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Applied and Environmental Microbiology, November 2000, p. 4916-4920, Vol. 66, No. 11
Harmful Phytoplankton Section, Harmful Algal
Bloom Division, National Research Institute of Fisheries and
Environment of Inland Sea, 2-17-5 Maruishi, Ohno, Saeki, Hiroshima
739-0452, Japan
Received 21 April 2000/Accepted 29 August 2000
Recent observations that viruses are very abundant and biologically
active components in marine ecosystems suggest that they probably
influence various biogeochemical and ecological processes. In this
study, the population dynamics of the harmful bloom-forming phytoplankton Heterosigma akashiwo (Raphidophyceae) and the
infectious H. akashiwo viruses (HaV) were monitored in
Hiroshima Bay, Japan, from May to July 1998. Concurrently, a number of
H. akashiwo and HaV clones were isolated, and their virus
susceptibilities and host ranges were determined through laboratory
cross-reactivity tests. A sudden decrease in cell density of
H. akashiwo was accompanied by a drastic increase
in the abundance of HaV, suggesting that viruses contributed
greatly to the disintegration of the H. akashiwo bloom as mortality agents. Despite the large
quantity of infectious HaV, however, a significant proportion of
H. akashiwo cells survived after the bloom
disintegration. The viral susceptibility of H. akashiwo isolates demonstrated that the majority of these
surviving cells were resistant to most of the HaV clones, whereas
resistant cells were a minor component during the bloom period.
Moreover, these resistant cells were displaced by susceptible cells,
presumably due to viral infection. These results demonstrated that
the properties of dominant cells within the H. akashiwo population change during the period when a bloom is
terminated by viral infection, suggesting that viruses also play an
important role in determining the clonal composition and maintaining
the clonal diversity of H. akashiwo populations.
Therefore, our data indicate that viral infection influences the
total abundance and the clonal composition of one host algal species,
suggesting that viruses are an important component in quantitatively
and qualitatively controlling phytoplankton populations in
natural marine environments.
0099-2240/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.
Viral Impacts on Total Abundance and Clonal
Composition of the Harmful Bloom-Forming Phytoplankton
Heterosigma akashiwo
*
Corresponding author. Mailing address: Harmful Algal
Bloom Division, National Research Institute of Fisheries and
Environment of Inland Sea, 2-17-5 Maruishi, Ohno, Saeki, Hiroshima
739-0452, Japan. Phone: 81-829-55-0666. Fax: 81-829-54-1216. E-mail: nagasaki{at}nnf.affrc.go.jp.
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