Dissolved Nitrogen Acquisition in the Symbioses of Soft and Hard Corals With Symbiodiniaceae: A Key to Understanding Their Different Nutritional Strategies?

Chloé A Pupier,Renaud Grover,Cécile Rottier,Jeroen A J M Van De Water,Maoz Fine,Christine Ferrier-Pagès

doi:10.3389/fmicb.2021.657759

Abstract

Nitrogen is one of the limiting nutrients for coral growth and primary productivity. Therefore, the capacity of different associations between corals and their algal symbionts (Symbiodiniaceae) to efficiently exploit the available nitrogen sources will influence their distribution and abundance. Recent studies have advanced our understanding of nitrogen assimilation in reef-building scleractinian (hard) coral-Symbiodiniaceae symbioses. However, the nutrient metabolism of other coral taxa, such as Alcyoniina (soft corals), remains underexplored. Using stable isotope labeling, we investigated the assimilation of dissolved nitrogen (i.e., ammonium, nitrate, and free amino acids) by multiple species of soft and hard corals sampled in the Gulf of Aqaba in shallow (8–10 m) and mesophotic (40–50 m) reefs. Our results show that dissolved nitrogen assimilation rates per tissue biomass were up to 10-fold higher in hard than in soft coral symbioses for all sources of nitrogen. Although such differences in assimilation rates could be linked to the Symbiodiniaceae density, Symbiodiniaceae species, or the C:N ratio of the host and algal symbiont fractions, none of these parameters were different between the two coral taxa. Instead, the lower assimilation rates in soft coral symbioses might be explained by their different nutritional strategy: whereas soft corals may obtain most of their nitrogen via the capture of planktonic prey by the coral host (heterotrophic feeding), hard corals may rely more on dissolved nitrogen assimilation by their algal symbionts to fulfill their needs. This study highlights different nutritional strategies in soft and hard coral symbioses. A higher reliance on heterotrophy may help soft corals to grow in reefs with higher turbidity, which have a high concentration of particles in suspension in seawater. Further, soft corals may benefit from lower dissolved nitrogen assimilation rates in areas with low water quality.

Highlights

Nitrogen (N) is essential for the life and growth of all organisms on Earth, as it is required for the biosynthesis of key cellular components
To gain better insights into the extent to which soft corals rely on their symbionts for their nutrition, we investigated how these symbiotic associations assimilate different dissolved N (DN) forms, and how this may be impacted by depth and temperature
Analyses of the chloroplast 23SrRNA gene sequences of the Symbiodiniaceae revealed that colonies of L. arboreum and

Summary

Introduction

Nitrogen (N) is essential for the life and growth of all organisms on Earth, as it is required for the biosynthesis of key cellular components. Corals are the primary reef-building and habitatforming species in these marine environments and require a steady supply of nutrients for growth and reproduction. As a consequence, they have evolved as meta-organisms or holobionts (Rohwer et al, 2002; Bosch and McFall-Ngai, 2011), in which the host is associated with an assemblage of microorganisms. They have evolved as meta-organisms or holobionts (Rohwer et al, 2002; Bosch and McFall-Ngai, 2011), in which the host is associated with an assemblage of microorganisms These microbial communities are notably involved in the protection against pathogens and in the efficient uptake and recycling of the few available nutrients (Rädecker et al, 2015). The algal symbionts belonging to the family Symbiodiniaceae (LaJeunesse et al, 2018) are the main assimilation site of the dissolved inorganic N forms (DIN), such as NH4+ and NO3− (Muscatine et al, 1979; Grover et al, 2002, 2003; Pernice et al, 2012)

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Conclusion