Putative extremely long evolutionary stasis in bacteria might be explained by serial convergence
Authors:
Dvorak et al
Abstract:
In a recent paper, Schopf et al. (1) analyzed 1.8-Ga-old fossil sulfur bacteria and found an intriguing morphological similarity between fossil and modern species. Moreover, the authors showed that the deep-water sulfur cycling environment, where these bacteria reside, has not significantly changed throughout time. Thus, the authors hypothesize that this phenomenon is a result of an extreme evolutionary stasis in these bacteria. Such a static evolution is termed hypobradytelic and it has also been described in some cyanobacteria (2), where an evolutionary stasis is expected to be more than 2 Ga. However, these conclusions rely only on geological and morphological evidence.
Counterattack!
Reply to Dvořák et al.: Apparent evolutionary stasis of ancient subseafloor sulfur cycling biocoenoses
Authors:
Schopf et al
Abstract:
We thank Dvořák et al. for their comment (1) on our paper (2), in which we compare sulfur-cycling ∼1.8- and ∼2.3-Ga fossil communities with their modern counterparts and report that the community fabric of the fossil and modern microbes, as well as their organismal and cellular morphology, their interlinked energy-production via anaerobic sulfate-reduction and sulfur species oxidation, and their use of sulfate and nitrate to fuel this sulfur cycle appear to have remained unchanged over a segment of geological time equivalent to half the age of the Earth.
Large sulfur isotope fractionations associated with Neoarchean microbial sulfate reduction
Authors:
Zhelezinskaia et al
Abstract:
The minor extent of sulfur isotope fractionation preserved in many Neoarchean sedimentary successions suggests that sulfate-reducing microorganisms played an insignificant role in ancient marine environments, despite evidence that these organisms evolved much earlier. We present bulk, microdrilled, and ion probe sulfur isotope data from carbonate-associated pyrite in the ~2.5-billion-year-old Batatal Formation of Brazil, revealing large mass-dependent fractionations (approaching 50 per mil) associated with microbial sulfate reduction, as well as consistently negative Δ33S values (~ –2 per mil) indicative of atmospheric photochemical reactions. Persistent 33S depletion through ~60 meters of shallow marine carbonate implies long-term stability of seawater sulfate abundance and isotope composition. In contrast, a negative Δ33S excursion in lower Batatal strata indicates a response time of ~40,000 to 150,000 years, suggesting Neoarchean sulfate concentrations between ~1 and 10 μM.
Palaeoecological and post-depositional changes recorded in Campanian–Maastrichtian black shales, Abu Tartur plateau, Egypt
Authors:
El-Shafeiy et al
Abstract:
The Upper Cretaceous black shales of Egypt are part of the worldwide belt of Late Cretaceous organic-rich shales. Black shales are particularly prominent in North Africa and the Middle East. In Egypt, these shales occur in an east-west trending belt extending from the Quseir-Safaga district along the Red Sea to the Kharga-Dakhla land-stretch passing through the Nile Valley. The black shales are hosted mainly in the Campanian to Maastrichtian Duwi and Dakhla formations. In order to reconstruct palaeoenvironmental conditions, the present work focuses on the distribution of organic matter including lipid biomarkers within the Abu Tartur borehole section, which was drilled in 2007 in the Maghrabi-Liffya area. The kerogen in the Abu Tartur section is of type III with the exception of sedimentary deposits at the Duwi/Dakhla transition. Low Tmax, odd-over-even predominance of n-alkanes with a commonly high Carbon Preference Index, good preservation of carboxylic acids and abundant 17β,21β-hopanes and -hopanoic acids indicate immaturity of the organic constituents in the bitumen. Although thermal maturation was only low, the preponderance of rearranged steranes (diasterenes) over regular steranes indicates enhanced clay catalysis. Significant allochthonous input typifies the Abu Tartur section deposits, which are characterized by high contents of long-chain n-alkanes and low carbonate contents. The high content of desmethyl steranes and diasterenes suggests that marine algae were the main marine primary producers. The presence of different isomers of hopanes (C27, C29–C31) and hopanoic acids (C31–C33) reveals input from various bacteria. The observed variation in the abundance of biomarkers corresponds to changes in planktic algal assemblages associated with sea level change and episodic photic zone anoxia, which are indicated by the occurrence of aryl isoprenoids, biomarkers of green sulphur bacteria.
