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

Date Published: March 14, 2019

Publisher: Public Library of Science

Author(s): Xiyang Zhang, Mingyue Dai, Min Wang, Yong’an Qi, O. Roger Anderson.

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

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.

Partial Text

Epiphyton Bornemann (1886) [1] is one of the best known calcareous genera from Early Paleozoic shallow-marine limestones [2]. These dendritic microfossils have been widely reported in Early-to-Middle Cambrian [3–6] and Early Ordovician reef systems [7], are also seen sporadically in the Silurian [8], and then undergo a resurgence in the Late Devonian [9, 10] before disappearing from the fossil record in the Cretaceous [11]. The phylogenetic position of this genus also 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 also shared by Pratt [4]. It is noteworthy that Epiphyton branches always co-occur with Renalcis chambers, and that both of these genera have been shown to be the most common end-members of a series of salient, partly intergrading morphotypes [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].

The North China Platform was located near the paleoequator during the Cambrian [19] and comprised a stable epeiric sea that was surrounded by abyssal troughs (e.g., the northern Paleo-Asian Ocean, the Southern Qinling Ocean, and the western Paleo-Qilian Ocean). Deposition of the whole of this block began during Cambrian Epoch 2 in the aftermath of widespread transgression, subsequent to a long period of weathering since ca. 850 Ma [20]. During the early Hsuchuangian within Cambrian Miaolingian, enormous tidal flats covered the whole of this platform, ranging from sand-mud areas in the western Ordos Basin to mud flats in the eastern Ji-Lu-Yu area, separated by the Laiyuan and Houma seas (Fig 1A). A carbonate platform was gradually established via sustained transgression from the southwest, and by Changhian time this was dominated by ooid shoals [21].

A total of 24 samples were collected for this study from each thrombolite unit. Each sample was then processed into at least eight thin sections, one polished slab and one rock platelets, before petrographic (ZEISS Axioskop 40 Pol, Germany) and stereoscopic microscopes (ZEISS SteREO Discovery V20, Germany) were utilized to observe the optical characteristics of Epiphyton. Rock platelets were etched with 5% formic acid for 5 s before being rinsed with distilled water. This etching method for the identification of cyanobacterial sheath and capsule remains has been utilized in previous studies on both modern and ancient calcified cyanobacterial mats [25–27]. Splintered samples were then sputtered with gold to enhance conductivity, and were examined using a scanning electron microscope (SEM; FEI quanta 250, USA) operating at 10–15 kV before an accessorial Energy Dispersive Spectrometer (EDS; Bruker, Germany) was applied for multi-element determinations. All micro-level examinations were carried out in the Key Laboratory of Biogenic Trace and Sedimentary Minerals, Henan Polytechnic University, Jiaozuo, China. All thrombolite samples and thin sections are publicly deposited in the ichnofossil showroom of Henan Polytechnic University, Jiaozuo, China under catalogue numbers PZ160612–PZ160641, and all of them are accessible by others. No permits were required for the described study, which complied with all relevant regulations.

Two thrombolite forms were recognized in the outcrop. The first of these, khaki-colour stratiform thrombolites (Fig 2C), are up to 40 cm in thickness and comprise multiple yellow and black clots. These bifurcating clots (like branches) are less than 5 cm in height and approximately equal in diameter, while branches run perpendicular to bedding surfaces and are truncated by the argillaceous stylolite above (Fig 2D). The second kind of thrombolite seen in this section comprises a series of domed mounds and argillaceous interstitial material (Fig 2E); each of these mounds is 20–40 cm in diameter and 60–100 cm in height, comprised of black clots and grayish yellow fillings (Fig 2F). These clots occupy up to 70% of the available space and are stacked vertical to bedding (Fig 2F).

This study presents evidence that Epiphyton was constructed by colonies of calcified coccoid cyanobacteria. In these cases, branches are stacked in the form of a string of end-to-end capsules regarded here as cyanobacterial thalli. Thus, each of these capsules comprises an outermost Al-Mg-Fe silicate envelope and inner calcite infillings which are constituted from mineralized thick EPS and calcified thin sheaths, respectively. Some pits within inner calcite in these cases have been dismissed as remnants of degraded cocci, while the metabolism of cyanobacterial photosynthesis and SRB sulfate reduction both contribute to in vivo calcification and post-mortem changes by elevating ambient alkaline levels to the supersaturated calcium carbonate point. Al-Mg-Fe silicate are the last stage of mineralization when metallic cations absorbed in the EPS react with aqueous SiO2 in the water. Thus, massively abundant occurrences of Epiphyton in the Cambrian reflect an excessive seawater carbonate saturation state which promoted bioinduced calcification, the first such act within the Phanerozoic [58].

 

Source:

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

 

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