Symbiotic stony and soft corals: Is their host‐algae relationship really mutualistic at lower mesophotic reefs?

Christine Ferrier‐Pagès,Vanessa Bednarz,Cecile Rottier,Yehuda Benayahu,Jean‐François Maguer,Joerg Wiedenmann,Maoz Fine,Renaud Grover

doi:10.1002/lno.11990

Abstract

AbstractMesophotic coral ecosystems (30–150 m depth) present a high oceanic biodiversity, but remain one of the most understudied reef habitats, especially below 60 m depth. Here, we have assessed the rates of photosynthesis and dissolved inorganic carbon (DIC) and nitrogen (DIN) assimilation by Symbiodiniaceae associated with four soft coral species of the genus Sinularia and two stony coral species of the genus Leptoseris collected respectively at 65 and 80–90 m depth in the Gulf of Eilat. Our study demonstrates that both Leptoseris and Sinularia species have limited autotrophic capacities at mid‐lower mesophotic depths. DIC and DIN assimilation rates were overall ~ 10 times lower compared to shallow corals from 10 m depth in the same reef. While Leptoseris symbionts transferred at least 50% of the acquired nitrogen to their host after 8‐h incubation, most of the nitrogen was retained in the symbionts of Sinularia. In addition, the host tissue of Sinularia species presented a very high structural carbon to nitrogen ratio (C : N) compared to Leptoseris or to the shallow coral species, suggesting nitrogen limitation in these mesophotic soft corals. The limited capacity of soft coral symbionts to acquire DIN and transfer it to the coral animal, as well as the high C : N ratios, might explain the scarcity of symbiotic soft corals at mid‐lower mesophotic depths compared to their prevalence in the shallower reef. Overall, this study highlights the significance of DIN for the distribution of the Cnidarian‐ Symbiodiniaceae association at mesophotic depth.

Highlights

All data were normalized to both surface area and ash free dry weight (AFDW) for Leptoseris species, whereas they were normalized to AFDW only for Sinularia species
Data normalized to AFDW show significantly lower rates of Pg and Pn in L. fragilis compared to L. glabra (Fig. 1, Supporting Information Table S1)
dissolved inorganic carbon (DIC) assimilation rates in Sinularia were not different between the four species, while the symbionts assimilated approximately twice as much DIC compared to the host compartment among all species

Summary

Introduction

Compared with shallow reefs, mesophotic reefs experience more stable and lower temperatures and a different light spectrum, to which species such as symbiotic corals have to adapt (Tamir et al 2019). Depth-specialist species such as Leptoseris spp., that are restricted almost exclusively to mesophotic environments exhibit horizontal plate-like morphologies, with a large coenosarc tissue, sparse corallites, and tiny tentacles (Englebert et al 2017); such morphology and polyp arrangement may increase mass transfer rates of inorganic carbon and nitrogen and the autotrophic nutrient assimilation. Sinularia species might depend even more on passive suspension feeding/ heterotrophy than Leptoseris species due to a different morphology and polyp arrangement. Such measurements will inform us on the strategies for energy acquisition in mesophotic corals. A better knowledge on the trophic ecology of habitat specialists is critical to understand their capacity to successfully establish, and compete in these light-limited environments and withstand future anthropogenic environmental changes

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Conclusion