Soft corals form aragonite-precipitated columnar spiculite in mesophotic reefs

Erez Shoham,Zahava Barkay,Thomas Prohaska,Andreas Zitek,Yehuda Benayahu

doi:10.1038/s41598-018-37696-z

Abstract

Surveys conducted in Eilat’s upper mesophotic coral ecosystem (MCE) revealed protruding columnar calcareous structures with a Sinularia octocoral colony growing atop of each. The current study addressed the hypothesis that these colonies produce spiculites, and sought to determine (a) the spatial occurrence and dimensions of the spiculite-forming colonies and their species affiliation; (b) their microstructural features; and (c) the elemental composition of the columnar spiculites in comparison to the sclerites of the colonies. All the spiculite-forming colonies were exclusively found in the upper MCEs and produced by S. vrijmoethi. This type of spiculite, including its elemental analysis, is reported here for the first time for coral reefs in general and for the MCE in particular. Examination of the spiculites by scanning electron microscopy and energy-dispersive X-ray spectroscopy revealed spindle shaped-sclerites cemented by crystallites. The elemental composition of the sclerites differed from that of the cementing crystallites, in featuring ~8% Mg in the former and none in the latter. Inductively coupled plasma mass spectrometry revealed fragments of spiculite to be composed of 35% sclerites and 65% crystallites. X-ray powder diffraction analysis of individual sclerites indicated that they are composed exclusively of magnesium-calcite, and the spiculite fragments to also feature 9.3 ± 4% aragonite and 5–7% amorphous calcium carbonate. Consequently, it is proposed that the formation of the crystallites, which lithify the sclerites, is caused by a non-biogenic aragonite precipitation, and that the living colony might benefit from this protruding spiculite structure by means of enhanced exposure to water flow.

Highlights

Soft corals of the genus Sinularia are widespread on the Indo-Pacific reefs and considered to be the most speciose among Octocorallia, with more than 185 described species[1]
The underwater survey yielded 60 calcareous columnar structures all bearing colonies identified as S. vrijmoethi: 22 at the Eilat Ashkelon Pipeline Company (EAPC), 19 at the Institute for Marine Sciences in Eilat (IUI), and 19 at the Nature Reserve (NR) upper mesophotic coral ecosystem (MCE) (Fig. 3)
The columnar structures exhibited an average height of 18 ± 0.9 cm (Fig. 3, n = 60, range 5–40 cm) and, there were no significant differences in height among these MCEs (Fig. 3, ANOVA, p = 0.1)

Summary

Introduction

Soft corals of the genus Sinularia (family Alcyoniidae) are widespread on the Indo-Pacific reefs and considered to be the most speciose among Octocorallia, with more than 185 described species[1]. Cores removed from living colonies there revealed a distinct transition in composition from the discrete spindle-shaped sclerites at the top of the colony, to the compact spiculite at the bottom, with its characteristic cementing calcium carbonate These Sinularia octocorals are capable of accreting material into significant reef structures over time, and can be considered as reef-builders. The results shed light on a peculiar feature of soft corals in the MCEs, reflected in the shape of unique calcareous columnar structures, which are suggested to benefit the living colony atop them These structures have been confirmed to be spiculites, composed of Sinularia spindle-shaped calcite sclerites lithified by fibrous aragonite crystallites, whose distinct elemental composition is presented here for the first time

Methods

Results

Conclusion