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 NH 4 + 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: NH 4 + enhancement depended on the nutritional history of the host anemones. Zooxanthellae from well‐fed laboratory populations showed little enhancement of dark 14 C fixation by NH 4 + , while mean dark enhancement ratios (NH 4 + 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 NH 4 + 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.