Abstract
Harmful algal blooms (HABs) can lead to the detrimental effects on co-occurringorganisms through oxygen depletion or toxin productions and alter food web dynamics
that could eventually induce illness or death on marine lives such as dolphins, manatees,sea lions and even human. Recent researches are indicating the increases in thefrequency, severity and geographic distribution of HABs in the world due to theeutrophication and climate change, etc. Therefore, developments of effective
countermeasures against the HAB events are crucial. However, no practical measures ofHABs control are established except clay dispersion in a limited extent in spite of global
expansions of HABs. The control of HABs requires direct interventions withecosystems which easily lead to public concerns and oppositions. Therefore, a measurethat is as environmentally friendly as possible is desired for the future establishment of
HABs control in the world. Biological controls are thought to be the least impact toecosystems in the present circumstances in that it harnesses natural living organisms. In
this dissertation, new prospective of phylogenetic and ecological characteristics ofnatural living bacteria that could potentially be utilized to suppress HABs in coastal sea,
seagrass and macro-algae beds were investigated and discussed through chapter Ⅱ tochapter Ⅴ.
Dynamics of algicidal bacteria (AB) and growth-inhibiting bacteria (GIB) in thecoastal area of Yatsushiro Sea, Japan were revealed in Chapter Ⅱ. These bacteria
showed a close relationship with chlorophyll a fluctuation during a period when noHABs were present indicating new ecological roles of those growth-limiting bacteria(AB and GIB) in regulating phytoplankton populations in natural environments during aperiod when no HABs were present, possibly acting as homeostasis of coastal sea to acertain extent. Moreover, much less frequencies and densities of AB were detected inoffshore waters than the coastal water implied the origin of these bacteria are primarilyin nearshore areas.
In Chapter Ⅲ, the temporal variations of algicidal and growth-inhibiting bacteria
were investigated at the seagrass bed in Miyazu Bay, Kumamoto Prefecture, Japan. It
was clarified that AB were most abundant in summer with the density reaching morethan ten million CFU per 1 g of seagrass blade which was thousand times higher thanthe density in the seawater. Growth-limiting bacteria (AB and/or GIB) were constantly
present in the seawater at the seagrass bed through sampling period except the last
sampling day (the end of September) when most of seagrasses were lost by thewithering. Changes in phytoplankton density and compositions at the seagrass bed
shifted low densities with pennate diatoms dominated to high density with centric
diatom domination corresponding with seagrass loss at the site. It was suggested that theloss of habitat for these bacteria influenced phytoplankton community structures in theseawater.
In Chapter Ⅳ, the distributions of AB and GIB associated with two seagrass species
Z. marina, Z. japonica and several macro-algal species were examined throughout fivedifferent basins in Puget Sound, WA, USA. Between hundred to ten thousand times
higher densities of growth-limiting bacteria were observed in the biofilm of twoseagrass species Z. marina and Z. japonica, and one green alga U. lactuca in Puget
Sound compared to the densities studied for nearshore water of Yatsushiro Sea, Japan(Chapter Ⅱ). The distribution and densities of these bacteria associated with seagrass
and macroalgae were revealed and described first time in USA. In particular, this is thefirst report showing the attachment of GIB on the biofilm of Z. japonica. These states ofseagrass beds as home of AB and GIB are probable to be common phenomena globally.
In Chapter Ⅴ, how predators and competitors of AB and GIB influence theirdynamics in the natural seawater at the seagrass bed were investigated to assess
capability of these bacterial assemblages for controlling the bloom of fish-killingraphidophyte C. antiqua. Bacterial communities living in the seawater at the seagrass
bed and biofilm formed on the seagrass blade effectively suppressed Chattonella bloom
under the presence of their competitors and predatory organisms. Phylogenetic analysis
suggested that considerable numbers of Chattonella growth-limiting bacteria isolated
from the seagrass biofilm and seawater at the seagrass beds are the decomposers of
lignocellulosic compounds that are likely to be abundant at seagrass beds. The directcomparison of 16S rRNA sequences of growth-limiting bacteria isolated from Z. marina
biofilm and seawater at seagrass bed (St. 1) and nearshore water (St. 5) revealed that 4
different strains of growth-limiting bacteria had identical gene sequences to the isolates

from seawater at seagrass bed (St. 1) and also at nearshore water at St. 5, respectively.
Notably, growth-limiting bacteria (strain ZM101) belonging to the genus of
Phaeobacter (alphaproteobacteria) isolated from the biofilm of Z. marine showed
100% similarity of 16S rRNA gene sequence with 4 strains and 3 strains ofgrowth-limiting bacteria isolated from seawater at Sts. 1 and 5, respectively. These
results give a quite reasonable answer to their origins as seagrass biofilms and/or
seawater at seagrass beds.
Seagrass beds are the one of the most important coastal constituents, not only serving
as critical habitat for marine and estuarine animals, providing nursery, migratory
grounds and food resources. In this dissertation, it was indicated that seagrass beds
provide a new important role as the nursery of these algal growth-limiting bacteria.
These new findings give more values to the vital nearshore habitat that could also
potentially enhance the movements toward the protection and restoration of seagrassbeds which may be applied to be environment-friendly HABs control.

********************************************************************************
次回のゼミ [2 月 10 日(水), 9:30～, 204 共用ゼミ室]は修論・卒論発表練習です。