Hoegh-Guldberg, Ove2008-08-052008-08-051994-07Hoegh-Guldberg O. 1994. Population dynamics of symbiotic zooxanthellae in the coral Pocillopora damicornis exposed to elevated ammonium [(NH4)2 SO4] concentrations. Pac Sci 48(3): 263-272.0030-8870http://hdl.handle.net/10125/2235Division 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.en-USArticle