jarmdars ragnir joseph
Emigdio Binet
As a graduate student at University of Wisconsin-Madison working with Professor Katherine McMahon, I study the microbiome of freshwater. A microbiome is the community of microbes (microscopic organisms, including bacteria) that live within an ecosystem, so I study the bacteria of freshwater systems.
Wetland ecosystems are extraordinarily useful communities (National Resource Counsel 1992). They perform vital environmental functions (denitrification, water purification, flood control, etc) and provide more services per hectare than any other ecosystem (Craig et al. 2008, Richardson 2008). Along with these natural benefits, wetlands also have the ability to reduce the effects of anthropogenic pollution, such as wastewater treatment and excessive fertilizer removal (Keeny 1973, Lee et al. 1969, Nichols 1983). One of the most important functions that wetlands perform is their role in the transformation of nitrogen. Fertilizers generate high nitrate loads and wetlands have the ability to transform this into less harmful forms of nitrogen.
Denitrification is an especially important function carried out by wetland communities (Smith and Ogram 2008, Forshay and Stanley 2005, Craig et al. 2008) as excessive nitrate in the water can contribute to eutrophication. Left unchecked, eutrophication can lead to extensive algal blooms, hypoxia following decomposition of algal biomass, and an abrupt change in the makeup of the overall ecosystem. This phenomenon has been observed in both the Gulf of Mexico and Chesapeake Bay, and is mostly caused by the excessive amounts of fertilizer that end up in the waterways from extensive farming (Hey, 2002) along the Mississippi and Potomac rivers respectively (Galeone et al. 2006, Howarth et al. 1996, Malakoff 1998). Natural wetlands remove nitrate from the water and can be used to alleviate eutrophication. However, because of extensive habitat loss, nitrification of waterways increased drastically during the 20th century (Malakoff 1998, Walter and Merritts 2008).
Wetlands are vital communities, and provide a multitude of services to ecosystem function. They are incredibly diverse ecosystems and have large roles in primary production and floodwater retention. Perhaps one of the most important functions of a wetland is the habitats ability to purify water. Wetlands have the ability to aid in pollutant removal, and microorganisms present in the saturated soils of these wetlands play a large role in performing that function.
These ecosystems are closely linked with estuary and salt marsh systems in that fresh water and salt water combine to form a wide array of salinities. In this environment, the constantly fluctuating water levels (from tidal action) and salt concentrations combine to form a difficult habitat. Certain plants have adapted to these variable conditions to form unique communities capable of flourishing in the extreme environment. These include mangroves, certain grasses, and other salt-tolerant trees and shrubs.
Unlike coastal wetlands, salinity is not as big a contributing factor for inland wetland systems. While salinity is important for various plant and microbial communities, wild fluctuations in the salt concentration are not seen as frequently as in estuarine habitats. Inland wetlands are most common on floodplains along rivers and streams (riparian wetlands), but can also be found in land depressions, surrounding lakes and ponds, and anywhere else where the soil environment is under constant, or near constant, saturation (vernal pools and bogs) (USEPA). Riparian wetlands are unique because they allow the water to percolate through the system slowly as opposed to rushing down a stream channel. Because the water is spread out over a large surface floodplain, the hydric soil microbial communities, along with the plants present are able to filter out nutrients and other pollutants to help purify the water. Because inland wetlands cover a wide range of environmental conditions, classification is broken down further into types of wetlands based on region.
Water availability plays a huge role in determining the processes that can be performed by a wetland. In general, more saturated environments (aquatic wetlands and flooded riparian wetlands) experience higher rates of anaerobic respiration - like dentrification, methanogenesis, iron reduction, and sulfate reduction, and depressed rates of aerobic processes - like nitrification. Constant saturation causes oxygen to be depleted quickly, causing microorganisms to turn to other substrates for energy (Balser, 2006). Microorganisms are quite adept at using other available substrates for energy. Environments that experience wetting and drying cycles tend to be able to perform both aerobic and anaerobic functions depending on the conditions experienced. During wet cycles, anaerobic pathways can be used for energy (dentrification, etc) while in dry cycles, oxygen is present allowing for aerobic cycles to present themselves again.
A variety of insect and animal species can inhabit wetland environments. The availability of standing water makes the habitat an ideal breeding ground for a host of insect species including mosquitoes and gnats. The overabundance of algae and photosynthetic bacteria also provides the insect populations with an easy source of food. Wetlands are particularly important habitats for amphibians and reptiles because of the proximity of open water to vegetated areas. Also, because of the wide array of insects inhabiting the ecosystem, a plentiful source of food is available for the amphibians and reptiles. Larger mammals and birds also are plentiful in marshy environments, again because of the abundance of food found. Overall, the food web found in wetland conditions is often the most complex and involved simply because of the abundance and diversity of life found in the area.
Wetlands microbes mediate many of the vital biogeochemical processes needed in the environment. The carbon, nitrogen, phosphorus, sulfur, and iron cycles all have some role in wetland communities and the bacteria present in the anoxic hydric soils are often responsible for the various oxidations and reductions that occur.
Microbes are very important in the carbon cycle. Many photoautotrophs are responsible for the initial fixing of carbon dioxide into useful sugars that can be used for energy. Aside from primary production, decomposition is also a function of microbial communities in wetland soils. Because of anaerobic conditions, decomposition rates are slow, but overall soil organic matter (SOM) is quite high. Microbial communities in hypoxic conditions have the ability to transform this organic matter into usable forms of mineralized dissolved organic carbon. This process allows plants and other organisms to use these substrates once again for energy. If mineralization did not occur, then carbon would stay in an organic form and be unusable to plants. Microbial communities in the soil can mineralize the SOM into inorganic forms of carbon, like carbon dioxide, that plants can then use for photosynthesis once again.
Under extremely reduced conditions, where no good terminal electron accepters are available, microbes can use carbon dioxide. These methanogenic bacteria use the CO2 as a TEA resulting in the production of methane (CH4) also known as swamp gas. Another group of bacteria, known as methanotrophs, use the methane as their energy source and oxidize it to CO2. In general, methanotrophs are obligate aerobes, meaning that in hydric soils, they will be active right above the aerobic/anaerobic dividing line. Methane is a major greenhouse gas, but because of the placement of methanotrophs, up to 90% CH4 generated in hydric soils can be consumed before it reaches the atmosphere (USDA, 2004).
A similar process to this is dissimilatory nitrate reduction in which bacteria convert nitrate all the way to ammonium, which is then released by the cell. This process is favored by a high ratio of available C to NO3-. This is because the microbes need useable forms of nitrogen, and the conversion all the way to ammonium creates and inorganic form of nitrogen usable to both microbes and plants. Also, a select few groups of chemoautotrophic bacteria can get energy from oxidizing ammonia to nitrite (NO2-) and subsequently nitrate.
Other organisms are capable of nitrification (the process of converting N2 to ammonia), but this process is not as prevalent a pathway as denitrification. Nitrification requires an extensive energy input to convert nitrogen gas to ammonia, and the process is usually only done under conditions of low nitrogen availability. In general, wetlands have high concentrations of available nitrogen (in the form of NO3- and NH3), so the nitrification pathway is not readily used.
Another possible compound that can be used by bacteria as a TEA is sulfate (SO42-). In the reduction process, sulfate is converted to either elemental sulfur or hydrogen sulfide (H2S), which gives off the characteristic smell of rotting eggs. Sulfur-oxidizing bacteria, on the other hand, have the ability to oxidize the sulfides and elemental sulfer back to sulfate, or some other partially oxidized form of sulfur. While this is a useful process, bacteria often will use any available oxidized substrate before sulfate as a TEA. The reduction of sulfate will give the organism energy, but it will be nowhere near the amount gained as if the organism had used oxygen, nitrate, iron, or manganese.
There are also photosynthetic bacteria present in wetlands. The primary photosynthetic bacteria group is cyanobacteria. Often time, these will form symbiotic relationships with plants, because of their capability to fix nitrogen into a useful inorganic form (ammonium).
Archaea are the organisms responsible for the sulfate reductions that occur in wetlands, along with a good portion of the ammonia reductions. These lithotrophic organisms are almost exclusively anaerobic in wetland environments and are classified as nitrifiers, methanogens, and anaerobic methane oxidizers. Some of the common organisms found in this domain include:
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