The Calcification Of Cyanobacteria From Microbial Carbonates In The Cambrian,Western Henan

Two cyanobacterial species,Girvanella and Epiphyton,are found in microbialites from the Cambrian,Western Henan.The analysis of morphological characters of cyanobacteria,mineral component and their calcification in order to rebuild the structure of cell and distinguish the biominerals can provide evidences for the study on“calcification act of cyanobacteria” in the early Phanerozoic.Girvanella is curving,non-branching and intertwined which consists of tube core and tube wall.Tube core is elongated calcite considered as product of diagenesis,10–20?m in diameter and 50-200 ?m in length.Tube wall,as biominerals by photosynthesis,consists of granular calcites which arranged radially around the tube core.Accordingly,the structure of cell can be rebuilt: trichome and external extracellular polymeric substance?EPS?,between which develops a thin and tough cell wall.The definition of Epiphyton actually disciplines “capsules”?the assembly of coccoid cyanobacteria?and branched structure stacked by capsules.The capsules,15-60 ?m in diameter,consist of several coccoid cyanobacteria and their EPS.Capsules and branched structures are brown color scattering in the black clot under a stereoscopic microscope.The edge of capsule is darker than inner part and the branch is dichotomy under a polarizing microscope.Capsules consist of thin pumice wall which encapsulate the calcite.The walls,1-5 ?m thick,are layered or radial.The encapsulated calcite contains many pits which represent the degraded cell of coccoid cyanobacteria.The surrounding calcites are calcified EPS by the process of photosynthesis and sulfate reduction.CO₂-concentrating mechanism?CCM?involves active carbon transport into the cell and conversions that liberate OH^-.OH^-transforms HCO3-into CO32-.Once cyanobacteria are dead,excretive endolysin will degrade the EPS leading to the release of Ca²⁺ which absorbed to EPS,and then promote the precipitation.This process takes place in the EPS,so the arrangements of the biomacromolecule in the EPS have a big influence on morphology of minerals.The regular granular calcites indicate biomacromolecules in Girvanella EPS are ordered,while dense calcites indicate biomacromolecules in Epiphyton EPS are disordered.The calcification of Epiphyton EPS may be caused by sulfate reduction when the Epiphyton are dead.This process generates OH^-that promotes the precipitation.The EPS provide the template for nucleation.Endolysin secreted by sulfate reducting bacteria prompt releasing Ca²⁺.