Calcified coccoid from Cambrian Miaolingian: Revealing the potential cellular structure of Epiphyton.

Xiyang Zhang,Min Wang,Yong’An Qi,Mingyue Dai,O Roger Anderson

doi:10.1371/journal.pone.0213695

Xiyang Zhang, Min Wang + Show 3 more

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https://doi.org/10.1371/journal.pone.0213695

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Abstract

Epiphyton, Renalcis, and Girvanella are ubiquitous genera of calcified cyanobacteria/algae from Early Paleozoic shallow-marine limestones. One genus, Epiphyton, is characterized by a particular dendritic outline, and extensive research has revealed the morphology of calcified remains although little information on cellular structure is known. The mass occurrence of calcified Epiphyton in microbialites from Cambrian Miaolingian, the Mianchi area of North China is preserved as black clots within thrombolites and have dendritic and spherical outlines when viewed with a petrographic microscope. These remains, visible under scanning electron microscope (SEM), also comprise spherical or rectangle capsules. These capsules are made up from external envelopes and internal calcite with numerous pits, which closely resemble modern benthic coccoid cyanobacteria. These pits are between 2 μm and 4 μm in diameter and are interpreted here to represent the remnants of degraded coccoid cells, while the calcite that surrounds these pits is interpreted as calcified thin extracellular polymeric substances (EPS). In contrast, associated capsular envelopes represent thick EPS mineralized by calcium carbonate with an admixture of Al-Mg-Fe silicates. Dendritic ‘thalli’ are typically stacked apically because of the repeated growth and calcification of these capsules. Carbon and oxygen isotope results are interpreted to indicate that both photosynthesis and heterotrophic bacterial metabolism (especially sulfate reducing bacteria) contributed to carbonate precipitation by elevated alkalinity. Epiphyton are therefore here interpreted as colonies of calcified coccoid cyanobacteria, and the carbonate-oversaturated seawater during the Cambrian was conducive to their mineralization.

Highlights

Epiphyton Bornemann (1886) [1] is one of the best known calcareous genera from Early Paleozoic shallow-marine limestones [2]
The aim of this paper is to identify both Epiphyton cells and their appendages based on observations and to hypothesize the classification of this genus
This study presents evidence that Epiphyton was constructed by colonies of calcified coccoid cyanobacteria

Summary

Introduction

Epiphyton Bornemann (1886) [1] is one of the best known calcareous genera from Early Paleozoic shallow-marine limestones [2]. The phylogenetic position of this genus remains debated; Epiphyton was originally classified within the red algae [12, 13] and was placed within the Rhodophyta by Luchinina and Terleev [14] who compared Cambrian samples from Siberia with thalli samples from living Corallina. In earlier work, both Hofmann [15] and Poncet [16] had assumed that Epiphyton formed by repeated growth and the synsedimentary calcification of colonies of coccoid blue green algae, a view that was later shared by Pratt [4]. Luchinina considered other calcimicrobes similar to Epiphyton to be growth stages within the life cycle of this genus, including Renalcis, Izhella, Chabakovia, Shuguria, Gemma, and the dendroid form Korilophyton [14]

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