Symbiont diversity is not involved in depth acclimation in the Mediterranean sea whip Eunicella singularis

Abstract

In symbiotic cnidarians, acclimation to depth and lower irradiance can involve physio logical changes in the photosynthetic dinoflagellate endosymbiont, such as increased chlorophyll content, or qualitative modifications in the symbiont population in favour of better adapted strains. It has been argued that a lack of capacity to acquire new symbionts could limit the bathymetric distribution of the host species, or compromise its long-term survival in a changing environment. But is that always true? To address this question, we investigated the symbiont genetic diversity in Eunicella singularis, a Mediterranean sea whip species with a wide bathymetric distribution (10 to 50 m depth), which has recently suffered from mass mortalities after periods of abnormally high sea temperatures. We measured symbiont population densities and chlorophyll content in natural populations, and followed the response of the holobionts after reciprocal transplantations to deep and shallow depths. A total of 161 colonies were sampled at 2 depths (10 and 30 m) at 5 sites in the northwestern Mediterranean. All colonies harboured a single ribosomal Symbiodinium clade (A'), but a relatively high within-clade genetic diversity was found among and within colonies. This diversity was not structured by depth, even though the deeper colonies contained significantly lower population densities of symbionts and less chlorophyll. We did, however, reveal host-symbiont specificity among E. singularis and other Mediterranean cnidarian species. Transplantation experiments revealed a limit of plasticity for symbiont population density and chlorophyll content, which in turn questions the importance of the trophic role of Symbiodinium in E. singularis.

Photophysiology and daily primary production of a temperate symbiotic gorgonian

“…Amongst the 20 gorgonian species of the Mediterranean, E. singularis is the only one living in symbiosis with unicellular dinoflagellates of the genus Symbiodinium sp, belonging only to the temperate clade A (Forcioli et al 2011). It is also found aposymbiotic in deep waters (Gori et al 2012a), as this gorgonian spreads from the surface down to the edge of the continental shelf (Gili et al 1989(Gili et al , 2011.…”

“…It is also found aposymbiotic in deep waters (Gori et al 2012a), as this gorgonian spreads from the surface down to the edge of the continental shelf (Gili et al 1989(Gili et al , 2011. Although its reproduction patterns (Gori et al 2007;Ribes et al 2007;Gori et al 2012b) and spatial distribution before or after a stress event (Coma et al 2006;Forcioli et al 2011;Gori et al 2011) have been investigated, the role of symbionts in the acquisition of nutrients has never been assessed. Two previous studies (Gori et al 2012b;Cocito et al 2013), based on the analysis of d 13 C and d 15 N of the gorgonian tissue, have however found a strong heterotrophic signature, even in summer during high light levels.…”

New insights into the genetic diversity of zooxanthellae in Mediterranean anthozoans

“…phylotypes and host taxa, mainly because the study of zooxanthellae diversity in that region is still in the early stages in comparison with studies available from tropical regions. Currently, there are only few studies characterizing the diversity of zooxanthellae from Western Mediterranean anthozoans (Savage et al 2002;Visram et al 2006;Forcioli et al Forcioli et al 2011), with all of them indicating the "Temperate A" clade as dominant in this basin.…”

Symbiodinium isolation by NaOH treatment

“…FIZ extractions from the three species were performed as described elsewhere (Richier et al, 2003;Forcioli et al, 2011). Anemonia viridis tentacles were cut and quickly dried on paper to remove excess seawater.…”

“…They include stripping coral tissues from the skeleton with a fine jet of seawater using a WaterPik (Johannes and Wiebe, 1970), grinding the whole coral (Santiago-Vázquez et al, 2006) or gorgonian tissue (Forcioli et al, 2011;Pey et al, 2011) in liquid nitrogen, manual tissue fractionation and potter homogenization of sea anemones (Richier et al, 2003). Although effective, these techniques could underestimate the zooxanthella biomass and require a large investment of time.…”