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Biogeosciences An interactive open-access journal of the European Geosciences Union
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https://doi.org/10.5194/bg-2019-181
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/bg-2019-181
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: technical note 12 Jun 2019

Submitted as: technical note | 12 Jun 2019

Review status
This discussion paper is a preprint. A revision of the manuscript is under review for the journal Biogeosciences (BG).

The silicon isotopic composition of choanoflagellates: implications for a mechanistic understanding of isotopic fractionation during biosilicification

Alan Marron1, Lucie Cassarino2, Jade Hatton2, Paul Curnow3, and Katharine R. Hendry2 Alan Marron et al.
  • 1Department of Applied Mathematics and Theoretical Physics, Centre for Mathematical Sciences,University of Cambridge,Wilberforce Road, Cambridge CB3 0WA, UK
  • 2University of Bristol, School of Earth Sciences, Wills Memorial Building, Queen’s Road, Bristol, BS8 1RJ, UK
  • 3School of Biochemistry, University of Bristol, Biomedical Sciences Building, University Walk, Bristol BS8 1TD, UK

Abstract. The marine silicon cycle is intrinsically linked with carbon cycling in the oceans via biological production of silica by a wide range of organisms. The stable silicon isotopic composition (denoted by δ30Si) of siliceous microfossils extracted from sediment cores can be used as an archive of past oceanic silicon cycling. However, the silicon isotopic composition of biogenic silica has only been measured in diatoms, sponges and radiolarians, and isotopic fractionation relative to seawater is entirely unknown for many other silicifiers. Furthermore, the biochemical pathways and mechanisms that determine isotopic fractionation during biosilicification remain poorly understood. Here, we present the first measurements of the silicon isotopic fractionation during biosilicification by loricate choanoflagellates, a group of protists closely related to animals. We cultured two species of choanoflagellates, Diaphanoeca grandis and Stephanoeca diplocostata, which showed consistently greater isotopic fractionation (approximately −5 to −7 ‰) than cultured diatoms (−0.5 to −2 ‰). Instead, choanoflagellate silicon isotopic fractionation appears to be more similar to sponges grown under similar DSi concentrations. Our results highlight that there is a taxonomic component to silicon isotope fractionation during biosilicification, possibly via a shared or related biochemical transport pathway. These findings have implications for the use of biogenic silica δ30Si produced by different silicifiers as proxies for past oceanic change.

Alan Marron et al.
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Status: final response (author comments only)
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Alan Marron et al.
Alan Marron et al.
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Isotopic signatures of silica fossils can be used as archives of past oceanic silicon cycling, which is linked to marine carbon uptake. However, the biochemistry that lies behind such chemical fingerprints remains poorly understood. We present the first measurements of silicon isotopes in a group of protists closely related to animals, choanoflagellates. Our results highlight a taxonomic basis to silica isotope signatures, possibly via a shared transport pathway in choanoflagellates and animals.
Isotopic signatures of silica fossils can be used as archives of past oceanic silicon cycling,...
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