84 4 Taxonomy, Physiology, and Ecology of Aquatic Microorganisms
Fig. 4.25 Some pigments and storage material of algae
4.1.7 Viruses
Until recently, viruses were not thought to be abundant
or important in the aquatic environment. We now know
that they are not only abundant, but that they pro
foundly influence the ecology and food status of the
aquatic environment including seas and oceans.
Viruses are lifeless crystals of nucleic acid which
are able to grow and reproduce only in living cells.
They differ from cells in the following ways, and also
have the following properties:
- Whereas cells contain both DNA and RNA, viruses
contain either DNA or RNA, never both.
2. Viruses have a nucleic acid inner core (the genome)
and outer protein cover, the capsid (see Fig. 4.2 6 ).
3. Viruses enter only susceptible cells: thus there are
viruses which will attack only plants while some
will attack only animals. Even among plants and
animals, some viruses will attack some members
and not the others.
4. All living things are attacked by viruses, including
the microorganisms: Bacteria, fungi, algae, and
protozoa. Viruses attacking bacteria and fungi are
bacteriophages and mycophages, respectively.
Viruses used to be classified on their diseases they
cause and their sizes and shapes, but these criteria have