Environmental Microbiology of Aquatic and Waste Systems

4.1 Taxonomy of Microorganisms in Aquatic Environments 105

including humans, as well as species that feed on bac­
teria, fungi, algae, and on other nematodes. Four out
of every five multicellular animals on the planet are
nematodes. The majority of nematodes are micro­
scopic, averaging less than a millimeter in length, but
some of the animal parasites are quite large and read­
ily visible to the naked eye. Nematodes that feed on
other organisms are important participants in the
cycling of minerals and nutrients in the ecosystem
that is fundamental to other biological activity. Some
of these nematodes may have major roles in decom­
position, including biodegradation of toxic com­
pounds. In fact, the incidence of certain nematode
species is sometimes used as an indicator of environ­
mental quality (Fig. 4.2 9 ).
Nematodes are, by nature, aquatic organisms. It is
estimated that about 50% of nematode species inhabit
marine environments, although many of these have yet

to be described and characterized. The remainder of the

species inhabit soil and freshwater. In the soil, their

aquatic requirements are satisfied by inhabiting the water

films around soil particles. Parasitic nematodes are bio­

logically active when bathed in moisture films supplied

by water in the tissues or body fluids of the host.

Nematodes in freshwater aquatic systems also serve

as a nutrient source for invertebrates, small vertebrates,

and fungi. The source of food for these nematodes is pri­

marily bacteria, but algae and fungi are also consumed.

The marine environment provides habitat for an

enormous diversity of nematodes, from surface, littoral,

and estuarine zones to the ocean depths. One interest­

ing group of deep sea nematodes are the Rhaptothyridae,

which have no mouth and a very reduced alimentary

tract. The digestive tract is filled with symbiotic

chemoaototrophic bacteria. A similar relationship

exists in the mouthless genus Astomonema.

Fig. 4.29 Marine and freshwater crustaceans and nematode (All items in the table reproduced with permission)

Table 4.19 Rotifers common in aquatic environments (Modified from Saksena 2006 )

S. No. Taxonomic group Common aquatic habitat Food/feeding type Examples

1. Seisonidea Marine epizoic Sessile parasite Seison


2. Bdelloidiae Freshwater planktonic, freshwater sessile, freshwater creeping Suspension feeders Philodina


3. Monogonata Freshwater planktonic Suspension feeders Brachionus
   Keratella
   Polyarthra
   Floscularia
   Raptorial predator Asplanchna