Regulation of the population of symbionts in Anemonia viridis
This study was undertaken to investigate the effects of different environmental variables on the association between the temperate anemone, Anemonia viridis and its symbionts. The effects of exposure to ammonium enrichment, changes in light intensity, feeding and starvation were studied. Many studies involving tropical associations have addressed this question by monitoring changes in the symbiont population density. However, the symbiont population density can change as a result of changes in the zooxanthella population or changes in the host biomass or host surface area. In addition, the zooxanthella population is determined by the rate of division and the rate at which cells are lost from the population. Anemones maintained at light intensities of 20 and 300E.m-2s-1 had similar population densities measured as cells.g host protein-1 in tentacles, however the zooxanthella division rate, measured as the mitotic index, increased from 2 to 4% with increasing light intensity within the range 20 to 300E.m-2s-1 after 3 weeks exposure. The specific expulsion rate also increased with light intensity over the range 50 to 300E.m-2s-1 from 0.0003 to 0.002 cells.cell-1.d-1. Although the mitotic index of zooxanthellae increased with increasing light intensity, the increase was much larger in animals receiving ammonium enrichment for 4 weeks. Under ammonium enrichment, the symbiont mitotic index increased from 2% at 20E.m-2.s-1 to 9% at 300E.m-2.s-1. The rate of ammonia uptake in A.viridis has previously been shown to be determined by the level of illumination and therefore the above observations indicate that the rate of cell division may be limited by low light intensity through limitation of ammonium uptake even under ammonium enrichment.