| Cyanobacteria vary in their degree of exposure to environmental sulfide, a toxin which kills these organisms by poisoning photosynthesis. Many species never experience this electron transport chain inhibitor in nature, but others do as a result of its presence in habitat source water or its production by the metabolism of other microbial community members. While oxygenic photosynthesis is irreversibly inhibited at low sulfide concentrations in most cyanobacteria, species which are frequently exposed to environmental sulfide have evolved different mechanisms for coping with this toxin. These include enhanced resistance of oxygenic photosynthesis and utilization of sulfide as an electron donor in photosystem I dependent, anoxygenic photosynthesis. By comparing physiological differences in sulfide tolerance across species within the context of their evolutionary histories, we can try to determine how frequently different mechanisms of tolerance have evolved and which tolerance mechanism evolved first. By analyzing these physiological data together with environmental sulfide data collected from our research sites using microelectrodes, we can also test whether the breadth of an organism's sulfide tolerance can be predicted from the magnitude of variation in environmental sulfide concentration it has experienced in its recent evolutionary past and whether greater average sulfide concentration and/or temporal variability in sulfide favors the evolution of a particular mechanism of sulfide tolerance. |
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