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

Abstract

Mesophotic 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.

Molecular insights into the Darwin paradox of coral reefs from the sea anemone Aiptasia

“…However, the provision of organic carbon is not the only important function attributed to the algal endosymbionts. Nitrogen is one of the primary growth-limiting nutrients in coral-reef ecosystems (2,6), and the algae have been thought to be the main contributors to nitrogen acquisition and recycling (7)(8)(9)(10)(11) due to their high capacity for ammonium assimilation (12). However, some evidence has also suggested an active role for the host in nitrogen assimilation (13), a view supported by the recent realization that the host also possesses the enzymatic machinery to recycle ammonium via the glutamine synthetase / glutamate synthase (GS/GOGAT) system (14)(15)(16).…”

Soft coral reproductive phenology along a depth gradient: Can “going deeper” provide a viable refuge?

“…The results of our study demonstrate that surface‐brooding in the MCE population consistently lags behind that of the shallow one (Figures 2 and 4), which might also be due to the exponential reduction in light intensity with depth (Kahng et al, 2019). This lag in timing could result from the longer time needed to acquire the photosynthetically‐derived assimilates necessary for gonad development and maturation by corals in deeper habitats, compared with those inhabiting the well illuminated shallow depths (Ferrier‐Pagès et al, 2021).…”

Linking photoacclimation responses and microbiome shifts between depth-segregated sibling species of reef corals

“…Another aspect that may influence the vertical distribution of Orbicella species is their heterotrophic feeding capacity. It has been indicated that corals from deep environments increase their metabolic reliance on heterotrophy to compensate for reduced photosynthesis [ 53 , 54 ]. The dominance of O. franksi in deep environments might be related to an increased heterotrophic feeding capacity relative to its sibling species.…”