Abstract
Little Salt Spring (Sarasota County, FL, USA) is a sinkhole with groundwater vents at ~77 m depth. The entire water column experiences sulfidic (~50 μM) conditions seasonally, resulting in a system poised between oxic and sulfidic conditions. Red pinnacle mats occupy the sediment–water interface in the sunlit upper basin of the sinkhole, and yielded 16S rRNA gene clones affiliated with Cyanobacteria, Chlorobi, and sulfate-reducing clades of Deltaproteobacteria. Nine bacteriochlorophyll e homologues and isorenieratene indicate contributions from Chlorobi, and abundant chlorophyll a and pheophytin a are consistent with the presence of Cyanobacteria. The red pinnacle mat contains hopanoids, including 2-methyl structures that have been interpreted as biomarkers for Cyanobacteria. A single sequence of hpnP, the gene required for methylation of hopanoids at the C-2 position, was recovered in both DNA and cDNA libraries from the red pinnacle mat. The hpnP sequence was most closely related to cyanobacterial hpnP sequences, implying that Cyanobacteria are a source of 2-methyl hopanoids present in the mat. The mats are capable of light-dependent primary productivity as evidenced by 13C-bicarbonate photoassimilation. We also observed 13C-bicarbonate photoassimilation in the presence of DCMU, an inhibitor of electron transfer to Photosystem II. Our results indicate that the mats carry out light-driven primary production in the absence of oxygen production—a mechanism that may have delayed the oxygenation of the Earth's oceans and atmosphere during the Proterozoic Eon. Furthermore, our observations of the production of 2-methyl hopanoids by Cyanobacteria under conditions of low oxygen and low light are consistent with the recovery of these structures from ancient black shales as well as their paucity in modern marine environments.
Original language | English (US) |
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Pages (from-to) | 784-797 |
Number of pages | 14 |
Journal | Geobiology |
Volume | 15 |
Issue number | 6 |
DOIs | |
State | Published - Nov 2017 |
Bibliographical note
Funding Information:National Science Foundation, Grant/ Award Number: NSF EAR-0525503; NASA Astrobiology Institute, Grant/Award Number: NNA04CC06A and NNA13AA90A
Funding Information:
Sampling at Little Salt Spring was carried out in cooperation with J. Gifford (U. Miami/RSMAS) and divers S. Koski (U. Miami/RSMAS), R. Riera-Gomez (U. Miami/RSMAS), and C. Coy (Florida Aquarium). We thank S. Koski for help with field operations, and K. Dawson, D. Jones, and D. Tobler for geochemical analyses. We are grateful to D. Walizer for invaluable technical assistance and S. Lincoln for insightful discussions and detailed assistance with the hopanoid data. We are grateful to S. Kopf and an anonymous reviewer for their constructive suggestions. This project was funded by the National Science Foundation (NSF EAR-0525503 to J.L.M.), the NASA Astrobiology Institute (PSARC, NNA04CC06A to J.L.M. and K.H.F.), and the PSU Science Diving Program. Work at MIT was supported by the NASA Astrobiology Institute (NNA13AA90A) Foundations of Complex Life, Evolution, Preservation, and Detection on Earth and Beyond to R.E.S. T.L.H. graciously acknowledges support from the NASA Astrobiology Institute Postdoctoral Program and the University of Cincinnati.
Publisher Copyright:
© 2017 The Authors. Geobiology Published by John Wiley & Sons Ltd.
Keywords
- Chlorobi
- Cyanobacteria
- Proterozoic
- anoxygenic
- biomarkers
- hopanoid
- hpnP
- oxygen
- oxygenic
- photosynthesis
- pinnacle
- sinkhole
- sulfide