Toxin production by Crocosphaera watsonii: structure and influence on nutrient cycling in the upper ocean

Cyanobacteria are abundant components of the biosphere and play a crucial role in carbon sequestration and oxygen supply to the atmosphere. In addition to their photosynthetic capabilities, some cyanobacteria contribute to global ocean productivity by fixing inert atmospheric nitrogen gas (N2) into a bioavailable form. Many of these nitrogen-fixing bacteria (diazotrophs) also produce toxic metabolites, whose physiological and ecological roles are poorly understood. Crocosphaera watsonii has recently been shown to possess non-ribosomal peptide synthetases and polyketide synthetases, which have been implicated in toxin production. Cell extracts of C. watsonii exhibited toxicity to human cell lines. We propose to purify and structurally characterize the toxin. In addition, experiments will be conducted to determine the conditions for optimal toxin production, shedding light on the significance of the toxin to the producing organism. As cyanobacterial toxins are capable of inhibiting the photosynthetic and nitrogen-fixing capabilities of other cyanobacteria, we will perform algicide assays to determine the potential effects of the toxin on the planktonic community. Nitrogen-fixing bacteria (diazotrophs) are thought to significantly contribute to global ocean productivity and carbon export to the deep sea, yet nitrogen-fixation rates and their control mechanisms are poorly understood. To improve our understanding of the controls on nitrogen fixation, we must understand the physiological role of toxins in diazotrophs. In addition, if we are to correctly gage the contribution of nitrogen-fixation to marine productivity, we must have a thorough understanding of the interactions between diazotrophs, competing photoautotrophs, and grazer populations in the open ocean.