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Mass Extinctions Past and Present Biogeosciences Discuss., 5, 2401–2423, 2008 Biogeosciences www.biogeosciences-discuss.net/5/2401/2008/ Discussions BGD © Author(s) 2008. This work is distributed under 5, 2401–2423, 2008 the Creative Commons Attribution 3.0 License. Biogeosciences Discussions is the access reviewed discussion forum of Biogeosciences Mass extinctions past and present S. A. Wooldridge Title Page Mass extinctions past and present: Abstract Introduction Conclusions References a unifying hypothesis Tables Figures S. A. Wooldridge J I Australian Institute of Marine Science, PMB #3, Townsville MC, 4810, QLD, Australia J I Received: 18 April 2008 – Accepted: 13 May 2008 – Published: 9 June 2008 Back Close Correspondence to: S. A. Wooldridge ([email protected]) Full Screen / Esc Published by Copernicus Publications on behalf of the European Geosciences Union. Printer-friendly Version Interactive Discussion 2401 Abstract BGD Enzymes are often referred to as the “agents of life” – a very apt term, since essentially all life processes are controlled by them. Typically, these enzymes only function across 5, 2401–2423, 2008 a narrow band of environmental conditions, particularly temperature and pH. Ambi- 5 ent conditions that challenge these operating conspecifics trigger enzyme dysfunction. Mass extinctions Here, it is proposed that the pH-dependent inactivation of a single enzyme, urease, past and present provides a unifying kill-mechanism for at least four of the “big five” mass extinctions of the past 560 million years. The triggering of this kill-mechanism is suggested to be S. A. Wooldridge sensitive to both gradualistic and catastrophic environmental disturbances that cause 10 the operating pH of urease-dependent organisms to cross enzymatic “dead zones”, one of which is suggested to exist at ∼pH 7.9. For a wide range of oceanic and ter- Title Page restrial ecosystems, this pH threshold coincides with an atmospheric CO2 partial pres- Abstract Introduction sure (pCO2) of ∼560 ppmv – a level that at current CO2 emission trajectories may be exceeded as early as 2050. The urease hypothesis thus predicts an impending An- Conclusions References 15 thropocene extinction event of equivalence to the “big five” unless future atmospheric Tables Figures pCO2 levels can be stabilised well below 560 ppmv. Immediate scientific discussion and testing is required to confirm the validity of the urease hypothesis. J I 1 Introduction J I Back Close Origination and extinction of species have occurred throughout the history of life, with 20 the continual increase of diversity indicating the predominance of origination (Raup and Full Screen / Esc Sepkoski, 1982). The only major reversals to this trend have occurred during mass ex- tinctions. Mass extinctions are defined as the extinction of a significant proportion of the Printer-friendly Version Earth’s biota over a geographically widespread area in a geological insignificant period of time, often one that appears instantaneous when viewed at the level provided by the Interactive Discussion 25 geological record. Known ominously as the “big five”, most authors accept the occur- rence of five such mass extinction events during the past 560 million years: the late- 2402 Ordovician (∼440 Ma), late-Devonian (∼370–350 Ma), end-Permian (∼250 Ma), end- Triassic (∼220–200 Ma) and end-Cretaceous (∼65 Ma) events (Raup and Sepkoski, BGD 1982; McGhee et al., 2004; Rohde and Muller, 2005). The marine extinction inten- 5, 2401–2423, 2008 sity across the Phanerozoic (Fig. 1a) highlights the infrequent but significant impact of 5 these events within the marine realm. Research into the cause of mass extinction events was tremendously invigorated Mass extinctions by the seminal work of Louis Alverez and colleagues, who proposed that a bolide im- past and present pact was ultimately responsible for the end-Cretaceous extinction event (Alverez et al., 1980). Alternative catastrophic scenarios events have been proposed, including major S. A. Wooldridge 10 volcanism, and the rapid release of seafloor methane hydrates. More gradualistic pro- cesses that induce global climate change (warming or cooling), fluctuating sea levels, Title Page or oceanic anoxia have also been implicated. Geochemical and paleontology evidence suggest that no single triggering mechanism successfully explains the onset of all of Abstract Introduction the “big five” mass extinction events (Elewa, 2008). However, it is important to draw a Conclusions References 15 clear distinction between kill and trigger mechanisms. A kill mechanism is the physio- logically disruptive process that causes death, whereas a trigger mechanism is the crit- Tables Figures ical disturbance that brings one or more kill mechanisms into play. Importantly, each kill mechanism predicts a physiology-dependent pattern of extinction and survival, thereby J I providing a test for geophysical hypotheses. The development of “testable” mechanistic 20 explanations for mass extinctions events thus requires that trigger mechanisms inferred J I from geological data are integrated with kill mechanisms that are buttressed by paleo- Back Close physiological support. What’s more, a credible kill mechanism should: (i) explain all of the losses, not just focus on a few iconic groups (such as dinosaurs); (ii) explain why Full Screen / Esc particular groups of organisms died out and why others survived – both on land as well 25 as in the ocean; and (iii) be based on events or processes that can be demonstrated Printer-friendly Version to happen, not just inferred from the extinction. Here, an evidential based approach is used to propose that the pH-dependent inac- Interactive Discussion tivation of a single specific enzyme, urease, satisfies all of the requirements necessary to be considered a potent kill mechanism for the late-Devonian, end-Permian, end- 2403 Triassic and end-Cretaceous events. The accuracy with which this new understanding allows these extinction events to be unified provides a strong rationale from which to BGD warn of an impending Anthropocene extinction event unless future atmospheric pCO 2 5, 2401–2423, 2008 levels can be stabilised below 560 ppmv. Mass extinctions 5 2 Urease inhibition: A potent kill mechanism? past and present Urease (EC 3.5.1.5, urea amidohydrolase) is a nickel-dependent metalloenzyme that S. A. Wooldridge catalyses the near instantaneous hydrolysis of urea (NH2CONH2) to form ammonia (NH3) and carbon dioxide (CO2). The half-time of the uncatalyzed decomposition of urea is ∼3.6 years, which confirms urease as the most proficient enzyme identified Title Page 10 to date (Estiu and Merz, 2004). Numerous organisms synthesize urease, including terrestrial plants, marine algae, bacteria, fungi and invertebrates (including marine Abstract Introduction crustaceans). This wide biological distribution does not however include vertebrates Conclusions References (Urich, 1994) – though bacteria found within the digestive tract of vertebrates is known to synthesize urease e.g. the human pathogen Helicobacter pylori (Ha et al., 2001). Tables Figures 15 Molecular analysis demonstrates that urease protein sequences are highly conserved across eukaryotic and prokaryotic kingdoms; with all known ureases being structurally J I similar, including superimposable active sites that employ the same catalytic mecha- nism (Krajewska and Ciurli, 2005; Witte et al., 2005). J I A key function of urease is to allow an organism to access external or internally Back Close 20 generated urea as a nitrogen source for cellular growth. Significant amounts of plant nitrogen flow through recycled urea (Sirko and Brodzik, 2000; Canton´ et al., 2005). In Full Screen / Esc this case, recycled urea derives from the catabolism of the amino acid, arginine, and possibly from the degradation of purines and ureides (Sirko and Brodzik, 2000). In ad- Printer-friendly Version dition to these pathways, organisms that possess the ornithine-urea cycle are also able Interactive Discussion 25 to recover urea from the routine catabolism of ingested and/or cellular protein (amino acid) sources. The role of urease in increasing the availability of nitrogen for cellular growth is particularly important during the early stages of an organism’s development. 2404 For example, a seed-specific urease converts the products arising from the breakdown of storage proteins into nitrogen sources that support the growth of both angiosperm BGD and gymnosperm seedlings (Jones and Boulter, 1968; Zonia et al., 1995; Todd et al., 5, 2401–2423, 2008 2001). Similarly, urease activity found within the eggs of marine invertebrates (e.g. sea 5 urchins: Brookbank and Whiteley, 1954) functions to mobilise nitrogen from storage compounds during the pre-feeding stages of embryo development. Mass extinctions A second important function of urease is in facilitating the biomineralisation of cal- past and present cium carbonate (CaCO3) by invertebrates (Campbell and Speeg, 1969; Crossland and Barnes, 1974). Indeed, urease inhibition has been shown to disrupt mineralisation in S. A. Wooldridge 10 land snails (Abolins-Krogis,ˇ 1986) and gastropod molluscs (Pedrozo et al., 1997). A recent biomineralisation model (Wooldridge, submitted1) supports the involvement of Title Page urease in the initiation of nascent CaCO3 nanograins that underpin the initial stages of the biomineralisation process within invertebrates (Dauphin et al., 2007). The model Abstract Introduction builds upon the theoretical understanding that NH3 produced by urease-mediated hy- Conclusions References 15 drolysis
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