Life and Nutrient Transformation Processes

The Cell

The cell is the smallest independent unit in all living organisms. The cell can also form an individual organism itself. Such organisms are referred to as microorganisms as they are not visible to the naked eye. Examination of the internal structure of the microbial cells reveals two structural types: the prokaryote (Bacteria or Archea) and the eukaryote (Eukarya) (Table 1). The previous group includes the bacteria while the latter contain protozoa, fungi, algae, plants, and animals. Prokaryotic cells have a very simple structure. They lack a membrane-enclosed nucleus and they are very small, typically being from less than 1 mm up to several micrometers. Eukaryotic cells are generally larger and structurally more complex. They contain a membrane-enclosed nucleus, and several membrane-enclosed organelles specialized in performing various cell tasks. The morphological differences between the two cell types have profound effect on their capacities to absorb and transform nutrients and energy. The pro-karyotes have a large surface in relation to their volume meaning short transportation distances within the cell not hindered by complex membrane systems. Their potential to transform and take up nutrients as well as to grow is very high; hence, they can be said to be tailor-made for high metabolic rates. Some bacteria may under optimal conditions multiply by binary division every 20 min. This will result in a rapid exponential increase in cells.

For its growth the cell needs energy, carbon, and macronutrients like nitrogen and phosphorus, and several elements in minor amounts. In addition, an adequate environment is needed, with oxygen, water, temperature, and pH being the most important regulators. Most microorganisms are heterotrophs and organotrophs meaning that they derive their energy and carbon, respectively, from organic molecules (Table 2). Other energy options available are inorganic chemicals (lithotrophs) and light (phototrophs). It is not uncommon that bacteria, like plants, can use carbon dioxide as the carbon source (auto-trophs). Though the most common trait of living is organo-heterotrophic, virtually all combinations above of energy and carbon derivation exist.

Classical taxonomy of microbes is based on phenotypic characters like shape and size, and their relation to oxygen, as well as way of utilizing the carbon and energy source. Two classical shapes of bacteria are the rod and coccus, but filamentous and appendaged forms are also common. In addition to the shape, production ofdifferent enzymes is an important parameter in grouping and identifying bacteria. Recent developments within the nucleic acid-based molecular biology have provided invaluable tools in the systematic of life by genotypic characters. By

Table 1 Cell types and some typical characteristics







Morphology and genetic Cell size

Cell wall components

Cell membrane lipids Membrane-enveloped organelles DNA


Biochemistry and physiology Methane production Nitrification Denitrification Nitrogen fixation Chlorophyll-based photosynthesis Fermentation end products

Small, mostly

0.5-5 mm Peptidoglucane

Ester-linked Absent

One chromosome, circular, naked Yes


Small, mostly

0.5-5 mm Protein, pseudopeptido-glucane Ether-linked Absent

One chromosome, circular, naked Yes


Larger, mostly

5-100 mm Absent, or cellulose or chitin


Mitochondrion, chloroplast, endoplasmatic reticulum, Golgi apparatus Several chromosomes, straight, enveloped


No No No No Yes

Lactate or ethanol

Table 2 Characterization of chemotrophic organisms according to their need of carbon and energy


Carbon source

Examples of primary electron donors

Examples of terminal electron acceptors

Energy metabolism Lithotrophs


Carbon metabolism



Fei, H2 Organic

Respiration: O2, NO3, NO2, S0, SO23, CO2

Respiration: O2, NO^ NO2, SO43, Fe3+, CO2, organic; fermentation: organic

-, not relevant to this term.

-, not relevant to this term.

comparing nucleotide sequences of not known organisms with the emerging database of sequence information, unknown organisms can be identified and/or classified.

The Microbial Community

Aggregated microbial communities called flocs or biofilms are the backbone of most WWT processes (Figures 1a and 1b). The source of microorganisms is soil and sewage coming in with influent wastewater. In the WWT system the organisms are subjected to high selective pressure. Those tolerating the new environment will develop and even thrive to form the basis for an effective WWT process. In any system organic molecules due to their chemical/energetic properties will accumulate at interfaces (gas/liquid or liquid/solid). Hence, these niches will be the first to be colonized and microorganisms with features for keeping the community tightly together, for example, production of extracellular polysaccharides acting as glue, will dominate. The microbial community so formed will consist of a web of different species of bacteria, protozoa, and metazoa. Though present, fungi, algae, and virus probably play a less important role. The communites can be observed as sludge flocks or biofilms. Another advantage of living in dense communities is that environmental gradients, for example, of oxygen and substrate, are formed, allowing many types of organisms to share the space. From the WWT point of view the cooperation of micoorganisms will result in an effective degradation and mineralization of organic matter.


Mineral particle

Protozoa Filamentous Air bubble Organic Polysaccharide matrix with bacteria fiber oxygen and chemical gradients


Mineral particle

Protozoa Filamentous Air bubble Organic Polysaccharide matrix with bacteria fiber oxygen and chemical gradients

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