Assessing symbiont diversity in restored and wild coral populations in Honduras

Elkhorn (Acropora palmata) and staghorn (A. cervicornis) corals are important reef builders on Honduran reefs, and their coverage has declined by >90% since the 1970s due to disease and bleaching. These corals form obligate symbioses with photosynthetic dinoflagellate endosymbionts, and different symbiont taxa provide the host coral with benefits that aid coral resilience, such as thermotolerance or disease resistance. Ongoing coral restoration projects in Honduras have not yet identified the symbiont taxa in their corals, which is helpful to ensure effective restoration. Here, we used restriction fragment length polymorphism analysis to screen and identify symbionts from 266 wild and restored corals across different reef sites. This information will be given to the restoration programs, enabling them to assess the genetic and symbiotic diversity of their restored corals and improve their approach to slowing the population decline of these important corals. 

Hello, everyone. My name is Karina Cabrera, and I am a Junior here at GMU pursuing a BS in Geology and a minor in oceanography. Today, I will be talking about the work I have done this semester to develop a protocol for identifying coral symbionts in elkhorn and staghorn corals.

Corals are important ecosystem engineers that build up coral reefs and provide habitat for extremely diverse organisms to live in, supporting as many as 1/3 of marine species. They also benefit human communities near the coast by supporting ecotourism and reducing coastal erosion. In the Caribbean, staghorn and elkhorn corals were historically dominant reef-builders but have experienced over 90% decline in the past 4 decades due to bleaching and disease.

This unfortunate decrease not only puts reef ecosystems at risk but also threatens the organisms that depend on reefs for survival, including humans. One way to combat this decline is through coral restoration, and specifically a method called coral gardening, in which samples are taken from wild corals and then grown in controlled conditions so that the coral population for staghorn and elkhorn corals are restored. Despite this collection method being an easy and fast way to restore corals additionally helping increase population numbers, because this process relies strictly on asexual reproduction, it also means that the coral host and symbiont diversity decreases over time.

These photosynthetic dinoflagellate symbionts form obligate symbiotic relationships with the corals, and different symbiont taxa provide the host coral with benefits that aid coral resilience, such as thermotolerance or disease resistance. Because of this, understanding the phylogenetic diversity of these symbionts will help improve the effectiveness of coral restoration efforts. I am working with four coral restoration programs in the Bay Islands of Honduras, seen on this map, but these restoration programs do not currently have the necessary molecular facilities or financial resources to perform molecular symbiont identification. To address this need, my URSP project focuses on developing a relatively cheap and efficient assay to identify the coral symbionts.

Samples were collected from wild and restored populations of the two coral species being restored in Honduras, staghorn and elkhorn corals. 100 wild corals were collected from sites all around the island of Roatan, and 166 restored corals were collected from the four different restoration programs on Roatan and Utila. To identify the symbionts in these samples, I developed a protocol based on polymerase chain reaction (or PCR) and restriction fragment length polymorphisms (RFLP), originally developed by Rowan and Powers. This protocol amplifies the 18S rRNA gene in the symbiont and then cuts up the DNA. These different length fragments from different DNA sequences are what cause different banding patterns. These different patterns then correlate to the taxonomic clades that the symbionts belong to. As you can see these are the banding patterns for clade a, b, c, and d. Getting into my results, I first optimized the PCR step. Based on the original protocol, which incorporated lower-quality DNA extractions, I was not getting good amplification of the target gene from most of the samples as shown in this PCR blank gel. This is due to the DNA being too short for the banding to show up. Because this gene is very long, I switched the protocol to use higher quality DNA instead and received much better results. In this optimized gel there are clear bandings due to the DNA being of higher quality and longer. I am now working to optimize the RFLP portion of the protocol. The restriction appears to be working from the gel there is some banding appearing at 30 minutes and there is some double banding patterns present, which is expected for these symbionts, but was not separated enough so I let the gel run for an hour and saw that it had become blurry. Because of this, my next steps are to try optimizing the time in which the gel is run since an hour seems too long, but 30 minutes is not enough for the bands to become clear, so hopefully reducing the time will give us better and more clear results. Once I have optimized this portion of the protocol, I will screen all the wild and restored corals and share my results and the protocol itself with the four restoration programs in Honduras. This will help them design out planting schemes that maximize genetic diversity and ensure that the restored populations mimic the diversity found in the wild. This will help improve the effectiveness of restoration efforts in Honduras and help to build future reef resilience against ongoing climate change.

This research would not have been possible without the OSCAR URSP Program and the environmental science and policy department here at mason. Thank you to Teagen Corpening, Jennifer Keck, and all of the RIMS interns who helped to collect samples and made this research possible. Finally, I acknowledge all the funders who supported this project. Thank you for your attention!