Abstract
Hopanoids are bacterial membrane lipid biomarker molecules that feature prominently in the molecular fossil record. In the modern marine water column, recent reports implicate bacteria inhabiting low-oxygen environments as important sources of hopanoids to marine sediments. However, the preliminary biogeography reported by recent studies and the environmental conditions governing such distributions can only be confirmed when the numerical abundance of these organisms is known with more certainty. In this study, we employ two different approaches to examine the quantitative significance of phylogenetically distinct hopanoid producers in low-oxygen environments. First, we develop a novel quantitative PCR (qPCR) assay for the squalene hopene cyclase (sqhC) gene, targeting a subset of hopanoid producers previously identified to be important in the eastern North Pacific Ocean. The results represent the first quantitative gene abundance data of any kind for hopanoid producers in the marine water column and show that these putative alphaproteobacterial hopanoid producers are rare, comprising at most 0.2 % of the total bacterial community in our samples. Second, a complementary analysis of existing low-oxygen metagenomic datasets further examined the generality of the qPCR observation. We find that the dominant sqhC sequences in these metagenomic datasets are associated with phyla such as Nitrospinae rather than Proteobacteria, consistent with the qPCR finding that alphaproteobacterial hopanoid producers are not very abundant in low-oxygen environments. In fact, positive correlations between sqhC gene abundance and environmental parameters in these samples identify nitrite availability as a potentially important factor in the ecology of hopanoid producers that dominate low-oxygen environments.
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Further information on the Line P program can be found on the program website at: http://www.pac.dfo-mpo.gc.ca/science/oceans/data-donnees/line-p/index-eng.html
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Acknowledgments
We thank Dr. William Haskell (University of Southern California) and the Up.R.I.S.E.E. sampling program for assisting in the collection of the sample from the San Pedro Basin and Dr. Alex Sessions (Cal-Tech) for providing the sediment sample from the Santa Barbara Basin. We thank Dr. Brian Palenik and Dr. Eric Allen at Scripps Institution of Oceanography for the use of lab space and instrumentation and for many helpful discussions. This work was funded by the National Science Foundation Graduate Research Fellowship Program, and the University of California San Diego’s UC Ship Funds program supported the 2010 Cal-ECHOES cruise.
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Kharbush, J.J., Kejriwal, K. & Aluwihare, L.I. Distribution and Abundance of Hopanoid Producers in Low-Oxygen Environments of the Eastern Pacific Ocean. Microb Ecol 71, 401–408 (2016). https://doi.org/10.1007/s00248-015-0671-y
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DOI: https://doi.org/10.1007/s00248-015-0671-y