The Chlorobiaceae, also known as green sulphur bacteria, are strictly anoxigenic photoautotrophic prokaryotes that inhabit the photic zone of euxinic basins usually at the interface between well-oxygenated surface layers and anoxic waters rich in H2S. Chlorobiaceans fix carbon through a reductive Krebs’ cycle and play a key role in marine ecosystems acting as an ecological barrier to the diffusion of H2S in surface layers, thereby allowing the presence of nektonic organisms. These bacteria are adapted to sustain photosynthesis at very low light intensities (Overmann et al. 1992) and produce characteristic pigments, including some aromatic carotenoids. Molecular fossils derived from such pigments can be used to document the distribution of these photosynthesizing prokaryotes in the fossil record, as well as to provide evidence of the occurrence of photic zone euxinia in ancient depositional environments (Summons & Powell 1986; Sinninghe Damsté et al. 1993; Koopmans et al. 1996). A vast array of diagenetic and catagenetic products derived from the pigments characteristic of the Chlorobiaceae have been documented in sediments ranging from Ordovician to Miocene (Koopmans et al. 1996). Examination by gas chromatography/mass spectrometry of Cenomanian-Turonian ichthyolitiferous black shales of Argille Varicolori of northeastern Sicily revealed the presence of isorenieratane and chlorobactane, two sedimentary derivatives of aromatic carotenoid pigments (isorenieratene and chlorobactene) specifically produced by chlorobiaceans. The presence of isorenieratane and chlorobactane is accompanied by a carbon isotopic anomaly (enrichment of 13C; Scopelliti et al. 2008), which may be attributable to the unusual carbon metabolism of these bacteria (Siveråg et al. 1977). Aromatic carotenoids, including isorenieratene, are generally labile compounds not very resistant to transport over long distances (Sissinghe Damsté & Koopmans 1997). Therefore, the presence of their derivatives in the sedimentary rocks must be considered as autochthonous. In summary, the analysis of molecular fossils in the black shales of northeastern Sicily has provided unambiguous evidence of the activity of microbial communities that recorded the development of euxinic conditions at shallow depths along the north African shelf at the Cenomanian-Turonian boundary (Sinninghe Damsté & Köster 1998).
Evidence of chlorobiacean activity in the Cenomanian-Turonian black shales of northeastern Sicily
CARNEVALE, Giorgio
2011-01-01
Abstract
The Chlorobiaceae, also known as green sulphur bacteria, are strictly anoxigenic photoautotrophic prokaryotes that inhabit the photic zone of euxinic basins usually at the interface between well-oxygenated surface layers and anoxic waters rich in H2S. Chlorobiaceans fix carbon through a reductive Krebs’ cycle and play a key role in marine ecosystems acting as an ecological barrier to the diffusion of H2S in surface layers, thereby allowing the presence of nektonic organisms. These bacteria are adapted to sustain photosynthesis at very low light intensities (Overmann et al. 1992) and produce characteristic pigments, including some aromatic carotenoids. Molecular fossils derived from such pigments can be used to document the distribution of these photosynthesizing prokaryotes in the fossil record, as well as to provide evidence of the occurrence of photic zone euxinia in ancient depositional environments (Summons & Powell 1986; Sinninghe Damsté et al. 1993; Koopmans et al. 1996). A vast array of diagenetic and catagenetic products derived from the pigments characteristic of the Chlorobiaceae have been documented in sediments ranging from Ordovician to Miocene (Koopmans et al. 1996). Examination by gas chromatography/mass spectrometry of Cenomanian-Turonian ichthyolitiferous black shales of Argille Varicolori of northeastern Sicily revealed the presence of isorenieratane and chlorobactane, two sedimentary derivatives of aromatic carotenoid pigments (isorenieratene and chlorobactene) specifically produced by chlorobiaceans. The presence of isorenieratane and chlorobactane is accompanied by a carbon isotopic anomaly (enrichment of 13C; Scopelliti et al. 2008), which may be attributable to the unusual carbon metabolism of these bacteria (Siveråg et al. 1977). Aromatic carotenoids, including isorenieratene, are generally labile compounds not very resistant to transport over long distances (Sissinghe Damsté & Koopmans 1997). Therefore, the presence of their derivatives in the sedimentary rocks must be considered as autochthonous. In summary, the analysis of molecular fossils in the black shales of northeastern Sicily has provided unambiguous evidence of the activity of microbial communities that recorded the development of euxinic conditions at shallow depths along the north African shelf at the Cenomanian-Turonian boundary (Sinninghe Damsté & Köster 1998).
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