Hoegh-Guldberg, O. 1994. The population dynamics of symbiotic
zooxanthellae in the coral Pocillopora damicornis exposed to elevated
ammonia.  J. Pacific Science 48: 263-272. 


ABSTRACT The division synchrony and growth rate of symbiotic zooxanthellae
was investigated for populations living in colonies of the reef-building
coral Pocillopora damicornis exposed to different concentrations of NH4Cl
in seawater.  The presence of low concentrations of NH4Cl (0.2 然) did not
affect (compared to corals growing in NH4+-stripped seawater) either
division synchrony or growth rate.  Exposure to higher concentrations of
NH4Cl (20 mM or 50 然), however, affected the population dynamics of the
zooxanthellae residing in P. damicornis.  Zooxanthellae in corals exposed
to 20 然 NH4Cl had mitotic indices (the percentage of the total cells
dividing) that were two to three times higher than the mitotic indices of
zooxanthellae in control (0.2 然) corals.  Although the division of
zooxanthellae was still phased in corals exposed to 20 然 NH4Cl, there
were many more cells dividing out of phase as compared to control corals. 
The division of zooxanthellae in corals exposed to 50 然 was not phased. 
The calculated growth rates of zooxanthellae exposed to 20 然 or 50 然
NH4Cl were higher than those representative of zooxanthellae living in
control corals, although the growth rate of both carbon and nitrogen pools
were lower in 50 然 as compared to 20 然 NH4Cl.  These data support the
conclusion that the population dynamics of symbiotic zooxanthellae within
P. damicornis are affected by concentrations of NH4Cl in seawater that are
equal to or higher than 20 然, and that 50 然 NH4Cl 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 NH4Cl under ambient conditions, and further
emphasize the importance of enrichment studies concentrating on growth and
nitrogen incorporation rates measured for the entire symbiotic
association.