1989 Fiscal Year Final Research Report Summary
Studies on environmental regulation mechanisms of picophytoplankton growth in the aquatic ecosystem
| Project/Area Number |
61440001
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| Research Category |
Grant-in-Aid for General Scientific Research (A)
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| Allocation Type | Single-year Grants |
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| Research Field |
生態学
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| Research Institution | The University of Tokyo |
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Principal Investigator |
TAKAHASHI Masayuki Botany Dept. Univ. Tokyo Ass. Prof., 理学部, 助教授 (50111357)
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| Project Period (FY) |
1986 – 1989
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| Keywords | aquatic ecosystem / picophytoplankton / cyanobacteria / grazing pressure / growth rate / environmental regulation |
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| Research Abstract |
Details of picophytoplankton were not clear other than their cell sizes of less than 2um at the start of this research. Then unicellular cyanobacteria, being known to be one of abundant picophytoplankton in the sea and being recognized easily under a fluorescent microscope, were focused attention. The unicellular cyanobacteria were observed their wide distributions covering from pelagic to neritic ocean and lakes, and showed a rather small spatial and temporal variations in biomass such as about 2 orders of magnitudes compared to a large variation range of such as 5 orders of magnitudes in chlorophyll a__- (total phytoplankton biomass). The cyanobacteria become an exclusive dominant group in pelagic waters where phytoplankton population is small in size.
In situ growth rates of the cyanobacteria determined by three different methods in oligotrophic pelagic ocean equally gave about one division per day, which was equivalent to the rate obtained by unicellular cultures of cyanobacteria un
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der suitable growth conditions in the laboratory. This high in situ rate was also supported by active in situ photosynthetic rates determined by ^<14>C and ^<13>C tracer techniques. From time series experiments of division frequency of the cyanobacterial cells under natural conditions, cell division occurred throughout a day with showing extensive cell division during a few hours around dusk.
Actual biomass changes of the cyanobacteria in the water were observed a rapid increase between dusk and dawn, and a decrease between dawn and dusk. The decrease was evaluated mainly due to grazing by protozoa of flagellates and ciliates.
From the present study it has become clear that increased biomass of the cyanobacteria by active growth has utilized by grazing of protozoa. The biomass of the cyanobacteria was then maintained at a constant steady level which seem to be mainly regulated by grazing pressure. The unicellular cyanobacteria are considered be one of active metabolid agents in pelagic ecosystems. Less
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