Getting to Grips with the Science inside the Reed Bed

Nitrogen is critical to organisms for the synthesis of proteins, nucleic acids and many other nitrogen containing compounds. The earth's atmosphere consists of 78% molecular nitrogen gas (N₂), but it cannot be used by most living organisms until it has been fixed; that is reduced to ammonia (NH₃).

The inability of most living organisms to utilize molecular nitrogen results in a high demand for already fixed forms of this essential element. Ecological systems are therefore, to a great extent, adapted to the recycling of the more reactive nitrogenous compounds and a shortage of nitrogen in the soil is often the major limiting factor in plant growth.  The process by which this limited amount of nitrogen is circulated and re-circulated throughout the world of living organism is known as the nitrogen cycle.

Sewage treatment within constructed wetlands, represent aquatic ecosystems where the physicochemical conditions and resources (nutrients) define the niche. The solid phases in contact with the aquatic system, represented in this case by the roots of Phragmitis australis and the gravel medium, act as important sites of microbial activity.

Nutrients flow into the system in the form of sewage. Decomposing microorganisms extract energy from these compounds during metabolic processes breaking down organic matter into the inorganic constituents, carbon, nitrogen, sulphur and phosphorus. The process, called mineralization, releases ammonium ions.

Nitrification involves the oxidation of nitrogen, from ammonia to nitrite, then from nitrite to nitrate. This process is carried out by specialized autotrophic bacteria such as Nitrosomonas.

The conversion of nitrate to gaseous forms of nitrogen is called denitrification. It occurs in anaerobic conditions in swamps and marshes.  In wastewater treatment denitrification is very important as it removes nitrate from acquatic environments and thus prevents eutrophication.

An experiment was carried out to apply a range of methods for studying microorganism involved in nitrogen-cycling within a Gravel Bed Hydroponic (GBH) system or constructed wetland. Count procedure based on the Most Probable Number (MPN) technique estimates the site of the microbial population responsible for the transformation of ammonia to nitrite, nitrite to nitrate and finally nitrogen gas for three specific substrate types, gravel, root, and thermanox cover slip, within the GBH system. Other methods used during this microbiological study include; automated spiral plate count, direct observation with-the use of a scanning electron microscope and metabolic and biochemical activity using  miniaturization rapid test.

Results and Discussion

A comparison of data based on results obtained using a variety of methods for estimating population size relating to microbes involved in the cycling of nitrogenous compounds from biofilm adhering to the surfaces of three types of medium, gravel, root, and thermanox cover slip.

Overall results indicate that the population sizes, and therefore total rate of nitrogen transforming processes, in biofilms attached to Phragmites rhizomes, were higher than those found in both the gravel and thermanox cover slip. This relates, firstly to the physicochemical conditions encountered on the respective surfaces, the root surface potentially offering the most heterogeneous environment of the three substrates, and secondly, to air diffusion, via the aerenchyma, and root exudation, stimulating the activity of nitrogen transforming bacteria in the rhizosphere.

Nitrification, an oxygen demanding process, is greater in environments where the level of dissolved oxygen is highest. This reaction is also temperature dependant, the higher temperatures stimulating activity or increasing bacterial populations. Physicochemical variations within biofilm lead to greater overall interactions between metabolically related bacteria, for example, the syntrophic relationship between Nitrosomonas and Nitrobacter. The decomposition of organic matter by microorganisms can be monitored by way of biochemical oxygen demand (BOD), which is the amount of dissolved oxygen (in mg/dm³ water) that disappears from a water sample in a given time at a certain temperature. This is often used as an index of organic pollution, especially sewage.

Extract taken from J D Cornick BSc (Hons) Environmental Biology paper -

An estimate of population size and diversity of microbial consortia involved in nitrogen cycling processes in Gravel Bed Hydroponic (GBH) Constructed Wetlands.

School of Biological Sciences, Portsmouth University – 2001.

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