CYANOPHAGES - Cyanobacteria - page 471

of the phytoplankton mortality), (ii) consumption of viruses by nanozooplankton (13% of the virus
production), (iii) increased bacterial production and respiration rates (33% above the level without
the viruses) and (iv) a loss nearly of 20% in nanozooplankton production.
Hannen et al . (1999) developed two artifi cial laboratory-scale enclosures fi lled with lake water
(130 litres each) to simulate the conditions of Lake Loosdrecht, The Netherlands and the succession
of microorganisms was followed consequent upon the lysis of fi lamentous cyanobacteria by the
cyanophages. The cyanobacterial populations consisted of Oscillatoria c.f. limnetica and Prochlorothix
hollandica . Electron microscopic observations revealed the presence of lambda-like cyanophage
particles inside the cells of the above cyanobacteria as well as attached to their fi laments. However,
the titre of free viral particles has not been assayed. The microbial communities were dominated by
heterotrophic bacteria belonging to the Cytophagales and actinomycetes, that are known to degrade
complex organic substances. These were followed by the appearance of rotifers that are known to
connect the microbial loop with higher trophic levels. DGGE was chosen to follow the bacterial and
eukaryotic communities and were analysed by the amplifi cation of the sequences of 16S and 18S
rDNA fragments. These observations confi rm and strongly support the idea that the marine viruses
in general can structure microbial communities.
Sano et al . (2004) repeated the experiments of Wilcox and Fuhrman (1994) and confi rmed the
fi nding that marine viruses need a minimal concentration (>3% of natural concentration) in order to
fi nd hosts. They demonstrated further that viruses from different habitats such as near-shore marine
sites, lakewater, marine sediments and soil could be propagated on microbial fl ora from marine water
sample. This led to the idea that viruses can move between different ecosystems and propagate and
that the possibility of LGT between microorganisms of different biomes exists. Breitbart et al . (2004)
found the distribution of podophage T7-like DNA polymerase sequences in major biomes including
marine, freshwater, sediment, terrestrial, extreme and metazoan-associated environments. A wealth
of knowledge has accumulated on various aspects of abundance and importance of viruses in the
environment, culture-based and culture-independent studies of viral diversity, the movement of
viruses between biomes, metagenomic studies of viral diversity, the role of mobile genetic elements
and LGT (Breitbart and Rohwer, 2005; Breitbart et al. , 2007). Current aspects of viral genomics that
remain to be elucidated are as to whether (i) the key “signature” cyanophage sequences confer any
benefi t on the phages and (ii) the role of such host genes acquired by the phage in phage ecology and
host range. The marine phages have developed adaptive mechanisms to infect their hosts growing
in oligotropic environments of marine waters (O'Connell, 2005). Benthic viruses (vibriobenthos)
accounted for 10 7 to 10 10 particles g -1 of benthos of fresh and marine water sediments. As revealed
by PFGE and analysis of their genomes, vibriobenthic assemblages are highly diverse and distinct
from virioplankton (Danovaro et al ., 2008). Breitbart (2012) highlighted many of the “established
truths” about marine viruses and few of the challenges that lie ahead for the future researchers in
this fi eld.
XIV. METAGENOMICS
Our understanding of microbial physiology, genetics and ecology is mainly based on only 1% of the
microbes obtained in pure cultures. So the vast majority of the microbes remain unexplored (Schloss
and Handelsman, 2005). The development of culture-independent approaches for the identifi cation
of microorganisms from their natural habitats with the analysis of 5S and 16S rRNA gene sequences
paved the way for the recognition of new gene sequences as well as new microbes (Lane et al ., 1985;
Giovannoni et al ., 1990; Schmidt et al ., 1991; Stahl et al ., 1985; Barns et al .,1994; Eden et al ., 1999).