Date Published: March 8, 2019
Publisher: Public Library of Science
Author(s): Mariana M. Teschima, Amana Garrido, Alexandra Paris, Flavia L. D. Nunes, Carla Zilberberg, James R. Guest.
Zooxanthellate corals live in symbiosis with phototrophic dinoflagellates of the family Symbiodiniaceae, enabling the host coral to dwell in shallow, nutrient-poor marine waters. The South Atlantic Ocean is characterized by low coral diversity with high levels of endemism. However, little is known about coral–dinoflagellate associations in the region. This study examined the diversity of Symbiodiniaceae associated with the scleractinian coral Favia gravida across its distributional range using the ITS-2 marker. This brooding coral endemic to the South Atlantic can be found across a wide range of latitudes and longitudes, including the Mid-Atlantic islands. Even though it occurs primarily in shallower environments, F. gravida is among the few coral species that live in habitats with extreme environmental conditions (high irradiance, temperature, and turbidity) such as very shallow tide pools. In the present study, we show that F. gravida exhibits some degree of flexibility in its symbiotic association with zooxanthellae across its range. F. gravida associates predominantly with Cladocopium C3 (ITS2 type Symbiodinium C3) but also with Symbiodinium A3, Symbiodinium linucheae (ITS2 type A4), Cladocopium C1, Cladocopium C130, and Fugacium F3. Symbiont diversity varied across biogeographic regions (Symbiodinium A3 and S. linucheae were found in the Tropical Eastern Atlantic, Cladocopium C1 in the Mid-Atlantic, and other subtypes in the Southwestern Atlantic) and was affected by local environmental conditions. In addition, Symbiodiniaceae diversity was highest in a southwestern Atlantic oceanic island (Rocas Atoll). Understanding the relationship between corals and their algal symbionts is critical in determining the factors that control the ecological niches of zooxanthellate corals and their symbionts, and identifying host-symbiont pairs that may be more resistant to environmental changes.
The relationship between corals and their intracellular symbiont dinoflagellates (family Symbiodiniaceae) is essential for the survival of shallow water corals and, consequently, the entire coral reef system. Because of this symbiotic relationship, corals can thrive in low-nutrient tropical waters as the endosymbiontics community provides most of the coral host’s metabolic energy requirements, enhancing calcification rates and coral growth . Symbiodiniaceae is a diverse family with different genera, species and strains having different physiologies, photosynthetic efficiencies, and habitat associations [2–4]. Previous phylogenetic studies based on a number of molecular markers have identified nine major ‘Symbiodinium’ clades assigned alphanumeric nomenclature (A–I) , followed by recent taxonomic revision which recognizes the clades as genera and has named them accordingly . Endosymbiontics dinoflagellates are found across a wide range of hosts (e.g., tridacnids, cardiids, sponges, and soft corals ), with the genera Symbiodinium Hansen & Daugbjerg 2009, Breviolum Parkinson & LaJeunesse 2018, Cladocopium LaJeunesse & Jeong 2018, Durusdinium LaJeunesse 2018, Fugacium LaJeunesse 2018, and Gerakladium LaJeunesse 2018 (formerly Clades A, B, C, D F and G, respectively) found associating with corals . Symbiodiniaceae have primarily been identified and described using molecular phylogenetics based on the nuclear ribosomal internal transcribed spacer region 2 (ITS2 rDNA) [2,6,9]. Further sampling and advances in Symbiodiniaceae taxonomy has the potential to reveal additional species and new genera.
Most RFLP analyses followed a pattern found for major Symbiodiniaceae genera by Baker and Rowan . In addition, sequencing data supported the RFLP results, which indicate that RFLP was a good general predictor for dominant endosymbionts in this study. Only four sites (RA, FN, RN, and PS) had samples with unclear RFLP patterns for which cloning was required. Bacterial cloning of PCR products revealed that, in most cases, RFLP patterns that did not clearly match an expected single-clade pattern were caused by incomplete digestion by the restriction enzyme. Nearly all corals examined harbored a single symbiont type. Only coral samples from the RA had mixtures of different Symbiodiniaceae genera in the same colony (one colony each hosted S. linucheae/Fugacium F3 and S. linucheae/Cladocopium C1, respectively).