Degradation of zooxanthellae in the coral Galaxea fascicularis

Abstract

Hermatypic corals harbor symbiotic dinoflagellates (Symbiodinium spp.), which are generally referred to as zooxanthellae, in their gastrodermal cells. Zooxanthellate corals depend on photosynthetic products translocated by symbiotic algae as a nutrient source and a loss of zooxanthellae may lead to death of bleached corals. However, corals contain degraded zooxanthellae in their tissues and expel degraded zooxanthellae as well as healthy-looking zooxanthellae even under nonstressed conditions (Titlyanov et al. 1996). Brown et al. (1995) reported that degraded zooxanthellae were mainly observed in the mesentery and that they might be released from a certain part of the mesentery. These observations suggest that at least some zooxanthellae are partially digested by their hosts. It is, however, not understood whether these degraded zooxanthellae are produced by host digestion or represent aged algal cells undergoing auto-degradation. In order to examine the possibility of host partial digestion of zooxanthellae, we studied the distribution pattern of degraded zooxanthellae within polyps of Galaxea fascicularis. Polyps of G. fascicularis are large and can be easily isolated (Hidaka and Yamazato 1981), thus providing suitable material for comparison of zooxanthella morphology at various regions of a polyp. We also investigated whether the vacuoles containing degraded zooxanthellae are more acidic than those containing healthy-looking zooxanthellae, using a pH-sensitive fluorescent dye, LysoSensor, which accumulate in acidic compartments of the cell.