Anoxia Evidence for Eukaryote Survival and Paleontological Strategie
ANOXIA defines the lack of free molecular oxygen in an environment. In the presence of organic matter, the metabolism of anaerobic prokaryotes soon produces compounds such as free radicals, hydrogen sulfide, or methane that are typically toxic to aerobes.
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h et al. (eds.), Anoxia: Evidence for Eukaryote Survival and Paleontological Strategies, Cellular Origin, Life in Extreme Habitats and Astrobiology 21, 17–38 DOI 10.1007/978-94-007-1896-8_2, © Springer Science+Business Media B.V. 2012
BIOGEOCHEMICAL REACTIONS IN MARINE SEDIMENTS UNDERLYING ANOXIC WATER BODIES
TINA TREUDE Department of Marine Biogeochemistry, Leibniz Institute of Marine Sciences (IFM-GEOMAR), 24148 Kiel, Germany 1. Introduction This chapter provides an overview of biogeochemical reactions in marine sediments underlying temporal or permanent hypoxic and anoxic water bodies in modern and past oceans. The aim of this review is to describe the chemical environment that organisms inhabiting surface sediments encounter during oxygen depletion or deficiency. It also introduces important metabolic processes that govern or are governed by different redox settings. In Sect. 2, biogeochemical processes in sediments underlying fully oxygenated water bodies are elucidated. Section 3 explains differences in biogeochemical reactions in hypoxic and anoxic environments as opposed to oxygenated environments. Modern oxygen minimum zones and permanent anoxic environments are introduced. In Sect. 4, biogeochemical processes during past anoxic events and during the era before the first rise of oxygen are reviewed.
2. Sediment Biogeochemistry in Modern Oxygenated Oceans In modern oxygenated oceans, a cascade of electron acceptors (oxygen, nitrate, manganese, iron, and sulfate – in this order) is utilized by prokaryotes to degrade organic matter on the ocean floor (Canfield et al. 2005; Jørgensen 2000 ) (Fig. 1 ). Of this repertoire, oxygen, and in some rare cases nitrate (Risgaard-Petersen et al. 2006) are so far the only known electron acceptors used by eukaryotes. The order of electron acceptor utilization, and hence the order of depletion in the sediment, is governed by the Gibbs free energy gain of the reaction (Fig. 2 ): the higher the energy gain (kJ mol−1), the more successful the consumer is in competition with other organisms for food, i.e., the more attractive the electron acceptor is (Canfield et al. 2005; Jørgensen 2000). Organic matter degradation with oxygen generates the highest energy output, making it the most attractive and first consumed electron acceptor. Furthermore, free oxygen radicals, such as the superoxide anion, hydrogen peroxide, and hydroxyl radicals, are very powerful tools for the breakdown of refractory organic compounds, 19
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TINA TREUDE
Figure 1. Biogeochemical processes in marine sediments. Organic matter, produced on the basis of photosynthesis, is degraded by microorganisms in a redox cascade of processes utilizing different electron acceptors (O2 respiration, NO3− reduction, Mn4+ reduction, Fe3+ reduction, SO42− reduction, CH4 formation). The reduced compounds (Mn2+, Fe2+, H2S, CH4) are reoxidized by abiotic and biotic (chemoautotrophic) processes (From Jørgensen (2000). With permission).
allowing aerobic organisms to use a wide spectrum of food sources (Hedges and Keil 1995; Canfie
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