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Fluctuation of the abundances of microbial communities in coral reef with the tidal cycle

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-Rbl-ll. -1lfar. Sci. Fie.ih.., Kochi Univ. No. 14, pp.31-34, 1994

Proceedings of the Concluding WorkshoP on Coral ReefStudies, Kochi, IVovember 1993

Fluctuation of the Abundances of Microbial Communities

in Coral Reef with the Tidal Cycle

FuKAMI Kimio

Laboratoiy ofAquatic Environmental Science, Kochi University, Nanleoleu, Kochi 783, JaPan

In oligotrophic tropical and subtropical areas, coral reef is considered to be of relatively high productivity and it has rather complicated ecosystems. Among the organisms living in the reef area, corals with their symbiotic microalgae of zooxanthellae are one of the most irnportant producers of organic matter. Our previous finding showed that coral produced much abundant organic matter as mucus, especially when it got some physical stimulation. Since the corals at reef edge part were affected by tidal water movement, significant amount of mucus was pro-duced and was transported into a lagoon. It was often visible that the viscosity of seawater in

lagoon increased by mucus so much as that traces of boat remained on seawater surfaces.

These facts lead us to suppose the fluctuation of the concentration of organic matter, which must result in the changes in microbial abundances and corrimunity structures.

In the present study, fiuctuations of bacteria, heterotrophic nanofiagellates, and pico-sized cyanobacteria on coral reef edge and jn lagoon water were investigated along with tidal cycles, and discussed the serial changes in growth and consumption of microbes.

HERON I. • -a----• BRISBANE

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Fig. 1. Location of seawater sampling stations at Heron Island, Great Barrier Reef, Australia. Stations 1 and 2 were situated on the reef edge, while Stn. 3 was in a lagoon.

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32

FUKAMI, K.

Materials and Methods

Water samples were collected from three stations situated on reef edge and in lagoon of Heron Island (151055'E, 23026.6'S) Great BarTier Reef, Australia (Fig. 1) on September 14 and 15, 1993. Along with tidal cycles, the surface water was sampled, immediately fixed with glu-taraldehyde (final conc. 1 9o), and brought back to the laboratory of Heron Island Research Sta-tion (HIRS), the University of Queensland. One to 3 rnl of subsamples were filtered through O.2 ym Nuclepore filters after stained with DAPI, and the abundances of bacteria, hetetot-rophic nanoflagellates (HNF), and pico-sized cyanobacteria were counted under an epifluoresc-ence microscopy with a conventional method.

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2. Fluctuations of the densities of bacteria, with tidal cycles at Stn. 1. Hi: high tide;

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Sept. 15 AM(Eb) 'PM(Fi)

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heterotrophic nanoflagellates, and Fl: fiood tide' Eb

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ABUNDANCES OF MICROBIAL COMMUNITIES IN CORAL REEF

33 4

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HV-HNF(xl02cells/ml) Fig.

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Sept. f4 AM (Hi) 4. Fluctuations of the densities

with tidal cycles at Stn. 3. Hi:

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Sept. 15 AM(Eb) PM(FI)

heterotrophic nanoflagellates, and FI: flood tide; Eb: ebb tide.

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Results and Discussion

Fluctuations of the abundances of microbes on reef edge and in lagoon were il}ustrated in Figs. 2, 3, and 4. Bacterial densities were in the range of 2 to 5 xlO5 cellsfmi, while those of HNF were in the level of 102 cells/ml. These numbers were comparable to those in coral reefs of Lizard Island, Great Barrier Reef (Moriarty 1979), or of others (summarized in a review, Ducklow, 1990), however, they were slightly lower than those in the open ocean (Yoshinaga et al., 1991), suggesting that abundance or biomass of microorganisms in coral reef ecosystems were not so high as to be expected. However, density differences between bacteria and E[NF were more or less three orders of magnitude, which are consistent with those obmined in the other marine environments (Nakamura et al., 1994).

Densities of pico-sized cyanobacteria, probably Synechococcus spp., were shown in Figs. 2 through 4 with shaded columns. The fluctuations were relatively smal1, around 2xlO` cellslml, except for in the lageon. These abundances were also well-coincided with the numbers pre-viously reported (Moriarty et al., 1985).

Abundances of these three categories of microorganisms were relatively constant at two sta-tions on the coral reef edge (Figs. 2 and 3), while changes in those parameters were significant at a lagoon station and were reciprocal along with tidal cycles (Fig. 4). This result impHes that an environment around a reef edge is like a continuous culture due to the usual water current coming in and out of lagoon over reef flat, while environmental conditions in the Iagoon fluctu-ate much with tidal current. In fact, abundances of bacteria and pico-sized cyanobacteria were low during the fiood tide period as seawater of oligotrophic open ocean came into the lagoon, while at the high and ebb tide period numbers of bacteria and cyanobacteria increased but those of HNF were stil1 low. These results were implicated as follows; bacteria and cyanobac-teria in lagoon grow by using organic mucus andlor inorganic nutrients produced and trans-ported from corals at reef flat, and reached maximum just after high tide (ebb tide), however,

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34

FUKAMI, K,

HNF did not yet respond and were still in Iow numbers, while during the flood tide Gust after low tide) HNF started to grow on bacteria and cyanobacteria, resulting in the increase of HNF and Iow abundances of bacteria and cyanobacteria.

In the present study, unfortunately the concentrations of organic matter were not

deter-mined. Moreover, hourly changes in densities of bacteria, HNF and cyanobacteria were not

monitored along with tidal cycles. To confrrm the speculation mentioned as above, densities of microbes must be detemined more frequently and relationship between microbial fluctuations and tidal cycles should be clarified.

References

DucKLow, H. W., 1990. The biomass, production and fate of bacteria in coral reefs. In: Coral reefs, edited by Z.

Dubinsky, Eisevier, Amsterdam, pp. 265-289.

MoRIARTy, D. J. W., 1979. Biomass of suspended bacteria over coral reefs. Mar. BioL, 53, 193-200.

MoRiARTy, D. J. W., P. C. PoLLARD and W. G. HuNT, 1985. Temporal and spatial variation in bacterial tion in the water colurnn over a coral reeL Mar. Biol., 85, 285-292.

NAKAMuRA, Y., K. FuKAMI, S. SAsAKI and J. HIRoMI, 1994. Population dynamics of bacteria and heterotrophic

nanoflagellates foilowing the summer diatom bloom in the Seto Inland Sea. Bull. Planleton Soc. IaPan, 41, 1-8.

YosHINAGA, I., K. FuKAMI and Y. IsHll]A, 1991. Comparison of DNA and protein synthesis rates of bacterial assemblages between coral reef waters and pelagic waters in tropical ocean. Mar. Ecol. Prog. Ser., 76, 167-174.

Fig. 1. Location of seawater sampling stations at Heron Island, Great Barrier Reef, Australia

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