The influence of 'host release factor' on carbon release by zooxanthellae isolated from fed and starved Aiptasia pallida (Verrill)

Symbiotic dinoflagellates (zooxanthellae) typically respond to extracts of host tissue with enhanced release of short-term photosynthetic products. We examined this 'host release factor' (HRF) response using freshly isolated zooxanthellae of differing nutritional status. The nutritional status was m anipulated by either feeding or starving the sea anemone Aiptasia pallida ( Verrill). The release of fixed carbon from isolated zooxanthellae was measu red using C-14 in 30 min experiments. Zooxanthellae in filtered seawater al one released approximately 5% of photosynthate irrespective of host feeding history. When we used a 10-kDa ultrafiltrate of A. pallida host tissue as a source of HRF, approximately 14% of photosynthate was released to the med ium. This increased to over 25% for zooxanthellae from anemones starved for 29 days or more. The cell-specific photosynthetic rate declined with starv ation in these filtrate experiments, but the decline was offset by the incr eased percentage release. Indeed, the total amount of released photosynthat e remained unchanged, or even increased, as zooxanthellae became more nutri ent deficient. Similar trends were also observed when zooxanthellae from A. pallida were incubated in a 3-kDa ultrafiltrate of the coral Montastraea a nnularis, suggesting that HRF in the different filtrates operated in a simi lar manner. Our results support the suggestion that HRF diverts surplus car bon away from storage compounds to translocated compounds such as glycerol. (C) 2001 Elsevier Science Inc. All rights reserved.