Diversity of Gambierdiscus Across a Temperature Gradient in Wai 'Opae, Hawai'i

Kathleen Pitz, Biology Department
Advisor: Don Anderson, Biology

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Diversity of Gambierdiscus Across a Temperature Gradient in Wai 'Opae, Hawaii

Ciguatera fish poisoning (CFP) is a syndrome caused by the bioaccumulation of lipophilic ciguatoxins1,2 in coral reef fish and subsequent consumption by humans. These phycotoxins are produced by Gambierdiscus, a tropical dinoflagellate genus that lives on many varieties of macroalgae but also may occur on dead corals and sand. Globally, tens of thousands of individuals are afflicted with ciguatera on an annual basis, with up to 10% of the local population on some islands in endemic areas becoming ill3. CFP affects more people worldwide than any other seafood illness caused by toxic algae, and is the most common cause of human illness associated with the consumption of fresh seafood.
 
CFP outbreaks have long been associated with ecological disturbances to reef environments, likely due to proliferation of Gambierdiscus’ preferred substrates of algae and dead coral4,5. This presents an added concern that as the health of global coral reefs suffer due to damage from overfishing, pollution, and bleaching events, the threat of CFP will increase concurrently. Due to its responsiveness to environmental disturbance, CFP has been described as a sensitive indicator of changing conditions in tropical marine ecosystems6. Several studies have predicted that as global sea surface temperatures increase, Gambierdiscus will expand its geographic range, increasing the number of people at risk of ciguatera fish poisoning7,8,9. Furthermore, since no current method is available to rapidly and cheaply test fish for ciguatoxin, fishermen instead use known poisoning histories of fishing areas to determine the risk of their catch. These histories may lose accuracy as Gambierdiscus shifts in abundance due to temperature change9,10, emphasizing the need to understand how increasing seawater temperatures change.
 
In the Caribbean, Gambierdiscus has been shown to be most prevalent in the warmest regions where there are relatively constant temperatures greater than 29°C; however, laboratory growth experiments have defined the temperature maxima of 31°C for growth of most Gambierdiscus species9. Whether this temperature barrier (determined from constant temperature experiments) is different for populations subjected to episodic shortterm exposure to high temperature remains to be elucidated. Evidence that Gambierdiscus can survive at elevated temperatures is given by the recent discovery of populations in the Red Sea (D.M. Anderson, unpublished data), where seasonal temperatures regularly exceed 31°C in some areas. Clearly, the degree to which different Gambierdiscus species and strains can tolerate changing temperatures (both long-term gradual increases, and short-term, large fluctuations) needs to be further defined. Examining Gambierdiscus in a region of highly variable temperature would help to predict the ways Gambierdiscus population structure and geographic range will change with rising seawater temperatures.
 
In the Wai ‘Opae Tidepools on Hawai’i, a unique temperature gradient exists that allows me to examine future climate change scenarios under natural, field conditions. With this system, I can test to see whether Gambierdiscus species composition and abundance differs across a natural temperature gradient from 24 to 29oC, thereby demonstrating selection of certain species or strains, as well as true temperature tolerances and thresholds. Tidepools in Wai ‘Opae are influenced by an influx of geothermally heated groundwater. My study compares the Gambierdiscus community along the temperature gradient, from the lowest temperatures in the pools nearest the fore reef to the higher temperatures in the heated inner tidepools. So far I have undertaken three trips to this study site to characterize the Gambierdiscus community in January, March and July of 2015. This was designed to follow the observed seasonality in Gambierdiscus abundance, with very few cells in January, and colonization and selection occurring in the subsequent months. Samples were collected from rubble, macroalgae, and screens that were deployed for 24 hours and 3 days at a time. Samples were fixed with formalin and methanol at the University of Hawaii, Hilo, and shipped back to WHOI for analysis.
Hobo temperature loggers are continuously documenting the temperatures in 4 experimental sites, and 2 control sites. I am using novel FISH probes developed for another chapter of my thesis to target differences in rRNA sequences to differentiate between species. In the samples from March and July, G. australes, G. caribaeus, G. carpenteri, and G. belizeanus have been present. No cells were present in January, as expected. So far in my analyzed samples I have seen the highest diversity of species within the coolest sites closest to the fore reef, with some compositional differences as well.
 
I would like to take a final sampling trip to Wai ‘Opae in October 2015 to see the effects of the record hot temperatures Hawaii is currently experiencing due to El Nino, which is also  threatening to cause a massive coral die off. This grant would help me defray costs of the trip that my advisor will be able to partially fund. This data will help to complete this last chapter of my thesis - I am expecting to graduate June 2016.

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