Question

Explain how the relative position of microbial communities within sediment profiles is likely to change as a result of seasonal inundation in wetland ecosystems.

Answer 1

• Wetland ecosystems are extremely useful communities, and they are ecologically as well as economically important systems due to their high productivity, their nutrient (re)cycling capacities, and their prominent contribution to global greenhouse gas emissions.
• The plant roots in the wetlands often create a oxic-anoxic condition, which facilitates the growth of aerobic as well as anaerobic micro-organisms.
• Input of nutrients and fast recycling due to active aerobes and anaerobes makes these systems highly productive and hence attractive for humans as well as many other organisms.
• Wetlands are globally under pressure due to certain anthropogenic activities, as well as climate changes. All this can lead to changes in the microbial communities residing in that area. In general, in ecosystems such as wetlands, as there is a more saturated environment, hence they experience higher rates of anaerobic respiration - like dentrification, methanogenesis, iron reduction, and sulfate reduction, and depressed rates of aerobic processes - like nitrification.
• In a study which was conducted on the Black Mangrove wetlands, diversity of ammonia-oxidizing Betaproteobacteria, which reflects the changes in hydrology in three different Black mangrove habitats, i.e., locations with dwarf, sparse, and dense trees was calculated. Small, but significant differences in the bacterial communities between the flooded and non-flooded water reservoirs were also detected.
• Whenever there is an anoxic environment(absence of oxygen) due to flooding conditions, ferric compounds are utilized by the resident microbial communities as electron acceptors which causes reduced Fe compounds to accumulate over time. When such soils are drained and the condition becomes aerobic, Fe-oxidizing bacteria become more active, leading to the production of ferric oxyhydroxides (FeOOH), which strongly bind phosphate.
• When the soil will become flooded once again, the cycle will be reversed. Once oxygen has been depleted, denitrification at the expense of soil carbon will dominate microbial respiration until nitrate is exhausted. Dissimilatory Fe reduction will then become energetically favorable, leading to conversion of the ferric-phosphate complexes mentioned earlier into soluble Fe2 +, and thus releasing inorganic phosphate into the groundwater.  
• Many changes can take place due to this Fe, N, P and S cycles. Once the soil becomes aerobic again after the water drains,  the FeS_x precipitates are oxidized by Fe-oxidizing bacteria, producing a pulse of sulfuric acid as a by-product of the process. The resultant acidification of the wetland soil can cause changes in both plant and microbial communities and can be sufficient to mobilize toxic metals, such as aluminium.
• There are photosynthetic bacteria present in wetlands as well. The primary photosynthetic bacteria group is Cyanobacteria. These will form symbiotic relationships with plants, because of their capability to fix nitrogen into a useful inorganic form (ammonium).
• Thus microorganisms play vital roles in the food web, and carry out many important process in nature.