Adaptations to nitrogen availability drive ecological divergence of chemosynthetic symbionts.

Bacterial symbionts, with their shorter generation times and capacity for horizontal gene transfer (HGT), play a critical role in allowing marine organisms to cope with environmental change. The closure of the Isthmus of Panama created distinct environmental conditions in the Tropical Eastern Pacific (TEP) and Caribbean, offering a "natural experiment" for studying how closely related animals evolve and adapt under environmental change. However, the role of bacterial symbionts in this process is often overlooked. We sequenced the genomes of endosymbiotic bacteria in two sets of sister species of chemosymbiotic bivalves from the genera Codakia and Ctena (family Lucinidae) collected on either side of the Isthmus, to investigate how differing environmental conditions have influenced the selection of symbionts and their metabolic capabilities. The lucinid sister species hosted different Candidatus Thiodiazotropha symbionts and only those from the Caribbean had the genetic potential for nitrogen fixation, while those from the TEP did not. Interestingly, this nitrogen-fixing ability did not correspond to symbiont phylogeny, suggesting convergent evolution of nitrogen fixation potential under nutrient-poor conditions. Reconstructing the evolutionary history of the nifHDKT operon by including other lucinid symbiont genomes from around the world further revealed that the last common ancestor (LCA) of Ca. Thiodiazotropha lacked nif genes, and populations in oligotrophic habitats later re-acquired the nif operon through HGT from the Sedimenticola symbiont lineage. Our study suggests that HGT of the nif operon has facilitated niche diversification of the globally distributed Ca. Thiodiazotropha endolucinida species clade. It highlights the importance of nitrogen availability in driving the ecological diversification of chemosynthetic symbiont species and the role that bacterial symbionts may play in the adaptation of marine organisms to changing environmental conditions.

Bosques de coral negro del Área de Conservación Guanacaste, Costa Rica: especies y distribución

Introducción: El conocimiento de los antipatarios en Costa Rica es muy escaso, se ha informado de tres especies en el Pacífico y de la existencia de bosques de coral en la Isla del Coco. En este estudio se informa de la presencia de bosques de coral negro y sus características encontrados en el Área de Conservación Guanacaste (ACG) sector marino. Objetivo: Determinar las especies de corales negros presentes en el Sector Marino del ACG y alrededores, y localizar geográficamente los sitios en que se desarrollan los bosques de coral negro. Métodos: Se hicieron censos visuales cualitativos mediante buceo, se tomaron muestras y analizaron en el laboratorio para su identificación. Resultados: Los bosques de corales negros están compuestos por dos especies: Myriopathes panamensis y Antipathes galapagensis las cuales son descritas e ilustradas. Los bosques de coral se encuentran distribuidos dentro y fuera del Sector Marino del ACG. Myriopathes panamensis se observó entre 15-45 m de profundidad y A. galapagensis entre 30-45 m. Entre las ramas de M. panamensis se observó macrofauna asociada cuyo estatus e importancia ecológica no han sido estudiados. Conclusiones: Las especies encontradas no tuvieron variaciones morfológicas con las descritas en otras regiones similares. Se sugiere la importancia de un estudio dirigido a evaluar el rol de la fauna asociada con los corales negros, principalmente de la anémona Nemanthus californicus, el bivalvo Pteria sterna y el pez halcón narigón Oxycirrhithes typus. Se enfatiza la importancia del Área de Conservación Guanacaste como sitio de conservación de la biodiversidad marina.

Introduction: Little is known about the antipatharians in Costa Rica, only three species are known from the Pacific and the occurrence of coral forests was previously reported only for Isla del Coco. Black coral forests were observed at several sites in the north Pacific of the country, within the Guanacaste Conservation Area (ACG), which are reported and characterized in this study. Objective: To determine the species of black corals present in the ACG Marine Sector and its surroundings, and geographically locate the sites where black coral forests grow. Methods: Visual censuses were made by diving, samples were taken and analyzed in the laboratory for identification. Results: These black coral forests are composed of two species: Myriopathes panamensis and Antipathes galapagensis, these species are described and illustrated. The coral forests were found within and outside ACG marine protected areas. Myriopathes panamensis was found between 15-35 m depth and A. galapagensis at 30-45 m. Associated macrofauna was observed among the branches of M. panamensis, their ecological importance and status have not been determined. Conclusions: The species found did not have morphological variations with those described in other similar regions. Associated fauna was found (mainly the anemone Nemanthus californicus, the bivalve Pteria sterna and the longnose hawkfish Oxycirrhithes typus) and needs to be studied to determine its role on black corals. The importance of the ACG as a marine biodiversity conservation site is emphasized.