Endolithic Algae Affect Modern Coral Carbonate Morphology and Chemistry

Stefan Krause,Anton Eisenhauer,Angela Vogts,Gernot Nehrke,Stanislav N Gorb,Volker Liebetrau,Timo Damm,Sebastian Büsse,Thomas Leipe

doi:10.3389/feart.2019.00304

Abstract

While burial diagenetic processes of tropical corals are well investigated, current knowledge about factors initiating early diagenesis remains fragmentary. In the present study we focus on recent Porites microatolls, growing in the intertidal zone. This growth form represents a model organism for elevated sea surface temperatures and provides important but rare archives for changes close to the seawater/atmosphere-interface with exceptional precision on sea level reconstruction. As other coral growth forms, microatolls are prone to the colonization by endolithic green algae. In this case the algae can facilitate earliest diagenetic alteration of the coral skeleton. Algae metabolic activity not only results in secondary coral porosity due to boring activities, but may also initiate reprecipitation of secondary aragonite within coral pore space, a process not exclusively restricted to microatoll settings. In the samples of this initial study, we quantified a mass transfer from primary to secondary aragonite of around 4% within endolithic green algae bands. Using δ18O, δ13C, Sr/Ca, U/Ca, Mg/Ca, and Li/Mg systematics suggest that the secondary aragonite precipitation followed abiotic precipitation principles. According to their individual distribution coefficients the different isotope and element ratios showed variable sensitivity to the presence of secondary aragonite in bulk samples, with implications for microatoll-based sea surface temperature (SST) reconstructions. The secondary precipitates formed on an organic template, presumably originating from endolithic green algae activity. Based on laboratory experiments with the green algae Ostreobium quekettii we propose a conceptual model that secondary aragonite formation is potentially accelerated by an active intracellular calcium transport through the algal thallus from the location of dissolution into coral pore spaces. The combined high-resolution imaging and geochemical approach applied in this study shows that endolithic algae can possibly act as a main driver for earliest diagenesis of coral aragonite starting already during a coral’s life span.

Highlights

Scleractinian corals have been shaping the Earth’s surface since the Triassic (Stanley, 2003), forming massive reef ecosystems
Areas inhabited by endolithic algae were characterized by numerous boring voids with diameters matching the cell diameter of O. quekettii, indicative for metabolically induced three concentric bands inhabited by Ostreobium were apparent (Figure 3A)
The present study shows evidence that endolithic algae can induce early diagenesis of primary coral aragonite starting almost immediately after skeleton formation

Summary

Introduction

Scleractinian corals have been shaping the Earth’s surface since the Triassic (Stanley, 2003), forming massive reef ecosystems. Besides their endosymbionts, scleractinian corals host a diverse microbial community (Rosenberg et al, 2007). Siphonal green algae of the genus Ostreobium are the most common ones (Halldal, 1968; Jeffrey, 1968), producing distinctive green bands in the coral skeleton (Halldal, 1968; Highsmith, 1981; Verbruggen and Tribollet, 2011). Known for many years (Duerden, 1902) the colonization dynamics, ecophysiology, and activity of endolithic phototrophs as Ostreobium are not entirely deciphered (Ralph et al, 2007). While evidence exists that colonization by the algae occurs mainly as a result of coral mechanical damage or coral section death (Titlyanov et al, 2008), the entry of Ostreobium during early coral ontogeny, within days after larval settlement, has been documented (Massé et al, 2018)

Methods

Results

Discussion

Conclusion