46838-AC2
Developing a Molecular Proxy for Marine Cyanobacteria
Roger Summons, Massachusetts Institute of Technology
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We proposed to investigate the source and biological function of hopanoid lipids in the modern oceans. By elucidating the relationship between the structure of geolipids and the taxonomic affinities of their biological precursors, we can begin to reconstruct the microbial ecology of past environments, and in turn, aspects of biochemical processes influential in driving ancient geochemical cycles. The diagenetic products of the biological bacteriohopanepolyols (BHPs), hopanes, have been found in ancient sediments and oils, and their structural diversity and abundance promise to inform us of biogeochemical processes that prevailed at times in the geologic past. 2-Methylhopanoids may provide the earliest evidence for the existence of cyanobacteria during the Archaean and the presence of diazotrophic cyanobacteria during Mesozoic oceanic anoxic events. Hopanes are also important in the characterization of petroleum and often used for oil-source correlation.
We presently lack a clear understanding of the sources and the environmental controls on the production of hopanoids in the modern oceans, and this has prevented our taking full advantage of the information contained in the geologic record of hopanoids. Given the ubiquity and abundance of cyanobacteria in the oceans, a cyanobacterial source would provide the most parsimonious explanation for 2-methylhopanoids in marine sediments now and in the past. But we cannot rule out a non-cyanobacterial source, and have not demonstrated that marine cyanobacteria have the capacity for 2-methylhopanoid biosynthesis. Before this study, it wasn’t even known whether 2-methylhopanoids were present in the upper ocean. Over the past year we have made significant progress towards answering these questions, as well as broader questions about the biological role of BHPs in cyanobacteria.
To date, we have conducted a survey of BHPs in samples from a range of environments including bacterial mat, marine sediment and soils from the Bahamas for which we have a manuscript in preparation. We have analysed stromatolites from Shark Bay, Australia, and water column particulate organic matter and sediments from the Tropical N. Atlantic. Of particular interest is our detection of BHPs in filtered cells from the upper water column of the Tropical N. Atlantic off the coast of N.W. Africa. 2-methylbacteriohopanetetrol is present in the photic zone and in underlying sediments, at a site where unicellular nitrogen fixing cyanobacteria have been demonstrated to occur. We detected several BHPs known to occur in cultured marine nitrogen fixing cyanobacteria in samples from the upper water column. Significantly, the structural diversity of BHPs that we observe is very low compared with soils, and lacustrine sediments, suggesting that the diversity of hopanoid producing bacteria is also very low, and possibly dominated by cyanobacteria. All of the BHP structures that we observe in the upper marine water column are present in the core top sediment sample. It is, therefore, likely that cyanobacterial BHPs are readily exported to sediments and thus preserved in the geologic record. We are continuing to analyze samples from four additional sites from a latitudinal transect. In addition we are undertaking a novel approach to determining the biological source of BHPs in situ, by analyzing the BHP content of cyanobacterial cells isolated directly from upper water column by flow cytometric sorting.
Working with cultures available in John Waterbury’s lab at WHOI, we have also screened over 30 strains of marine cyanobacteria. To date, we have only identified BHPs in six strains, all of which are capable of nitrogen fixation. However, initial growth experiments with Crocosphaera WH8501 demonstrate that BHP production decreases by an order of magnitude during nitrogen limiting conditions when nitrogen fixation is occurring. We had hypothesized that hopanoids serve as a permeability barrier to oxygen to prevent inhibition of nitrogen fixation but this seems not to be the case. We are currently working to identify other environmental stresses that might control the cellular abundance and structural diversity of BHPs. What conditions are conducive to the production of 2-methylhopnoids and what implications does this have for conditions that prevailed when 2-methylhopanes are prevalent in the geologic record?
Over the past year we have made significant progress towards our goal of understanding the distribution and source of BHPs in the modern ocean, and the environmental conditions that affect the production of hopanoids by marine cyanobacteria. In the coming year, we expect to have gained additional insights to the problem and have at least one paper in the published literature.
