Part A | 4.4
36 Part A Marine Flora and Fauna
10
30 , and every second about 10
23 viral infections occur in the ocean. Therefore, the monitoring of the
virus population in the marine environment and an
understanding of the virus replication and host specificity are vital and emerging fields in the study of
marine viruses. However, our understanding of the
impact of viruses on global systems is still incomplete [4.4].
4.2 General Characteristic Features of Viruses
Viruses consist of genetic materials such as nucleic
acids, which are either DNA or RNA, surrounded by
a protein coat. They can replicate as obligate intracellular parasites by taking on the biosynthetic functions
within a host cell. All forms of cellular life are susceptible to virus infection. Therefore, we can speculate that
every type of marine organism is host to at least one
type of virus [4.5]. The genome of the DNA virus may
be single or double-stranded, and the structure may be
linear with free 5
0 and 3
0 ends or the ends have a covalently closed circular form. RNA viruses can also
be categorized based on their genome structure: singlestranded RNA (ssRNA), double-stranded (dsRNA), linear, and circular. Single-stranded genomes can be designated as positive (exactly the same as messenger RNA
(mRNA) except thymine replaces uracil in the ssDNA
genome) or negative (complementary to the mRNA)
sense genomes [4.6]. The genome size of viruses
varies from a smaller size of about 1:7 kb (Porcine circovirus (ssDNA) and hepatitis delta virus (ssRNA))
to a larger genome size of approximately 1:2 Mb in
mimivirus double-stranded DNA (dsDNA) [4.7]. The
virus gene expression differs depending on the viral
genome nature. For instance, dsDNA viruses encode
genes similarly to cellular organisms such as plants,
animals, and bacteria. Nevertheless, ssDNA, ssRNA,
and dsRNA viruses following a unique gene expression. Furthermore, most prokaryote, plant, and fungal
virus genomes are dsDNA, ssRNA, and dsRNA, respectively [4.6]. The structure and morphology of viruses
are also comparable with other virus groups. Some of
viruses have a lipid layer or compound which is responsible for the development of an envelope along
with proteins on the surface of virions. The envelope involves the attachment and entry of virions into the host
cells.
4.3 Host Specificity
Every virus has a host specificity and a particular
virus can infect a particular species or cells. However, some viruses can infect multiple organisms or
a wide range of species. The wide host range suggests that viruses might be involved in different host
shift events. Viruses must adapt to the new genetic
and immunologic environment of their hosts in order to replicate [4.8]. Furthermore, the interactions
between virus and host are crucial for viral replication. Moreover, the proteins or receptor molecules,
which are not uniform for all species for virus attachments, may play an important role in host specificity [4.9, 10]. Thus, viruses must often co-evolve with
their hosts, which often leads to a species and host
specificity [4.8]. Marine environments contain huge
life-forms and many different species living in close
proximity, which may allow the possibility of viral host
shift events.
4.4 Viral Families in Marine Ecosystems
About 15 DNA viral families persist in the marine environment; however, the majority of the families are
of the dsDNA virus family rather than ssDNA viruses.
Microviridae and parvoviridae belong to the ssDNA
virus family among the DNA virus families in the
marine environment. Microviridae is one of the families of bacteriophages with single-stranded circular
DNA genome. The virions are nonenveloped, icosahedral with spikes and a size of about 2527 nm
in diameter and lack tails. The genome sizes range
36 Part A Marine Flora and Fauna
10
30 , and every second about 10
23 viral infections occur in the ocean. Therefore, the monitoring of the
virus population in the marine environment and an
understanding of the virus replication and host specificity are vital and emerging fields in the study of
marine viruses. However, our understanding of the
impact of viruses on global systems is still incomplete [4.4].
4.2 General Characteristic Features of Viruses
Viruses consist of genetic materials such as nucleic
acids, which are either DNA or RNA, surrounded by
a protein coat. They can replicate as obligate intracellular parasites by taking on the biosynthetic functions
within a host cell. All forms of cellular life are susceptible to virus infection. Therefore, we can speculate that
every type of marine organism is host to at least one
type of virus [4.5]. The genome of the DNA virus may
be single or double-stranded, and the structure may be
linear with free 5
0 and 3
0 ends or the ends have a covalently closed circular form. RNA viruses can also
be categorized based on their genome structure: singlestranded RNA (ssRNA), double-stranded (dsRNA), linear, and circular. Single-stranded genomes can be designated as positive (exactly the same as messenger RNA
(mRNA) except thymine replaces uracil in the ssDNA
genome) or negative (complementary to the mRNA)
sense genomes [4.6]. The genome size of viruses
varies from a smaller size of about 1:7 kb (Porcine circovirus (ssDNA) and hepatitis delta virus (ssRNA))
to a larger genome size of approximately 1:2 Mb in
mimivirus double-stranded DNA (dsDNA) [4.7]. The
virus gene expression differs depending on the viral
genome nature. For instance, dsDNA viruses encode
genes similarly to cellular organisms such as plants,
animals, and bacteria. Nevertheless, ssDNA, ssRNA,
and dsRNA viruses following a unique gene expression. Furthermore, most prokaryote, plant, and fungal
virus genomes are dsDNA, ssRNA, and dsRNA, respectively [4.6]. The structure and morphology of viruses
are also comparable with other virus groups. Some of
viruses have a lipid layer or compound which is responsible for the development of an envelope along
with proteins on the surface of virions. The envelope involves the attachment and entry of virions into the host
cells.
4.3 Host Specificity
Every virus has a host specificity and a particular
virus can infect a particular species or cells. However, some viruses can infect multiple organisms or
a wide range of species. The wide host range suggests that viruses might be involved in different host
shift events. Viruses must adapt to the new genetic
and immunologic environment of their hosts in order to replicate [4.8]. Furthermore, the interactions
between virus and host are crucial for viral replication. Moreover, the proteins or receptor molecules,
which are not uniform for all species for virus attachments, may play an important role in host specificity [4.9, 10]. Thus, viruses must often co-evolve with
their hosts, which often leads to a species and host
specificity [4.8]. Marine environments contain huge
life-forms and many different species living in close
proximity, which may allow the possibility of viral host
shift events.
4.4 Viral Families in Marine Ecosystems
About 15 DNA viral families persist in the marine environment; however, the majority of the families are
of the dsDNA virus family rather than ssDNA viruses.
Microviridae and parvoviridae belong to the ssDNA
virus family among the DNA virus families in the
marine environment. Microviridae is one of the families of bacteriophages with single-stranded circular
DNA genome. The virions are nonenveloped, icosahedral with spikes and a size of about 2527 nm
in diameter and lack tails. The genome sizes range