Symbiotic anemones can grow when starved: nitrogen budget for Anemonia viridis in ammonium-supplemented seawater

The ability of endosymbioses between anthozoans and dinoflagellate algae (z ooxanthellae) to retain excretory nitrogen and take up ammonium from seawat er has been well documented. However, the quantitative importance of these processes to the nitrogen budget of such symbioses is poorly understood. Wh en starved symbiotic Anemonia viridis were incubated in a flow-through syst em in seawater supplemented with 20 mu M ammonium for 91 d under a light re gime of 12 h light at 150 mu mol photons m(-2) s(-1) and 12 h darkness, the y showed a mean net growth of 0.197% of their initial weight per day. Contr ol anemones in unsupplemented seawater with an ammonium concentration of <1 mu M lost weight by a mean of 0.263% of their initial weight per day. Atte mpts to construct a nitrogen budget showed that, over a 14 d period, simila r or equal to 40% of the ammonium taken up could be accounted for by growth of zooxanthellae. It was assumed that the remainder was translocated from zooxanthellae to host. However, since the budget does not balance, only 60% of the growth of host tissue was accounted for by this translocation. The value for host excretory nitrogen which was recycled to the symbionts equal led that taken in by ammonium uptake from the supplemented seawater, indica ting the importance of nitrogen retention to the symbiotic association.