CORAL HEALTH ASSESSMENT PROGRAM STUDIES

The diversity of the coral endosymbiont Symbiodinium and their role in disease susceptibility

Dinoflagellates from the genus Symbiodinium form mutualistic associations with coral. This symbiosis allows for the growth and development of coral reefs, which is an ecologically important habitat for marine invertebrates and fish. The genus Symbiodinium contains a diverse number of genetic varieties which have been shown to affect the biology of the coral host, including growth rate and tolerance to elevated sea surface temperatures. The genus has been grouped into eight clades (A-H), which in turn contain multiple sub-clade types based on rDNA. The diversity of Symbiodinium harbored by corals in the Northwestern Hawaiian Islands (NWHI) and Johnston Atoll (JA) remains unexplored. This research aims to: 1) investigate whether the genotype of Symbiodinium affects the health status of coral; and 2) describe the diversity of Symbiodinium in corals in the NWHI and JA and in the surrounding reef waters.

1: Investigating whether the genotype of Symbiodinium affects the health status of coral.

Figure 1. The genotype of Symbiodinium will be identified from colonies of Acropora cytherea that appear: a) healthy, b) bleached, and c) diseased (white syndrome). (Click on the image to open a larger version.)

We have chosen Acropora cytherea to investigate whether the Symbiodinium genotype affects the health status of the coral host. A. cytherea is common at French Frigate Shoals in the NWHI and at JA. We collected samples from colonies of A. cytherea that appeared healthy, bleached and diseased (see Figure 1). Restriction fragment length polymorphism (RFLP) of ssu rDNA will be used to determine the clade of Symbiodinium present in coral hosts. To date, our results indicate that colonies of A. cytherea harboring clade A Symbiodinium are more susceptible to disease, mainly white syndrome (see Table 1). We are currently processing additional samples that will confirm whether the association between A. cytherea and clade A Symbiodinium renders a coral more susceptible to disease.

Table 1. A. cytherea colonies, health state, and the clade of
Symbiodinium harbored.

Reef	Location	Date collected	No. of colonies	*Symbiodinium* clade	Host Health Status
French Frigate Shoals	FFS R46	09/05	4	C	Healthy
	FFS R46	09/05	1	A	Healthy
	FFS 12	09/05	5	C	Healthy
	FFS 12	09/05	4	A	Diseased
	FFS R16	05/06	1	A	Healthy
	FFS 30	05/06	1	A	Healthy
Johnston Atoll	JOH 20	05/06	1	C	Healthy

2: Exploring the diversity of Symbiodinium from corals in the NWHI and JI and the surrounding reef waters.

We will investigate the diversity of Symbiodinium in coral hosts from the NWHI and JA and the surrounding reef waters to determine the biogeographic distribution of symbiont types and potential genetic varieties that render a coral susceptible to disease. RFLP of Symbiodinium ssu rDNA from coral hosts show that corals predominantly harbor clade C Symbiodinium (Table 2). The Symbiodinium sub-clade will be identified by sequence analysis of more variable gene markers. We have sequenced the chloroplast 23S rDNA from Symbiodinium in Pocillopora meandrina collected at Necker at Kure. The results indicate that a single sub-clade chl23S rDNA genotype is dominant in these coral hosts. We will compare these results to sub-clade genotypes characterized by the Internal Transcribed Spacer region (ITS2) to determine whether: 1) the Symbiodinium population in Pocillopora meandrina is very low, or 2) the chl23S rDNA gene marker does not resolve Symbiodinium into distinct genotypes and has lower resolving power compared to the ITS2 gene marker. The results from this analysis will determine which gene marker we will use to investigate the diversity of Symbiodinium in all coral hosts and the water environment.

Table 2: Coral hosts and the Symbiodinium clade harbored

Species	Location, number of host colonies sampled, and clade of Symbiodinium harbored
	Necker	FFS	Maro	P&H	Mid	Kure	JA
Pocillopora meandrina	4: C					7: C
Porites compressa				5: C		4: C
Porites lobata	4: C	9: C	6: C	24: C	12: C	6: C
Acropora cytherea		10: C 7: A					1: C
Montipora capitata			1: C		3: C

FFS: French Frigate Shoals, P&H: Pearl & Hermes, Mid: Midway, JA: Johnston Atoll.

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