The relationship between nutritional status and carbon flux in the zooxanthellate sea anemone Aiptasia pallida

Abstract

It has been proposed that translocation of photosynthate from dinoflagellate endosymbionts ("zooxanthellae") to their cnidarian hosts is inversely related to zooxanthellar nutrient status. We investigated this relationship in the zooxanthellate sea anemone Aiptasia pallida (Verrill). Zooxanthellar nutritional status was manipulated by either feeding or starving sea anemones, and the flux of 14C-labelled photosynthate was measured in the intact association (in hospite). The percent translocation remained constant at about 16%, irrespective of nutritional history. This trend was not related to levels of "host release factor" activity in anemone tissues, as this was unaffected by starvation, and the possibility that increased anemone respiration masked a positive correlation between percent translocation and starvation was deemed unlikely, given that respiratory rate usually declines with starvation. The cell-specific photosynthetic rate increased after prolonged starvation, despite a likely decrease in chlorophyll a concentration. This may be explained by reduced competition for CO2, as the density of zooxanthellae decreased by up to 50% with starvation. Despite the loss of symbionts, unfed anemones received as much carbon on a per protein basis as fed ones, since the amount of carbon translocated per zooxanthella increased. We propose that the enhancement of carbon translocation by nutrient limitation in this sea anemone results from the balance between zooxanthella density and CO2 availability.