Collaborative Research: Biology and phylogeny of marine planktonic cyanobacterial symbioses

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This is a study of symbioses between cyanobacteria and planktonic diatoms, dinoflagellates, radiolarians, silicoflagellates and other planktonic protozoans in the equatorial oligotrophic Atlantic and Pacific Oceans. The investigators have recently observed that these symbioses are diverse and abundant, and some have not yet been reported in the scientific literature. The biology and the phylogeny of these symbioses are virtually unstudied. The 16S rDNA sequences of cyanobacterial symbionts within a diatom (Climacodium) showed that they are closely related to the N2 fixing genus Cyanothece (which fixes during the night), which suggests that some of these symbioses may involve N2 fixation.Drs. Carpenter and Zehr hypothesize that cyanelle symbionts may benefit host species either via incorporation of fixed N or C (via DON or DOC release or by being phagocyticized). The research approach will use combined microscopy and molecular biology approaches to link observed relationships between cyanobacteria and the eukaryotic microalgae with phylogenetic information and detection of the genes involved in nitrogen fixation. Using samples collected directly by microscopy, and bulk filtered water samples, the identity, nitrogen fixation potential (presence of nitrogenase genes) and expression of nitrogen fixation genes will be related to specific organisms and quantified in the water column. These approaches will be based on amplification and sequencing of 16S rDNA from the symbionts to understand the phylogeny of the cyanobacteria, and detection of the presence of nifH as an indicator of capacity to fix N2, examination of the ultrastructure of host and cyanelle, use of 14 C autoradiography to examine C transfer, measurements of abundance and distribution to quantify ecological importance, along with culture attempts to allow laboratory studies directed at determining the nature of the symbiotic interactions using GC/MS. The research will take advantage of four already- funded (NSF) research cruises scheduled over the next three years in the Atlantic and Pacific Oceans.