Effects of increased heat on fluorescence and dinoflagellate density in the captive coral, Anthelia sp .

The obligate mutualistic relationship between corals and dinoflagellates is a classic example of symbiosis. Coral reefs have been devastated by warm water temperatures at a rapid pace and currently, there is no effective method to predict a mass bleaching event. Fluorescence is a potential indicator of coral health but very few studies have attempted to utilize it as a proxy for dinoflagellate density, which was the scope of the present research. Here, we determined the effects of elevated water temperatures on fluorescence, dinoflagellate density, and mitotic index in the captive coral, Anthelia sp. Over a six-week period, tanks filled with Anthelia sp. underwent gradual increases in water temperature beginning at 28ºC and ending at 34ºC . Coral samples were quantified for fluorescence, dinoflagellate density, and mitotic index by fluorescence microscopy of whole specimens and a maceration method to examine dinoflagellates. As a result of the gradual increase in water temperature, corrected fluorescence decreased over time. In contrast, dinoflagellate density first increased until reaching a thermal limit that was then followed by a decrease. This was driven by an increase in the rate of cell division before the limit and a decrease afterwards. These results suggest that symbionts may be increasing their mitotic rate in response to elevated water temperatures to compensate for the shortage of photosynthate supply for their host. Because fluorescence dropped simultaneously with the increase in dinoflagellate density and mitotic index, coral fluorescence may be utilized as a prognostic tool for coral bleaching in Anthelia sp.