AMMONIUM ENHANCEMENT OF DARK CARBON FIXATION AND NITROGEN LIMITATION IN SYMBIOTIC ZOOXANTHELLAE - EFFECTS OF FEEDING AND STARVATION OF THE SEA-ANEMONE AIPTASIA-PALLIDA

The nutrient status of algae symbiotic with marine invertebrates is controversial. We assessed the nitrogen status of zooxanthellae symbiotic with the sea anemone Aiptasia pallida using NH4+ enhancement of dark C fixation; enhancement increases with N limitation in other microalgae. Freshly isolated symbionts obtained from laboratory populations of known feeding history and from field populations were assayed: NH4+ enhancement depended on the nutritional history of the host anemones. Zooxanthellae from well-fed laboratory populations showed little enhancement of dark C-14 fixation by NH4+, while mean dark enhancement ratios (NH4+ rates: seawater rates) exceeded 2.3 for zooxanthellae from anemones unfed for 1-6 weeks. Photosynthetic rates of the isolated algae declined with starvation. V(D):V(L), an index which includes both dark NH4+ enhancement and photosynthesis, increased markedly with time since feeding. Three of four field samples of A. pallida yielded zooxanthellae with dark enhancement ratios similar to those of fed laboratory populations; V(D):V(L) values were somewhat higher. Symbionts from anemones collected on a fourth date had a mean enhancement ratio of 2.88, and V(D):V(L) values similar to those from laboratory anemones that had not fed for 1-2 weeks, indicative of increased N limitation.