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

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.

Tubular microfossils from the Ediacaran Weng’an Biota (Doushantuo Formation, South China) are not early animals

“…The patterns of polarized terminal growth and dichotomous branching exclude it from a cyanobacterial affinity, and we have already presented evidence precluding a cnidarian interpretation. As a matter of fact, the branching morphology and growth mode of Ramitubus strongly resembles branching Cambrian Epiphyton, an enigmatic group of benthic, colonial reef building organisms that have not only been compared to the living red alga Corallina (Riding and Toomey, 1972;Luchinina and Terleev, 2008), but also to cyanobacterial calcimicrobes (Pratt, 1984;Zhang et al, 2019). The angle between sister branches of Epiphyton ranges 20° to 37°, similar to that of Ramitubus.…”

Contrasting morphology and growth habits of Frutexites in Late Devonian reef complexes of the Canning Basin, northwestern Australia

“…Zhang et al. (2019) proposed that Epiphyton branch formation was controlled by the growth hormones of coccoid colonies. Biofilms developed in laboratory conditions have shown that nutrient limitations control their morphology (Picioreanu et al., 1998; Wimpenny, 2000; Wimpenny & Colasanti, 1997).…”

Calcilobes wangshenghaii n. gen., n. sp., microbial constructor of Permian–Triassic boundary microbialites of South China, and its place in microbialite classification

“…Consequently, there is no supporting evidence that would demonstrate whether it was photosynthesizing or not, thus, interpretations of a cyanobacterial origin may or may not be correct. Nevertheless, interpretation that such similar structures as Tarthinia, Renalcis and Epiphyton may have been cyanobacteria is well known in literature (see discussions in Riding 2011b; Lee et al 2014;Liu et al 2016;Zhang et al 2019), so do not preclude the option of a cyanobacterial nature for the C. wangshenghaii in the PTBMs. Implications for palaeo-environmental reconstructions are obvious and will need further work to determine this aspect.…”