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The Global Biogeochemical Silicon Cycle

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The Global Biogeochemical Silicon Cycle Silicon (2009) 1:207–213 DOI 10.1007/s12633-010-9035-x ORIGINAL PAPER The Global Biogeochemical Silicon Cycle Eric Struyf & Adriaan Smis & Stefan Van Damme & Patrick Meire & Daniel J. Conley Received: 29 September 2009 /Accepted: 23 November 2009 /Published online: 24 February 2010 # Springer Science+Business Media B.V. 2010 Abstract Silicon is one of the most important elements in structure of silicate minerals. In the environment dissolved the current age of the anthropocene. It has numerous silicate (DSi), i.e. ortho-silicic acid (H4SiO4), is transported industrial applications, and supports a high-tech multi- through soil and exported to rivers and eventually the ocean billion Euro industry. Silicon has a fascinating biological [2] (Fig. 1). The silicate weathering process consumes CO2. and geological cycle, interacting with other globally For example, in the weathering of anorthite (over kaolinite) important biogeochemical cycles. In this review, we bring to gibbsite, DSi is produced and CO2 is consumed [3]: together both biological and geological aspects of the 2 CaAl2Si2O8 2:CO2 8:H2O Ca þ 2:Al OH 3 2:H4SiO4 2:HCO3À silicon cycle to provide a general, comprehensive review þ 2þ ! þ ðÞþ þ Ca þ 2HCOÀ CaCO3 H2CO3 of the cycling of silicon in the environment. We hope this þ 3 ! þ review will provide inspiration for researchers to study this The weathering of silicates is thus an important sink for fascinating element, as well as providing a background atmospheric CO2 on geological timescales (thousands to environmental context to those interested in silicon. millions of years) and exerts a large influence on the global climate [4]. Currently, global CO consumption by . 2 Keywords Silicon biogeochemistry Geology Biology chemical weathering is estimated to be ca. 0.26 Gt C.yr−1 Carbon . Nitrogen . Phosphorus [5]. Ultimately, CO2 is recycled back to the atmosphere by tectonic processes (metamorphic decarbonation and volcanic outgassing). The concentration of CO2 influences the rate 1 An Important Biogeochemical Cycle of silicate weathering; the weathering rates of silicates have changed significantly during the development of the The Earth’s crust consists primarily of silicates (Si oxides, biosphere [6]. 90% of all minerals); consequently silicon (hereafter referred DSi plays an important role in the production of phyto- to as Si) is the second-most abundant element in the earth’s plankton and the burial of carbon in the coastal zone and in crust (28.8%) after oxygen [1]. During silicate weathering deep-sea sediments [7]. Terrestrial ecosystems buffer the dissolved soil CO2 is used in a reaction where ortho-silicic weathering flux of silicon through the terrestrial biogeo- acid (H4SiO4) is dissolved and released from the crystalline chemical cycle through uptake, storage and recycling [8]. The biological control on DSi exports from watersheds has E. Struyf (*) : A. Smis : S. Van Damme : P. Meire only recently started to achieve scientific attention [9, 10]. Department of Biology, Ecosystem Management Research Group, University of Antwerp, Universiteitsplein 1C, 2610 Wilrijk, Antwerp, Belgium 2 Silicon Cycling in the Ocean and Coastal Ecosystems e-mail: [email protected] 2.1 The Biological Si-pump D. J. Conley GeoBiosphere Science Centre, Lund University, Sölvegatan 12, Silicon is an essential nutrient for the growth of diatoms 22362 Lund, Sweden (Bacillariophyceae). Diatoms take up DSi and use it to build 208 Silicon (2009) 1:207–213 Fig. 1 The global biogeochem- eolian transport ical cycle of Si. The two left ECOSYSTEM SOIL boxes represent the main conti- 0.5 Tmole yr-1 nental Si pools, the two right plant uptake OCEAN SURFACE boxes the oceanic Si pools. Rectangular black boxes repre- soil DSi 60-200 plant ASi Tmole yr-1 Net riverine sent Si fluxes between the -1 diss. diatom primary Si pools. Circular black transport DSi 240 Tmole yr-1 ASi boxes represent Si fluxes within soil ASi litterfall, … 6Tmoleyr-1 (*) the primary Si pools. (“diss.” weathering stands for the dissolution) (*) The 6 Tmole.yr−1 flux is parti- Hydro- 19-46 Tmole yr EARTH CRUST & SUBSOIL ASi burial tioned between the net riverine thermal input 6.5 Tmole yr-1 transport (excluding retained -primaryminerals ASi in estuaries) and the flux & -secondaryminerals resulting from hydrothermal ac- Seafloor OCEAN FLOOR tivity and seafloor weathering weathering - d iatom ASi in sea- floor sediment plate tectonics their siliceous cell wall or “frustule”. Consequently, transport ecosystems [17, 18]. In most major rivers worldwide, of continental DSi to the oceans is an important component concentrations of N and P have at least doubled as the in oceanic primary production, a large part of which consists result of anthropogenic inputs [19]. Whereas total algal of diatoms [11]. Forty percent of all oceanic C sequestration growth is primarily regulated by the availability of N and P, (∼1.5–2.8 Gton C yr−1) can be attributed to the growth and the relative availability of Si and the availability of Si sedimentation of diatoms [12, 13]. Although primary relative to N and P, e.g. the Si:N and Si:P ratios, can production through different groups of marine phytoplankton influence the composition of the phytoplankton community also results in a net CO2 flux towards the sea bottom (the [18]. The lack of Si can change aquatic ecosystems from “biological carbon pump”)[14], a crucial difference exists those dominated by diatoms to non-diatom based aquatic between diatoms and coccolithophores, an important sub- ecosystems usually dominated by flagellates [20]. Based on group of non-siliceous phytoplankton. Coccolithophores are an evaluation of long-term algal blooms and nutrient characterized by calcite shells (=coccoliths); CO2 is pro- conditions in different regions, it can be concluded that duced when calcium reacts with hydrogen carbonate during decreased Si:N and Si:P ratios can give rise to Si limitation calcite formation (the “carbonate counter pump”)[15]. of diatoms and the reduction of diatoms in the phytoplank- Therefore, an increased dominance of coccolithophores ton community. In addition, subsequent non-diatom blooms decreases the net sequestration of CO2 and consequently can contain harmful algal species such as Phaeocystis sp., the flux of CO2 from the atmosphere towards the ocean floor Gonyaulax sp., Chrysochromulina sp. [21]. [11]. The biological carbon pump in the ocean is often Diatoms are the primary energetic source for estuarine referred to as the “biological Si pump” [7]. and coastal food chains [22]. Transfer of energy to higher Changes in Si inputs to marine ecosystems, especially in trophic levels is enhanced by diatoms through their higher the coastal ocean, can significantly influence the species nutritional value [23] and the limited amount of trophic composition of oceanic primary producers, especially the steps between diatoms and higher trophic levels [24]. Non- balance of production between diatoms and non-siliceous diatom species are known to be less available to higher phytoplankton [16]. It has been hypothesized that a higher trophic levels [21, 25] and some non-diatom based food contribution of diatoms to total oceanic phytoplankton webs are economically undesirable [20]. Therefore, the biomass occurred during the Last Glacial Maximum (79% proportion of diatoms in the phytoplankton community is vs. 54% today) as the result of increased eolian inputs of Si of primary importance for many fisheries globally [20]. [11]. This demonstrates that a link exists between Si Furthermore, DSi limitation of diatoms and resultant transport from terrestrial to oceanic systems, atmospheric blooms dominated by non-diatom species can result in CO2 concentrations and variations in global climate. anoxic conditions, increased water turbidity and excessive production of toxic components [26]. 2.2 Silicon and Eutrophication of Coastal and Lake Increases in diatom biomass as a result of higher N and P Ecosystems inputs results in increased diatom sinking rates and increased diatom burial in bottom sediments [27]. Conse- Silicon plays an important role in the current eutrophication quently, in anthropogenically eutrophied systems that have problems of numerous lacustrine, estuarine and coastal experienced increases in N and P loading from human Silicon (2009) 1:207–213 209 activities with sufficiently long hydrodynamic residence organic compounds, carbonates, Al-hydroxides and Fe- times, the aquatic DSi stock decreases and eutrophication oxides has been observed, although these physico-chemical problems are worsened [18]. A similar effect has been controls on DSi in soil water are poorly quantified [30–32]. described for dams, e.g. the artificial lake effect [28]. Dams increase the residence time of water in river ecosystems, 3.1 The Mineralogical Soil Pool of Ecosystem Silicon which stimulates phytoplankton productivity [29]. This results in the increased trapping of biogenic Si in lake The mineralogical pool of Si can be subdivided into sediments, and decreased transport of DSi downstream. primary minerals, secondary crystalline minerals (mainly This effect has been described for major dams worldwide, clay minerals) and secondary poorly to non-crystalline and is an important component of changed N:P:Si ratio’s in (amorphous) phases [33, 34]. The first group can be coastal ecosystems. indicated as parent material, the other two groups find their origin during processes of ecosystem soil formation. Aluminium hydroxides, iron oxides and carbonates play a 3 The Uptake
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