Significance of photosynthetic endosymbionts to the carbon budget of the scyphomedusa Linuche unguiculata

Symbiotic dinoflagellates (zooxanthellae) associated with the medusa of Linuche unguiculata constituted 22–26% of the total C and N biomass. The maximal net production for intact medusae (Pm) averaged 4.5 mg O2 (mg Chl a)−1 h−1, equivalent to an assimilation number of 2.0 mg C (mg Chl a)−1 h−1. At the midday maximum, gross oxygen production averaged four times the respiration rate. The saturating light intensity was 200–300 µEinst m−2 s−1 (Einstein = mol quanta) and the compensation light intensity (dO2 = 0) was ∼20 µEinst mn−2 s−1. On a diel basis Pgrow : R averaged 1.7 for intact medusae at ambient near‐surface light. Diel periodicity with a midday maximum was measured for the production potential under artificial, saturating light. Both intact medusae and freshly isolated zooxanthellae demonstrated similar diel production patterns. Respiration did not show the same strong diel pattern as net production, but respiration declined steadily when medusae were held unfed in the dark. Respiration was independent of the ambient oxygen concentration over a range from 4 to 16 mg O2 liter−1. Diel periodicity was measured in egg release and division rates of zooxanthellae. Estimates of both algal and host growth ranged from 0.01 to 0.04 d−1. Calculations indicated that only 21% of the C fixed in photosynthesis was used in algal respiration and growth. If the remaining photosynthate were translocated, it could provide all the C required for host respiration and somatic growth. Egg production represented another large C flux in adult females, with a daily release equivalent to 4–8% of host body C for medusae of 0.2–1.2 ml in displacement volume.