Tropical Marine Cyanobacteria in Coastal Environments: Ecological Significance and Molecular Fingerprints


Timothy I. Eglinton, Marine Chemistry and Geochemistry Department, WHOI
John B. Waterbury, Biology Department, WHOI

Cyanobacteria are thought to have been the first oxygen producing organisms in the history of life on Earth, and they continue to play an important role in the modern oceans as the dominant phytoplankton throughout most of the ocean. Their role in nitrogen fixation makes them especially important in the global carbon and nitrogen cycles, particularly in tropical regions of the ocean. As such, there is great interest in understanding how cyanobacterial productivity and nitrogen fixation influences and is influenced by ocean chemistry and climate. However, compared with processes in the open ocean, the biogeochemical and ecological role of coastal cyanobacteria in the tropics is poorly understood.

We aim to utilize a novel suite of molecular lipid “biomarkers” to evaluate the ecological and biogeochemical significance of cyanobacteria in coastal tropical waters, and to use this information to establish their use as fingerprints of ancient cyanobacterial activity preserved in marine sediments. We are particularly interested in developing a molecular proxy for marine cyanobacteria that could ultimately be used to explore links between past changes in ocean chemistry and climate, the ecology of nitrogen fixing cyanobacteria and the oceanic N budget. In addition to studying relatively modern processes, coastal matforming benthic cyanobacteria represent analogues for some of the earliest forms of life on Earth. By studying the dynamics of these organisms and their associated patterns of lipid biosynthesis, we can begin to unravel the significance of cyanobacterial lipids preserved in ancient rocks dating back to 2.7 billion years before present. We propose to examine the bacteriohopanepolyols (BHPs), a class of lipids also  known as hopanoids, as candidate cyanobacterial biomarker lipids since they have been found in diazotrophic cyanobacteria including Trichodesmium and Crocosphaera, they are preserved in sediments, and variations in their structure could contain unique chemotaxonomic information.

In this proposal, we seek funds for a sampling expedition to the Liquid Jungle Laboratory, as well  as support for instrument time to analyze these and existing samples for hopanoid content. At present, we have acquired water column particulate matter, and cyanobacterial mat samples from tropical waters that cover a spectrum of coastal settings. Our approach will be two-fold: (i) to investigate the environmental distribution, and ecological diversity of hopanoids and their cyanobacterial precursors in different tropical coastal systems, and (ii) to explore environmental controls on hopanoid production by culturing cyanobacteria under different conditions in the laboratory. Using this approach, we seek to provide a robust foundation for the application of these novel proxies for cyanobacterial productivity, and improve our understanding on the ecological and biogeochemical controls of cyanobacterial lipid signatures in coastal tropical environments, particularly in regions where nitrogen-fixing cyanobacteria are prevalent.