Prevalence and photobiology of photosynthetic endosymbionts of Exaiptasia diaphana in Berghia stephanieae

• Autor(a) principal: Silva, Ruben Xavier Grave da
• Data de Publicação: 2021
• Tipo de documento: Dissertação
• Idioma: eng
• Título da fonte: Repositório Científico de Acesso Aberto de Portugal (Repositórios Cientìficos)
• Texto Completo: http://hdl.handle.net/10773/30750
• topic: Endosymbiotic photosynthetic dynoflagellates; Stenophagous species; Photobiology; Bleaching

Nudibranchs are soft bodied, shell-less marine gastropod molluscs. They are widespread over a wide variety of environments. Some of these species are known due to their stenophagous dietary habits and their ability to retain photosynthetic endosymbionts from their preys. One of such nudibranchs is the aelolid Berghia stephanieae, a stenophagous nudibranch that retains both the photosynthetic endosymbionts and the nematocysts of its prey, the glass anemone Exaiptasia diaphana. Berghia stephanieae is known to be one of the best models for ecological studies addressing the mollusc-photosynthetic dinoflagellate endosymbiosis. However, these studies are often made with the use of invasive techniques, that commonly harm the study subject, and may even lead to their sacrifice. The glass anemone, E. diaphana, is seen as a model species to the study of cnidarian-microbiome symbiosis. As this species establishes a symbiosis with photosynthetic dinoflagellates, it has been used to study bleaching, the phenomenon that results from the loss of photosynthetic endosymbionts or pigments from the animal host. The present work addresses the maintenance of the symbiosis between B. stephanieae and its photosynthetic dinoflagellate endosymbionts acquired through the predation of E. diaphana using a non-invasive methodology, the measurement of pulse amplitude modulated chlorophyll fluorescence. While this methodology is commonly employed on plants and algae, it has already been successfully used to study kleptoplasty in sacoglossan sea slugs, as well as the maintenance of photosynthetic efficiency of photosynthetic dinoflagellate endosymbionts in some nudibranchs. Our experimental trials made possible to detect a positive correlation between the abundance of photosynthetic dinoflagellate endosymbionts within B. stephanieae and the minimum fluorescence of dark-adapted animals (F₀). The abundance of photosynthetic dinoflagellate endosymbionts within B. stephanieae and their photosynthetic efficiency were studied in sea slugs reared under different light intensities (high light, 80 µmol m- ² s - ¹; low light, 10 µmol m- ² s - ¹) and access to food. Exposure to different light intensities had no significant impact in the maintenance of B. stephanieae’s photosynthetic dinoflagellate endosymbionts, nor on their photosynthetic efficiency. However, starvation was shown to have a significant impact by decreasing both the abundance of photosynthetic dinoflagellate endosymbionts within B. stephanieae and their photosynthetic efficiency. The effect of different trophic regimes in the abundance of photosynthetic dinoflagellate endosymbionts within B. stephanieae and their photosynthetic efficiency was also tested. It was possible to show that sea slugs fed with bleached anemones (anemones lacking photosynthetic dinoflagellate endosymbionts) were impacted in a similar way to starved sea slugs, as they both exhibited a decrease in the abundance of photosynthetic dinoflagellate endosymbionts and a lower photosynthetic efficiency. This finding shows that the photosynthetic dinoflagellate endosymbionts present within B. stephanieae need to be replenished regularly by these nudibranchs by preying on symbiotic glass anemones (anemones that have photosynthetic dinoflagellate endosymbionts). Overall, the present study is the starting point to future works on which chlorophyll fluorescence parameter F₀ may be used to infer the abundance, maintenance and photosynthetic efficiency of endosymbionts in nudibranchs. This methodology may be a powerful tool to study the consequences of bleaching of marine organisms on their stenophagous predators, as these are widely unknown and need to be further investigated.