Nutrition of the temperate Australian soft coral Capnella gaboensis

Abstract

Capnella gaboensis Verseveldt, 1977 (Coelenterata: Octocorallia: Alcyonacea: Nephtheidae) is an abundant soft coral in the temperate waters of south-eastern Australia. From 1981 to 1984, using material collected from Sydney Harbour (33°50′S; 151°15′E), we investigated certain aspects of its apparently obligate association with its symbiotic zooxanthellae, Symbiodinium sp. Numbers of zooxanthellae and chlorophyll content were recorded throughout the first year, then net photosynthesis and respiratory rates of the coral as a function of photo-flux densities, temperature and season were measured in later years. The fractions into which photosynthetically fixed carbon was incorporated were also determined. The zooxanthellae contained a mean of 4.1 μg chlorophyll a 10-6 zooxanthellae. Neither the numbers of zooxanthellae in C. gaboensis nor the chlorophyll a content varied on a seasonal basis. Photon-flux densities in the field ranged from 5 to 120 μE m-2 s-1 over the year. The maximum net oxygen-exchange rate recorded for C. gaboensis was 9.4 μmol O2 mg-1 chlorophyll a h-1 at 871 μE m-2 s-1. The maximum carbon fixation rate obtained was 65.6 μmol C mg-1 chlorophyll a h-1 at 100 μE m-2 s-1. Photosynthesis of C. gaboensis was not light-saturated at 871 μE m-2 s-1; the light compensation point was in the range 50 to 90 μE m-2 s-1 and the optimum temperature was 25°C. Photosynthetic rates were highest in populations sampled in summer. Labelling with 14C showed that photosynthetically fixed carbon was initially incorporated into the aqueous-methanol (low molecular weight) fraction of the coral tissues. From 20 to 100 min after the introduction of the 14C label the rate of incorporation was fairly evenly divided between the aqueous methanol-soluble, the chloroform-soluble (lipid) and the insoluble fractions. In the light, little 14C was released as particulate and/or dissolved organic carbon. Translocation of products of photosynthesis represented up to approximately 10% of the total fixation.