Introduction to Ecological Risk Assessment - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 581

including a cell core, internal membranes and cytoskeleton. Eukaryotes are sub-
divided into four kingdoms: animals ( Animalia ), plants ( Plantae ), fungi ( Fungi ),
and protists ( Protista ). The second domain, the Eubacteria , comprises prokary-
otes, organisms that do not have a cell core and whose DNA is available within
the cell cytoplasm. The third domain consists of the Archaea , a group of single-
celled prokaryotic organisms that distinguish themselves from the other domains
by their extreme habitats and a biochemistry which is very different from the other
domains. They live at high temperatures (sometimes over 100 ◦ C), in hot springs, in
the deep sea, or in extreme alkaline, saline and acid water, and are therefore often
called 'life's extremists'. Most of the Archaea live in anaerobic conditions.
13.2.2.2 Fungi
Fungi (from the Greek word sphongos , meaning sponge) are characterized by a
chitinous cell wall, and often have a filamentous growth pattern. Unlike plants, they
are not able to participate in photosynthesis (Lomako et al. 2004 ) and usually lack
an efficient xylem- or phloem-like vascular system (Mihail and Bruhn 2005 ). Fungi
are the largest organisms of the three groups of common soil microorganisms. Since
the structure of fungi can range from individual cells to enormous chains of cells,
there is a huge variation in size.
Fungi live for the most part in soil (e.g., yeasts and moulds) almost everywhere
in the world, even in desert soils, and they play a very important role in most ecosys-
tems. Around 70,000 fungal species have been formally described by taxonomists,
but the true dimension of fungal diversity is still unknown and might be twice as big
(Mueller and Schmit 2006 ). Many fungi have important symbiotic relationships,
either mutualistic or antagonistic, with many other organisms (Perotto and Bonfante
1997 ).
The amount of fungal biomass in soil may be very high and one gram of soil may
contain 10 to 1000 meters of hyphal threads.
13.2.2.3 Bacteria
Bacteria (from the Greek word bacterion , meaning 'small staff') are prokaryotic
organisms (i.e., do not possess a cell nucleus) and differ from eukaryotic organisms
(organisms with a cell nucleus), in many biochemical aspects. The most typical
characteristics of bacteria is that they are unicellular, have no nucleus and no inter-
nal membranes. Usually they are one to a few micrometers in length (roughly about
50 times smaller than the width of a human hair) and have a wide range of shapes,
ranging from spheres to rods to spirals. Bacteria are found everywhere, in every
habitat, also in every soil in the world. They are the most numerous types of organ-
isms present in soils. Most of the bacteria have not been characterized. Estimates of
the number of bacteria in a gram of soil vary between 40 million (Whitman et al.
1998 ) to 1 billion (Conklin 2002 ). Bacteria are capable of very rapid reproduction,
by dividing in two, when the conditions are favourable. One bacterium is capable of
producing 16 million more in just 24 h (Bacteria Guide 2009 ); however, in soil the