Bacteria wetland soils

They concluded that both soils, but particularly the wetland soil, had a high richness of nir genes, most of which have not yet been found in cultivated denitrihers. This contrasts with a similar survey by Rosch et al. (2002) in which denitrihcation was not a genetic trait of most of the uncultured bacteria in a hardwood forest soil. 

Upland soils can be transformed into wetland soils as a result of excessive rainfall, poor drainage, and high oxygen demand in the soil. Under these conditions, oxidized forms are reduced as a result of the respiratory requirements of anaerobic bacteria. Similarly, when wetland soils are drained, they function as upland soils, and under these conditions many of the reduced compounds are oxidized by either chemical or biochemical reactions. 

Fungi, which are active in upland environments, cease to exist in wetland soils. This is primarily due to the absence of oxygen and alteration in soil pH (acid to neutral) under anaerobic conditions. Overall, microbial biomass decreases under saturated soil conditions. The metabolic activities of anaerobic bacteria depend on alternate electron acceptors, such as oxidized forms of nitrogen, iron, manganese, and sulfur. Under wetland soil conditions, rates of many microbially mediated reactions decline, and some reactions may be eliminated and replaced by new ones. New microbial reactions are involved in the reduction of oxidized compounds during respiratory processes, resulting in the production of reduced compounds. 

When oxygen is limited, as is the case in wetland soils, unique conditions are set in motion that differentiate wetlands from uplands in snch a way as to increase organic matter in the soil, which may even resnlt in the formation of thick layers of peat, and a change in the distribntion of microorganisms (with anaerobic bacteria being more active) and chemical properties of wetland soil. 

Plants have adapted to the harsh anaerobic conditions of wetland soils. Development of aerenchyma tissnes permit oxygen pumping to the roots, to support root respiration and aerobic bacteria in the root zone. 

Fermenting bacteria in wetland soils are largely obligate anaerobes of the following genera (Molongoski and Klug, 1976) ... 

In biological systems, oxygen serves as an electron acceptor during respiration by bacteria and as a reactant in certain biochemical reactions. In addition, oxygen can be involved in chemical oxidation of reduced species in wetland soils. 

Nitrate reduction rates have been shown to be highly correlated to soil organic matter and soluble or available organic carbon (determined as extractable organic carbon) in soils. Several studies have shown a strong relationship between nitrate reduction and available carbon in soils (Buford and Bremner, 1975 Reddy et al., 1982). Nitrate reduction in wetland soils can be coupled to organic matter mineralization (see Chapter 5), as facultative bacteria use... 

The following biochemical reactions show the utilization of select electron donors during inorganic sulfur reduction by obligate anaerobic bacteria. Listed below are few examples of reactions common to all wetland soils where sulfate is present as an electron acceptor ... 

There is a link between sulfate levels and methyl mercury production in wetland ecosystems (Figure 11.12). High sulfate levels (or salinity levels) inhibit methylation whereas methylation increases when the sulfate levels are low. The increased sulfide formation at high sulfate levels actually inhibits the MeHg production. The sulfide produced reacts with mercury to form HjS and makes mercury less available for the sulfate-reducing bacteria, which methylate the mercury present in wetland soils. 

In wetlands N2 fixation can occur in the water colnmn, in the aerobic water-soil interface, in the anaerobic soil bulk, in the rhizosphere, and on the leaves and stems of plants. Phototrophic bacteria in the water and at the water-soil interface are generally more important than non-photosynthetic, heterotrophic bacteria in the soil and on plant roots (Buresh et al, 1980 Roger 1996). The phototrophs comprise bacteria that are epiphytic on plants and cyanobacteria that are both free-living and epiphytic. A particularly favourable site for cyanobacteria is below the leaf surface of the water fern Azolla, which forms a very efficient symbiosis with the cyanobacterinm Anabaena azollae. This symbiosis and those in various leguminous plants have been exploited in traditional rice prodnction systems to sustain yields of 2 to 4 t ha of grain withont fertilizer for hnndreds of years. 

Snbmerged soils are important sinks for atmospheric snlfnr (Howarth et al 1992). Snlfate washed into wetlands or deposited from the atmosphere is largely rednced to snlflde by sulfate-reducing bacteria. Subseqnent precipitation with metals, especially as FeS, results in more or less permanent removal of the S from the global S cycle. 

Methanotrophs occur at the oxic-anoxic interface of methanogenic habitats, in symbiotic association with animals (Kochevar et al., 1992), and inside wetland plants (Bosse and Erenzel, 1997). Although methanotrophs dominate aerobic CH4 oxidation, NH4-oxidizing bacteria may account for a small amount of the CH4 oxidation activity in soils and sediments (Bodelier and Erenzel, 1999). 

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