cycle: sources and consequences. Ecological Applications 7, 737–750 (1997).Vitousek, P. M. et al. Towards an ecological understanding of biological nitrogen fixation. Biogeochemistry 57, 1–45 (2002).Ward, B. B. et al. Denitrification as the dominant nitrogen loss process in the Arabian Sea. Nature 460, 78–81 (2009).Zehr, J. P. et al. Nitrogenase gene diversity and microbial community structure: a cross-system comparison. Environmental Microbiology 5, 539–554 (2003).

Bacteria and Archaea and the Cycles of Elements in the Environment (page 4) 

(This chapter has 4 pages) 

© Kenneth Todar, PhD 

The Sulfur Cycle

Sulfur is a component of a couple of vitamins and essential metabolites and it occurs in two amino acids, cysteine and methionine. In spite of its paucity in cells, it is an absolutely essential element for living systems. Like nitrogen and carbon, the microbes can transform sulfur from its most oxidized form (sulfate or SO4) to its most reduced state (sulfide or H2S). The sulfur cycle, in particular, involves some unique groups of procaryotes and procaryotic processes. Two unrelated groups of procaryotes oxidize H2S to S and S to SO4. The first is the anoxygenic photosynthetic purple and green sulfur bacteria that oxidize H2S as a source of electrons for cyclic photophosphorylation. The second is the "colorless sulfur bacteria" (now a misnomer because the group contains many Archaea) which oxidize H2S and S as sources of energy. In either case, the organisms can usually mediate the complete oxidation of H2S to SO4.

H2S----------------> S ----------------> SO4 litho or phototrophic sulfur oxidation

Sulfur-oxidizing procaryotes are frequently thermophiles found in hot (volcanic) springs and near deep sea thermal vents that are rich in H2S. They may be acidophiles, as well, since they acidify their own environment by the production of sulfuric acid.

Since SO4 and S may be used as electron acceptors for respiration, sulfate reducing bacteria produce H2S during a process of anaerobic respiration analogous to denitrification. The use of SO4 as an electron acceptor is an obligatory process that takes place only in anaerobic environments. The process results in the distinctive odor of H2S in anaerobic bogs, soils and sediments where it occurs.

Sulfur is assimilated by bacteria and plants as SO4 for use and reduction to sulfide. Animals and bacteria can remove the sulfide group from proteins as a source of S during decomposition. These processes complete the sulfur cycle.

Figure 3. The Sulfur Cycle

The Phosphorus cycle

The phosphorus cycle is comparatively simple. Inorganic phosphate exists in only one form. It is interconverted from an inorganic to an organic form and back again, and there is no gaseous intermediate.

Phosphorus is an essential element in biological systems because it is a constituent of nucleic acids, (DNA and RNA) and it occurs in the phospholipids of cell membranes. Phosphate is also a constituent of ADP and ATP which are universally involved in energy exchange in biological systems.

Dissolved phosphate (PO4) inevitably ends up in the oceans. It is returned to land  by shore animals and birds that feed on phosphorus containing sea creatures and then deposit their feces on land. Dissolved PO4 is also returned to land by a geological process, the uplift of ocean floors to form land masses, but the process is very slow. However, the figure below considers how PO4  is recycled among land-based groups of organisms.

Figure 4.The Phosphorus Cycle. Plants, algae and photosynthetic bacteria can absorb phosphate (PO4) dissolved in water, or if it washes out of rocks and soils. They incorporate the PO4 into various organic forms, including such molecules as DNA, RNA, ATP, and phospholipid. The plants are consumed by animals wherein the organic phosphate in the plant becomes organic phosphate in the animal and in the bacteria that live with the animal. Animal waste returns inorganic PO4 to the environment and also organic phosphate in the form of microbial cells. Dead plants and animals, as well as animal waste, are decomposed by microbes in the soil. The phosphate eventually is mineralized to the soluble PO4 form in water and soil, to be taken up again by photosynthetic organisms.  

Ecology of a Stratified Lake

The role of microbes in the global cycle of elements (described above) can be visited on a smaller scale, in a lake, for example, like Lake Mendota, which may become stratified as illustrated in Figure 5. The surface of the lake is well-lighted by the sun and is aerobic. The bottom of the lake and its sediments are dark and anaerobic. Generally there is less O2 and less light as the water column is penetrated from the surface. Assuming that the nutrient supply is stable and there is no mixing between layers of lake water, we should, for the time being, have a stable ecosystem with recycling of essential elements among the living systems. Here is how it would work.

At the surface, light and O2 are plentiful, CO2 is fixed and O2 is produced. Photosynthetic plants, algae and cyanobacteria produce O2, cyanobacteria can even fix N2; aerobic bacteria, insects, animals and plants live here.

At the bottom of the lake and in the sediments, conditions are dark and anaerobic. Fermentative bacteria produce fatty acids, H2 and CO2, which are used by methanogens to produce CH4. Anaerobic respiring bacteria use NO3 and SO4 as electron acceptors, producing NH3 and H2S. Several soluble gases are in the water: H2, CO2, CH4, NH3 and H2S.

The biological activity at the surface of the lake and at the bottom of the lake may have a lot to do with what will be going on in the middle of the water column, especially near the interface of the aerobic and anaerobic zones. This area, called the thermocline, is biologically very active. Bacterial photosynthesis, which is anaerobic, occurs here, using longer wave lengths of light that will penetrate the water column and are not absorbed by all the plant chlorophyll above. The methanotrophs will stay just within the aerobic area taking up the CH4 from the sediments as a carbon source, and returning it as CO2. Lithotrophic nitrogen and sulfur utilizing bacteria do something analogous: they are aerobes that use NH3and H2S from the sediments, returning them to NO3 and SO4.

Dr. Kenneth TodarUniversity of WisconsinDepartment of Bacteriology1550 Linden DriveMadison, Wisconsin 53706e-mail: kgtodar@wisc.edu 

http://textbookofbacteriology.net/environment_4.html

Biology I

B. Fleming

Ecology: Nutrient Cycling

 

Download this Lesson Plan in a Microsoft Word Document

Download Nutrient Cycling PowerPoint

 

Overview:

Presents the four primary nutrient cycles (water, carbon, nitrogen, and phosphorous) through discussion and an interactive PowerPoint presentation in a 50 minute class period. 

 Objectives:

  Content Standards

• Students describe how carbon and soil nutrients cycle through selected ecosystems (Michigan Benchmarks LEC III.5-5)

        Technology Standards

• Teacher demonstrates introductory knowledge, skills, and understanding of concepts related to technology (ISTE NETS I.A.).

• Teacher identifies and locates technology resources and evaluates them for accuracy and suitability (ISTE NETS II.C.)

Activities:

5 min.               What Do You Know?  Starter Question

• The water you drink every day has probably passed through at least one other person’s body.  (Eww!)  Describe how this could happen (safely). 

20 min.            PowerPoint presentation of nutrient cycling. 

• Water Cycle (should be review—used to access and activate prior knowledge of nutrient cycling and use it as a foundation for the next two cycles)

• Carbon Cycle (Start with map, discuss presence of carbon in the world around us.  Go through main steps and touch briefly on the substeps of the cycle.)

• Nitrogen Cycle (Start with map, discuss presence of nitrogen in the world around us.  Go through the main steps and touch briefly on the substeps of the cycle.)

Alternately, this lesson can be navigated by students on individual computers to go along with a set of guiding questions. 

15 min.             Assessment. 

• Phosphorous Cycle.  Have students look at the map and try to describe what they think is happening, based on the cycles they have just learned about.  (This is a simple cycle, with two separate parts.  Have them choose a branch and just describe that one, or do both if time permits.)

• Question: What would happen in the Nitrogen Cycle if the nitrogen-fixing bacteria were not working properly?  Describe the impact on both producers and consumers. 

• Question: Hypothetical scenario: Grandma Johnson had very sentimental feelings toward Johnson Canyon, Utah, where she and her late husband had honeymooned long ago. Her feelings toward this spot were such that upon her death she requested to be buried under a creosote bush overlooking the canyon. Trace the path of a carbon atom from Grandma Johnson’s remains to where it could become part of a hawk. Note: hawk dig not dig up and consume Grandma Johnson’s remains.

10 min.             Homework.

• Students should complete classwork and any other work appropriate at this point in the unit.

http://www.britannica.com/EBchecked/topic/572740/sulfur-cycle