Tracking the rise of eukaryotes to ecological dominance with zinc isotopes

Abstract

The biogeochemical cycling of zinc (Zn) is intimately coupled with organic carbon in the ocean. Based on an extensive new sedimentary Zn isotope record across Earth's history, we provide evidence for a fundamental shift in the marine Zn cycle ~800 million years ago. We discuss a wide range of potential drivers for this transition and propose that, within available constraints, a restructuring of marine ecosystems is the most parsimonious explanation for this shift. Using a global isotope mass balance approach, we show that a change in the organic Zn/C ratio is required to account for observed Zn isotope trends through time. Given the higher affinity of eukaryotes for Zn relative to prokaryotes, we suggest that a shift toward a more eukaryote-rich ecosystem could have provided a means of more efficiently sequestering organic-derived Zn. Despite the much earlier appearance of eukaryotes in the microfossil record (~1700 to 1600 million years ago), our data suggest a delayed rise to ecological prominence during the Neoproterozoic, consistent with the currently accepted organic biomarker records.

Absence of biomarker evidence for early eukaryotic life from the Mesoproterozoic Roper Group: Searching across a marine redox gradient in mid‐Proterozoic habitability

“…In contrast, abundant hopanes derived from diverse bacterial source are ubiquitous in immature rocks as far back as 1.64 Ga (Brocks et al., ). The most parsimonious interpretation of the lack of detectable steranes in our view is that aerobic eukaryotes did not become ecologically widespread and abundant in the global oceans until well into the Neoproterozoic era (Brocks et al., ) and that source biota in mid‐Proterozoic and older oceans were dominated by bacteria (Brocks et al., , ; Flannery & George, ; Isson et al., ; Luo et al., ). A recent molecular clock analysis (Gibson et al., ) predicts that photosynthesis did not actually emerge in Eukarya until ca.…”

“…A recent consensus has emerged from careful analysis of low maturity ancient sedimentary rocks that C 27 –C 30 steranes, biomarkers sourced from eukaryotes, are below detection in rock bitumens for mid‐Proterozoic strata and more generally in rocks older than ca. 800 Ma (Blumenberg, Thiel, Riegel, Kah, & Reitner, ; Brocks et al., , , ; Flannery & George, ; French et al., ; Isson et al., ; Luo et al., ; Pawlowska et al., ). Indeed, the oldest robust reports of ancient steranes derive from rocks of the Chuar Group and Visingsö Group (700–800 Ma) using the high sensitivity of MRM‐GC‐MS for biomarker detection (Brocks et al., , ).…”

“…One important caveat is that lowoxygen-adapted eukaryotes which did not possess sterols may have been part of the community structure, since a few anaerobic protists have recently been shown to lack either sterols or tetrahymanol in their cell membranes (Takashita et al, 2017). Both a low abundance of microbial eukaryotes relative to bacteria and the possibility of adapted eukaryotic protists that did not make steroids or tetrahymanol can potentially reconcile the finding of eukaryotic microfossils in Roper Group strata (Javaux, Knoll, & Walter, 2004) , 2015Flannery & George, 2014;French et al, 2015;Isson et al, 2018;Luo et al, 2015;Pawlowska et al, 2013).…”

“…However, the true geochemical nature of Earth's “boring billion” remains poorly constrained. There are many lines of evidence that document an increase in Earth's surface oxidation state approximately 2.4–2.3 Ga (Bekker et al., ; Luo et al., ), and another biospheric oxygenation estimated between 800 and 550 Ma (reviewed in Och & Shields‐Zhou, ) that was coincident with a global expansion of eukaryotic abundance in the marine realm (Brocks et al., ; Isson et al., ). Conditions in the intervening mid‐Proterozoic are less studied and correspondingly less known, with the likelihood of intermediate redox states in the ocean and atmosphere.…”

The impact of marine nutrient abundance on early eukaryotic ecosystems

“…We find that the chronic P limitation characteristic of most of Earth's history (Bjerrum & Canfield, 2002;Derry, 2015;Jones, Nomosatryo, Crowe, Bjerrum, & Canfield, 2015;Laakso & Schrag, 2014Ozaki et al, 2019;Planavsky, 2015;Reinhard et al, 2017) would have yielded generally small Phytoplankton cell size, limiting the trophic scope of early eukaryotic ecosystems in surface ocean environments and attenuating production and export of biomass from the photic zone. Building on this framework, we suggest that the temporal correspondence between a significant shift in the Earth surface P cycle (Planavsky et al, 2010;Reinhard et al, 2017), the expansion of eukaryotic algae (Brocks et al, 2017;Cohen & Macdonald, 2015;Gueneli et al, 2018;Isson et al, 2018), and increasing evidence of predation in surface ocean ecosystems (Porter, 2016;Porter, Meisterfeld, & Knoll, 2003) after nearly 1 billion years of apparent ecological stasis (Knoll, 2014) may have been a natural outcome of an increase in marine nutrient supply linked to the long-term oxygenation of Earth's ocean-atmosphere system.…”