TEM evidence for eukaryotic diversity in mid‐Proterozoic oceans

Abstract

Biomarker molecular fossils in 2770 Ma shales suggest that the Eucarya diverged from other principal domains early in Earth history. Nonetheless, at present, the oldest fossils that can be assigned to an extant eukaryotic clade are filamentous red algae preserved in ca. 1200 Ma cherts from Arctic Canada. Between these records lies a rich assortment of potentially protistan microfossils. Combined light microscopy, scanning electron microscopy, and transmission electron microscopy on 1500‐1400 Ma fossils from the Roper Group, Australia, and broadly coeval rocks from China show that these intermediate assemblages do indeed include a moderate diversity of eukaryotic remains. In particular, preserved cell wall ultrastructure, observed using transmission electron microscopy (TEM), can help to bridge the current stratigraphic gap between the unambiguous eukaryotic morphologies of later Proterozoic assemblages and molecular biomarkers in much older rocks.

New records of late Ediacaran microbiota from Poland

“…Leiosphaerids are conventionally interpreted as cysts, i.e. dormant/reproductive stages in the life cycle of microalgae, and this is well supported by the presence of an excystment opening, comparative morphology, recalcitrant cell wall and their abundance (Vidal and Ford, 1985;Vidal and Moczydłowska, 1997;Butterfield et al, 1994;Javaux et al, 2004;Marshall et al, 2005Marshall et al, , 2006. They show a random and widespread distribution in shallow marine sediments, which is consistent with a planktic mode of life and accumulation of resting cysts in seafloor sediments across shelves and open-marine facies.…”

“…3). Leiosphaeridia crassa, another species in the Lublin association, also possesses the multilayered wall ultrastructure, with a possible although partially preserved TLS observed in specimens from the 1.4-1.5 Ga Roper Group in Australia, and an affinity to green algae (Javaux et al, 2004). Microchemical analysis of three species of Leiosphaeridia (L. crassa, L. jacutica and L. tenuissima) from the same Australian material showed similar chemical composition of their wall (Marshall et al, 2005).…”

“…The eukaryotic algal affinity of several representative specimens of the genus Leiosphaeridia has recently been well supported by ultrastructural studies of the cell wall (Talyzina and Moczydłowska, 2000;Arouri et al, 2000;Javaux et al, 2004), but it has also been assumed on the basis of morphology, size, chemically resistant sporopollenin-like biopolymers of the cell wall and palaeoecologic distribution (Tappan, 1980;Vidal, 1994;Colbath and Grenfell, 1995). Leiosphaerids are conventionally interpreted as cysts, i.e.…”

“…The wall ultrastructure, which may reveal various biological affinities of leiosphaerids and allow their meaningful taxonomic segregation, is known at the moment in a very few morphotypes (Talyzina and Moczydłowska, 2000;Javaux et al, 2004). This mostly non-diagnostic morphology and lack of objective features in leiosphaerids resulted in application of arbitrary chosen size classes, together with approximate wall thickness, to define various species.…”

Paleoproterozoic sterol biosynthesis and the rise of oxygen

“…Yet the majority of Paleoproterozoic and Mesoproterozoic sedimentary rocks described that are known to preserve biomarkers appear to be devoid of eukaryotic steranes, but do contain C 27 -C 35 hopane hydrocarbons of bacterial origin (Blumenberg et al, 2012;Brocks et al, 2005;Flannery and George, 2014;Luo et al, 2015). From a micro-and macrofossil perspective, ample evidence exists for the presence of unambiguously eukaryotic fossil assemblages in the Mesoproterozoic (Butterfield, 2000;Javaux et al, 2001;Javaux et al, 2004;Knoll et al, 2006;Zhu et al, 2016). Accordingly, the failure to detect a concomitant record of eukaryotic steranes suggests that their membrane lipids contained limited amounts of steroids.…”