Photophysiology and daily primary production of a temperate symbiotic gorgonian

Abstract

Gorgonians are one of the most important benthic components of tropical and temperate areas, and play a fundamental role as ecosystem engineers. Although global warming and pollution increasingly threaten them, the acquisition of nutrients, which is a key process in fitness and stress resistance, has been poorly investigated in such species. This study has thus used an advanced in situ incubation chamber for the first time with gorgonians, to assess the daily acquisition of nutrients and the photophysiology of the Mediterranean symbiotic species, Eunicella singularis. The xanthophyll cycle was assessed in parallel. This work has revealed that E. singularis presents a different functioning than the Mediterranean symbiotic corals. This gorgonian indeed relies on both autotrophy and heterotrophy in summer to optimize its energetic budget, while corals mainly shift to autotrophy for their respiratory needs and tissue growth. In addition, although E. singularis lives in the same depths/locations, and harbours the same symbiont genotype than the corals, the photosynthetic performances of their respective symbionts are significantly different. Indeed, E. singularis acquired 2-3 times less autotrophic carbon from its symbionts than corals, but maintained a positive carbon budget by reducing respiration rates, and by presenting maximal photosynthetic rates throughout the day, suggesting a very efficient light utilization. Almost no photoinhibition was observed under very high light levels, because of the induction of a xanthophyll photoprotection process. These results help understanding why gorgonians often dominate many benthic ecosystems.

Host-microbe interactions in octocoral holobionts - recent advances and perspectives

“…A P:R > 1 indicates that the holobiont acquires more photosynthetic organic material than it consumes and can therefore rely on autotrophy for its energetic needs. The first few studies which have assessed the rates of photosynthesis and respiration of zooxanthellate octocorals measured very low rates of primary productivity compared to scleractinian corals, both in the temperate [ 44 ] and tropical [ 7 , 45 ] areas. It was also observed that the presence of Symbiodinium increased metabolic costs, and thereby respiration rates [ 46 ], indeed suggesting that these octocorals have to rely on both auto-and heterotrophy to sustain their metabolic needs.…”

“…Subsequent studies, performed on a greater number of species, however, showed that octocoral primary productivity depends on the environment, the polyp activity, the Symbiodinium clade identity, and the host morphology. For example, the P:R ratio tends to be the lowest in summer for tropical species (due to photoinhibition of the symbionts’ activity) [ 47 ], while it is the highest in summer for temperate areas, which are light limited during the other seasons [ 44 , 48 , 49 ]. For the same environment, the P:R ratio is also linked to the surface area:volume ratio (SA:V ratio).…”

Symbiotic Versus Non-symbiotic Octocorals: Physiological and Ecological Implications

“…11 Trade-off between polyp size and feeding effectiveness over a range of flow velocities (Adapted from Dai and Lin 1993) Fig. 12 Comparison of respiratory rates of symbiotic (dark yellow) and nonsymbiotic species (dark red) from the Great Barrier Reef (Fabricius and Klumpp 1995), the Red Sea (Svoboda 1978;, and the Mediterranean (Brafield and Chapman 1965;Coma et al 1998;Ribes et al 2003;Previati et al 2010;Ferrier-Pagès et al 2015). Numbers of species included are given in parentheses Fig.…”

“…Except in early spring, feeding on zooplankton and seston in general has a low contribution to the respiratory demands of this species (Previati et al 2010). Therefore, to meet basal metabolic requirements, this species depends on autotrophic input through symbiont photosynthesis, which has been shown to fulfill E. singularis respiratory demands during summer (Ezzat et al 2013;Ferrier-Pagès et al 2015). In contrast, the temperate Capnella gabonensis relies very little on carbon transfer from symbionts (translocation of approximately 10% of photosynthates; Farrant et al 1987) and uses heterotrophic input to fulfill its energy demands.…”

“…The presence of Symbiodinium is beneficial due to the energetic surplus from symbiont photosynthesis, but they can also impose certain restrictions (e.g., bathymetric distribution) and risks (e.g., higher increase in reactive oxygen species under stress conditions due to O 2 released through photosynthesis). The relative contribution of photosynthetically fixed carbon provided by Symbiodinium to their octocoral host seems to be species specific (Sorokin 1991), though it is generally accepted that the trophic contribution of the symbiotic dinoflagellates to the octocoral energy budget is low in comparison to scleractinian corals (Fabricius and Klumpp 1995;Ferrier-Pagès et al 2015). Therefore, octocoral dependency on heterotrophy is higher compared to scleractinians (Fabricius and Klumpp 1995), with high variability in the contribution of heterotrophic versus autotrophic nutrition, depending on species, depth, season, and coral health.…”