Pacific Science Volume 48, Number 3, 1994

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Pacific Science is a quarterly publication devoted to the biological and physical sciences of the Pacific Region.


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    Ratio of Energy and Nutrient Fluxes Regulates Symbiosis between Zooxanthellae and Corals
    (University of Hawaii Press, 1994-07) Dubinsky, Z. ; Jokiel, P.L.
    Ambient irradiance levels determine the rate of carbon influx into zooxanthellae at any given time, and thereby the energy available for the whole coral symbiotic association. Long-term photoacclimation of zooxanthellae to the time-averaged light regime at which the host coral grows results in optimization of light harvesting and utilization. Under high irradiance light harvesting is reduced, thereby avoiding photodynamic damage, whereas under low light, photon capture and quantum yield are maximized. Most of the photosynthate produced by the algae is respired. However, the capability of the zooxanthellae and the coral to retain carbon beyond that required to meet their respiratory needs depends on the availability of the commonly limiting nutrients, nitrogen and phosphorus. Therefore, the ratio of the flux of these nutrients into the colony to that of the photosynthetically driven carbon flux will regulate the growth of the zooxanthellae and of the animal. Nutrients acquired by predation of the coral on zooplankton are available first to the animal, whereas those absorbed by the zooxanthellae from seawater as inorganic compounds lead first to growth of the algae.
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    Resource Partitioning by Reef Corals as Determined from Stable Isotope Composition II. 15N of Zooxanthellae and Animal Tissue versus Depth
    (University of Hawaii Press, 1994-07) Muscatine, L. ; Kaplan, I.R.
    The pattern of resource partitioning versus depth for corals collected in February, 1983, from Jamaica was investigated by analyzing their stable nitrogen isotope composition. Observations were made on isolated zooxanthellae and corresponding algae-free animal tissue from nine species of symbiotic corals at four depths over a 50-m bathymetric range, and from a nonsymbiotic coral at 1 m. 5 15N values versus depth ranged from +3.54 to -2.15 %0 for zooxanthellae and from +4.71 to +0.23 %0 for animal tissue. In those species that occurred over a 30- to 50-m depth range, both animal tissue and zooxanthellae tended to be depleted in 15N as depth increased to 30 m. In a few species animal tissue was enriched in 15N from 30 to 50 m. Depletion of 15N in zooxanthellae with increasing depth may be the result of depth-dependent differences in their nitrogen-specific growth rates. Animal tissue was consistently more depleted in 15N than for the nonsymbiotic coral Tubastrea coccinea (Ellis) at the same depth, but it was still slightly more enriched in 15N than corresponding zooxanthellae in 16 of 25 paired samples. The latter trend was not correlated with depth. A comparison of 5 13C and 5 15 N for zooxanthellae and animal tissue over 50 m revealed a tendency toward depletion of heavy isotopes as depth increases. Increased carbon fixation appears to be accompanied by decreased nitrogen fractionation.
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    Effects of Water Velocity on Respiration, Calcification, and Ammonium Uptake of a Porites compressa Community
    (University of Hawaii Press, 1994-07) Atkinson, M.J. ; Kotler, E. ; Newton, P.
    Colonies of Porites compressa Dana were placed in a 10-m-long flume to form a community of coral. Ammonium uptake (N uptake) rate, respiration rate, and calcification rate were measured at different water velocities, ranging from 1 to 57 cm sec-1. N uptake was proportional to concentration from 20 to 0.15 uM N. The first-order rate constant for N uptake varied from 6.8 to 15.6 day-1, only an average of 2.1 times over a l0-fold change in water velocity. First-order rate constants for respiration were less than those for N uptake and ranged from 4.8 to 6.6 day-1. Respiration rate and calcification rate were not correlated with water velocity. The relative turnover of N compared with oxygen (O2) indicates that 94-98% of N flux must be retained within this coral community.
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    Effect of Ammonium-supplemented Seawater on Glutamine Synthetase and Glutamate Dehydrogenase Activities in Host Tissue and Zooxanthellae of Pocillopora damicornis and on Ammonium Uptake Rates of the Zooxanthellae
    (University of Hawaii Press, 1994-07) Yellowlees, D. ; Rees, TAV ; Fitt, W.K.
    Host glutamine synthetase activity decreases in Pocillopora damicornis (Linnaeus) following exposure of the coral to seawater containing elevated ammonium (20 uM). Zooxanthellae isolated from these corals exhibited lower ammonium uptake capacity and glutamine synthetase activity compared with those from the control corals. Ammonium concentration of the surrounding seawater had no effect on the NADPH-dependent glutamate dehydrogenase activity in the host.
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    Effect of Ammonium Enrichment on Respiration, Zooxanthellar Densities, and Pigment Concentrations in Two Species of Hawaiian Corals
    (University of Hawaii Press, 1994-07) Stambler, Noga ; Cox, Evelyn F. ; Vago, Razi
    Small branch tips or "nubbins" of two species of Hawaiian corals, Pocillopora damicornis (Linnaeus) and Montipora verrucosa Vaughan, were exposed to four ammonium concentrations, ammonium-stripped < 2 uM), ambient (~2 uM), and two enriched (20 uM and 50 uM) in microcosm tanks. Nubbins represent replicates of a single coral colony. We examined the effect of ammonium enrichment on zooxanthellar densities, pigment concentrations, and respiration rates of the nubbins. Nubbins of both P. damicornis and M. verrucosa showed a trend of increased pigment concentration with elevated ammonium concentration. Pocillopora damicornis increased from 9.3 ug chlorophyll a cm-2 in the ammonium-stripped treatment to 24.8 ug cm-2 in the 50-uM ammonium treatment. Similarly, M. verrucosa increased from 1.9 to 19.4 ug chlorophyll a cm-2. There were no significant differences in algal densities, pigment concentrations per cell, pigment ratios, or respiration rates.
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    Effect of Ammonium Enrichment on Animal and Algal Biomass of the Coral Pocillopora damicornis
    (University of Hawaii Press, 1994-07) Muller-Parker, G. ; McCloskey, L.R. ; Hoegh-Guldberg, O. ; McAuley, P.J.
    Algal and animal biomass parameters of colonies of the Pacific coral Pocillopora damicornis (Linnaeus) were measured as a function of time of exposure to elevated concentrations of seawater ammonium (20 and 50 uM [(NH4)2S04]) ranging from 2 to 8 weeks. Areal concentrations of zooxanthellae, chlorophyll, and protein increased with 20 uM ammonium addition. During the 8-week period of exposure to 20 uM ammonium, the population density of zooxanthellae increased from 3.5 to 7.5 x 105 cells cm-2, chlorophyll a content of zooxanthellae increased from 5.7 to 8.6 pg, and animal protein concentration doubled (from 0.74 to 1.38 mg cm-2). These data indicate that both the coral animal and the zooxanthellae respond to the addition of exogenous dissolved inorganic nitrogen provided as 20 uM ammonium. Growth of the symbiotic association in response to the addition of 20 uM ammonium adds further evidence to support the argument that growth of tropical symbioses is limited by the availability of nitrogen. However, the coral response is likely to depend on the concentration of ammonium provided, because the biomass parameters of corals held at 50 uM ammonium did not change significantly with time of exposure to the added nutrient.
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    Population Dynamics of Symbiotic Zooxanthellae in the Coral Pocillopora damicornis Exposed to Elevated Ammonium [(NH4)2 SO4] Concentrations
    (University of Hawaii Press, 1994-07) Hoegh-Guldberg, Ove
    Division synchrony and growth rate of symbiotic zooxanthellae was investigated for populations living in colonies of the reef-building coral Pocillopora damicornis (Linnaeus) exposed to different concentrations of ammonium [(NH4)2S04] in seawater. Presence of low concentrations of ammonium (0.2 uM) did not affect (compared with corals growing in ammoniumstripped seawater) either division synchrony or growth rate. Exposure to higher concentrations of ammonium (20 or 50 uM), however, affected the population dynamics of the zooxanthellae residing in P. damicornis. Zooxanthellae in corals exposed to 20 uM ammonium had mitotic indices (percentage of total cells dividing) that were two to three times higher than mitotic indices of zooxanthellae in control (0.2 uM) corals. Although division of zooxanthellae was still phased in corals exposed to 20 uM ammonium, there were many more cells dividing out of phase compared with control corals. Division of zooxanthellae in corals exposed to 50 uM was not phased. Calculated growth rates of zooxanthellae exposed to 20 or 50 uM ammonium were higher than those representative of zooxanthellae living in control corals, although growth rate of both carbon and nitrogen pools was lower in 50 uM as compared with 20 uM ammonium. These data support the conclusion that the population dynamics of symbiotic zooxanthellae within P. damicornis are affected by concentrations of ammonium in seawater that are equal to or higher than 20 uM and that 50 uM ammonium concentrations may be toxic to some extent. These data taken in isolation, however, do not constitute an effective test of the hypothesis that zooxanthellae are limited by the supply of ammonium under ambient conditions and further emphasize the importance of enrichment studies concentrating on growth and nitrogen incorporation rates measured for the entire symbiotic association.
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    Effect of Exogenous Nitrogen Levels on Ultrastructure of Zooxanthellae from the Hermatypic Coral Pocillopora damicornis
    (University of Hawaii Press, 1994-07) Berner, Tamar ; Izhaki, Ido
    Branches of the hermatypic coral Pocillopora damicornis (Linnaeus) were exposed for 2, 4, 6, and 8 weeks to ammonium concentrations of < 1 uM (nutrient-stripped), 2 uM (seawater as a control), 20 uM, and 50 uM (enriched), after which their symbiotic zooxanthellae were examined for changes in their ultrastructure. No significant differences among treatments were detected in cell diameter or in relative volume of any of the cellular organelles of zooxanthellae subjected to the various nitrogen levels. The surface density of thylakoids was higher in cells from the elevated-nitrogen treatments. However, there was a significant increase in accumulation of starch grains and lipid droplets in zooxanthellae in corals maintained in unenriched and nutrient-stripped seawater, occupying about 15% of the cell volume. Storage of these N-free compounds showed that under N-limited conditions photosynthate cannot be used as carbon skeletons in synthesis of amino and nucleic acids, both required for cell doubling. We believe that our results further demonstrate the uncoupling of photosynthesis from population growth under C : N ratios deviating from those needed to support balanced growth.
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    Amino Acid Content of Zooxanthellae Freshly Isolated from Pocillopora damicornis
    (University of Hawaii Press, 1994-07) McAuley, P.J.
    Total amino-N content and glutamine to glutamate ratios (gln: glu) were determined in zooxanthellae freshly isolated from colonies of the coral Pocillopora damicornis (Linnaeus) incubated in ambient seawater or in seawater supplemented with ammonium to give a final concentration of 20 or 50 uM. Addition of ammonium did not change total amino-N content but did increase gln: glu from 0.25 to 0.47-0.48, suggesting that ammonium was directly utilized by the symbiotic zooxanthellae. Gln: glu in zooxanthellae from corals maintained in seawater "stripped" of ammonium fell to 0.18. Sizes of pools of most free amino acids in zooxanthellae from P. damicornis were roughly two to five times those of zooxanthellae from the temperate sea anenome Anemonia viridis, but the latter, which is not believed to be N-limited, exhibited higher gln: glu ratios. These data indicate that gln: glu is a sensitive measure of the response of symbiotic zooxanthellae to exogenous dissolved nitrogen, but despite an increase in gln: glu when seawater is supplemented with ammonium, it cannot be concluded that individual zooxanthellae are normally N-limited.
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    Elemental Composition of the Coral Pocillopora damicornis Exposed to Elevated Seawater Ammonium
    (University of Hawaii Press, 1994-07) Muller-Parker, G. ; Cook, C.B. ; D'Elia, C.F.
    The elemental composition (C, N, and P) of zooxanthellae and host tissue from the coral Pocillopora damicornis (Linnaeus) was determined after maintenance in flowing seawater with 20-uM and 50-M ammonium enrichments for periods of 2 to 8 weeks. Compared with ambient seawater controls, total zooxanthellar nitrogen (ug N cm-2 colony surface) increased four-fold during exposure to 20-uM ammonium. This resulted from increases in N content of zooxanthellae and in zooxanthellae population densities. C: N ratios of zooxanthellae decreased from 19.7 (±4.0) to 10.3 (±3.0), and N: P ratios increased from 21.4 (± 3.1) to 30.4 (± 2.2) after 8 weeks in 20 uM ammonium. Zooxanthellae from the 8-week 50-uM ammonium corals had values of 8.9 (±0.6) for C: Nand 40.4 (±2.3) for N: P. Coral animal C, N, and P content were not affected by ammonium-enriched seawater. The C :N ratio of coral animal tissue was 5.2 (±O.O), and the N: P ratio was 20.1 (±0.2) after 8 weeks in 20-uM ammonium seawater. There were no changes in host C: N, N: P, or C: P with ammonium enrichment. Thus, most of the N from the elevated seawater ammonium is retained by the zooxanthellae of P. damicornis, rather than by the animal tissue. Accordingly, sustained high concentrations of ammonium are likely to result in increased N storage by zooxanthellae and to affect the relative size of zooxanthellar to animal N pools.
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