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. Uchida et al. J. Exp. Mar. Biol. Ecol. 241 1999 285 –299
substance on the other species cultured in the chamber. These procedures were conducted aseptically throughout the experiments. These cultures were carried out under
the same conditions as the bialgal cultures detailed above. After each set of the cultures, cell morphology was carefully observed each day under an inverted microscope. Then
cell density was determined by counting cells in 0.01–0.1-ml culture samples 4 days after the inoculation.
3. Results
3.1. Bialgal cultures Results of the bialgal cultures are shown in Fig. 1. When H
. circularisquama and G.
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mikimotoi were inoculated at densities of 200 cells ml , the growth of H
. circularis- quama in bialgal cultures was almost the same as in the monoalgal cultures control
Fig. 1A. However, the growth of G . mikimotoi was dramatically suppressed within 2
days after inoculation in the bialgal cultures with H . circularisquama, although in
monoalgal culture G . mikimotoi showed exponential growth to the end of the experi-
ment. G . mikimotoi cells had completely died 4 days after inoculation in the bialgal
culture. In this case, the cells became round and finally burst. Fig. 1B shows the growth of each species in bialgal and monoalgal cultures when the
21
initial cell density of G . mikimotoi was 1000 cells ml
, and H . circularisquamama at
21
200 cells ml . The results show almost the same tendency as initial cell densities of
21
200 cells ml . The growth of G
. mikimotoi was extremely depressed, however, the decrease of cell density was relatively moderate in this case.
21
When the initial cell density of G . mikimotoi was increased to 2000 cells ml
, the growth profile of both species entirely changed. The growth of H
. circularisquama was suppressed, and G
. mikimotoi showed active growth compared to the former two cases Fig. 1C. Furthermore, cells of H
. circularisqua became immotile and round to spherical in shape as observed in bialgal cultures with some diatom species Uchida et
al., 1996. These immotile cells of H . circularisqua recovered to the motile form again
when isolated and cultured in fresh medium. Some of these immotile cells became motile within 24 h after isolation in fresh medium.
3.2. Growth in culture filtrates Fig. 2 shows the growth of G
. mikimotoi in the filtrate of H. circularisquama culture. G
. mikimotoi grew as well as the control although the culture filtrate had not been enriched.
H . circularisquama in the culture filtrate of G. mikimotoi, prepared from a G.
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mikimotoi culture of 4600 cells ml , grew as well as the control during the first 6 days
from the inoculation although the final growth was slightly depressed compared to the control Fig. 3A. However, the filtrate of G
. mikimotoi culture with a density of 44 000
21
cells ml was found to considerably suppress the growth of H
. circularisquama Fig.
T . Uchida et al. J. Exp. Mar. Biol. Ecol. 241 1999 285 –299
289
Fig. 1. Growth of Heterocapsa circularisquama and Gymnodinium mikimotoi when cultured alone h, H .
circularisquama; s, G . mikimotoi and together j, H. circularisquama; d, G. mikimotoi varying the initial
cell density of G . mikimotoi in each experiment. Initial cell density of G. mikimotoi was adjusted to 200 A,
21 21
1000 B, and 2000 cells ml C, while that of H
. circularisquama was constant at 200 cells ml in each
experiment. Vertical lines show the standard deviation of the mean n 53. When cell densities were lower than
21
1 cell ml , they were expressed as 0 at the axis of the ordinates.
290 T
. Uchida et al. J. Exp. Mar. Biol. Ecol. 241 1999 285 –299
Fig. 2. Growth of Gymnodinium mikimotoi in culture filtrate of Heterocapsa circularisquama. H . circularis-
21
quama culture of 23 800 cells ml was used for the preparation of the filtrate s, control modified SWM3;
d, culture filtrate of H. circularisquama. Vertical lines show the standard deviation n53.
21
3B. The final cell density reached was only about 10 of the control. Most of the H
. circularisquama cells were in a motile form, and immotile round-elliptical cells were
less than 40 of the total cells. 3.3. Growth in culture when separated with a membrane filter
Fig. 4 shows the growth of G . mikimotoi when cultured with a high cell density of H.
21
circularisquama initial cell density 5 30 000 cells ml separated by a membrane
filter. The growth of G . mikimotoi 4 days after the start of the experiment does not
significantly differ from the control p . 0.2. That is, growth inhibition of G . mikimotoi
by H . circularisquama was not observed.
Fig. 5 shows the growth of H . circularisquama cultured with G. mikimotoi at various
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initial cell densities 4300, 9700, and 29 000 cells ml separated by a membrane filter.
T . Uchida et al. J. Exp. Mar. Biol. Ecol. 241 1999 285 –299
291
Fig. 3. Growth of Heterocapsa circularisquama in culture filtrate of Gymnodinium mikimotoi. G . mikimotoi
21 21
culture of a cell density of 4600 cells ml was used in A, and that of 44 000 cells ml
in B, respectively h, control modified SWM3; j, culture filtrate of Gymnodinium mikimotoi; m, enriched culture filtrate of
Gymnodinium mikimotoi . Vertical lines show the standard deviation n 53.
The growth of H . circularisquama with G. mikimotoi at an initial cell density of 4300
21
and 9700 cells ml was not significantly different from that of the control p . 0.2 and
. 0.05, respectively. On the other hand, the growth of H. circularisquama was suppressed significantly p , 0.05 when the initial cell density of G
. mikimotoi was
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29 000 cells ml . In this case, however, most cells of H
. circularisquama showed active motility, and did not form immotile round cells as observed in bialgal cultures.
4. Discussion
