Contrasting seasonal responses in dinitrogen fixation between shallow and deep-water colonies of the model coral Stylophora pistillata in the northern Red Sea.

Vanessa N Bednarz,Mamoon M D Al-Rshaidat,Laura Rix,Christian Wild,Ulisse Cardini,Nanne Van Hoytema,Malik S Naumann

doi:10.1371/journal.pone.0199022

Abstract

Tropical corals are often associated with dinitrogen (N2)-fixing bacteria (diazotrophs), and seasonal changes in key environmental parameters, such as dissolved inorganic nitrogen (DIN) availability and seawater temperature, are known to affect N2 fixation in coral-microbial holobionts. Despite, then, such potential for seasonal and depth-related changes in N2 fixation in reef corals, such variation has not yet been investigated. Therefore, this study quantified seasonal (winter vs. summer) N2 fixation rates associated with the reef-building coral Stylophora pistillata collected from depths of 5, 10 and 20 m in the northern Gulf of Aqaba (Red Sea). Findings revealed that corals from all depths exhibited the highest N2 fixation rates during the oligotrophic summer season, when up to 11% of their photo-metabolic nitrogen demand (CPND) could be met by N2 fixation. While N2 fixation remained seasonally stable for deep corals (20 m), it significantly decreased for the shallow corals (5 and 10 m) during the DIN-enriched winter season, accounting for less than 2% of the corals’ CPND. This contrasting seasonal response in N2 fixation across corals of different depths could be driven by 1) release rates of coral-derived organic matter, 2) the community composition of the associated diazotrophs, and/or 3) nutrient acquisition by the Symbiodinium community.

Highlights

Scleractinian corals are effectively composed of an assemblage of diverse organisms including the cnidarian host, endosymbiotic dinoflagellates, bacteria, archaea and fungi [1]
In order to tease apart the effects of light and other seasonal factors in conspecifics with hypothetically similar nutritional strategies, we investigated N2 fixation by the scleractinian coral S. pistillata along a shallower depth gradient
Previous studies have described either seasonal or depth-specific differences in coral-associated N2 fixation rates, while the present study provides a comparison of seasonal differences across corals from different depths

Summary

Introduction

Scleractinian corals are effectively composed of an assemblage of diverse organisms (often referred to as the coral ‘holobiont’) including the cnidarian host, endosymbiotic dinoflagellates (of the genus Symbiodinium), bacteria, archaea and fungi [1]. Previous studies on coral-associated diazotrophs in the northern Red Sea report highest N2 fixation rates during summer, when light levels and temperature are highest and DIN concentrations are lowest [12,21].

Results

Conclusion