Fate of photosynthetic fixed carbon in light- and shade-adapted colonies of the symbiotic coral Stylophora pistillata

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Abstract
The total daily flux of photosynthetically fixed carbon in light- and shade-adapted phenotypes of the symbiotic coral, Stylophora pistillata , was quantified. Light adapted corals fixed four times as much carbon and respired twice as much as shade corals. Specific growth rates of zooxanthellae in situ were estimated from average daily mitotic indices and from ammonium uptake rates (nitrate uptake or nitrate reductase activity could not be demonstrated). Specific growth rates were very low, demonstrating that of the total net carbon fixed daily, only a small fraction (less than 5 %) goes into zooxanthellae cell growth. The balance of the net fixed carbon (more than 95 %) is translocated to the host. New and conventional methods of measuring total daily translocation were compared. The ‘growth rate’ method, which does not employ 14 C, emerged as superior to the conventional in vitro and in vivo methods. The contribution of translocated carbon to animal maintenance res­piration (czar) was 143 % in light corals and 58 % in shade corals. Thus, translocation in the former could supply not only the total daily carbon needed for respiration but also a fraction of the carbon needed for growth. Whereas light-adapted corals released only 6%, shade-adapted corals released almost half of their total fixed carbon as dissolved or particulate organic material. This much higher throughput of organic carbon may possibly benefit the heterotrophic microbial community in shade environments.