Abstract
Elemental sulfur (S0) is produced and degraded by phylogenetically diverse groups of microorganisms. For Chlorobaculum tepidum, an anoxygenic phototroph, sulfide is oxidized to produce extracellular S0 globules, which can be further oxidized to sulfate. While some sulfur-oxidizing bacteria (e.g., Allochromatium vinosum) are also capable of growth on commercial S0 as an electron donor, C. tepidum is not. Even colloidal sulfur sols, which appear indistinguishable from biogenic globules, do not support the growth of C. tepidum. Here, we investigate the properties that make biogenic S0 globules distinct from abiotic forms of S0. We found that S0 globules produced by C. tepidum and abiotic S0 sols are quite similar in terms of mineralogy and material properties, but the two are distinguished primarily by the properties of their surfaces. C. tepidum’s globules are enveloped by a layer of organics (protein and polysaccharides), which results in a surface that is fundamentally different from that of abiotic S0 sols. The organic coating on the globules appears to slow the aging and crystallization of amorphous sulfur, perhaps providing an extended window of time for microbes in the environment to access the more labile forms of sulfur as needed. Overall, our results suggest that the surface of biogenic S0 globules may be key to cell–sulfur interactions and the reactivity of biogenic S0 in the environment.
Highlights
Globules of elemental sulfur (S0) are frequently produced as a required intermediate in the oxidation of sulfide to sulfate
We found that globules are soft and flexible, yet solid biominerals composed of nanocrystalline α-S8 and enveloped by a recalcitrant organic coating
Biogenic S0 globules produced by C. tepidum are at least in part composed of α-S8
Summary
Globules of elemental sulfur (S0) are frequently produced as a required intermediate in the oxidation of sulfide to sulfate. The formation and degradation of S0 as an intermediate is special among lithoautotrophs and is facilitated by the many oxidation states of sulfur. Surface Chemistry of Cba. tepidum S0 Globules biogenic S0 is mobile, can serve as reactive surface, and could, in principle, be used by organisms other than the producing species. Chemical speciation of sulfur in globules produced by several taxa has been debated, complicated by the fact that many sulfur intermediates are short lived, highly reactive, and difficult to quantify in living systems. We investigated the properties of the extracellular S0 globules of a model sulfur-oxidizing bacterium Chlorobaculum tepidum in an effort to understand how these properties facilitate globule formation and degradation, and the persistence and reactivity of biogenic S0 in the environment
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