Sulfur is a fundamental element for life, serving as a component of proteins like the amino acids cysteine and methionine. The sulfur cycle is the continuous movement and transformation of this element through Earth’s atmosphere, hydrosphere, lithosphere, and biosphere. This biogeochemical cycle ensures that sulfur, which is finite and often locked in inaccessible forms, is constantly recycled into compounds that can be readily taken up by living organisms.
Forms of Sulfur and Global Storage
Sulfur exists in various chemical states, ranging from the most oxidized form, sulfate (\(\text{SO}_4^{2-}\)), to the most reduced form, sulfide (\(\text{S}^{2-}\)), and as organic sulfur. The vast majority of the world’s sulfur is held in the lithosphere, primarily locked within sedimentary rocks and minerals such as gypsum and pyrite. The ocean is the second largest reservoir, holding dissolved inorganic sulfate (\(\text{SO}_4^{2-}\)), which is highly stable in oxygenated seawater.
Smaller, biologically active reservoirs include the soil, where sulfur is present in inorganic and organic compounds derived from decaying matter. In living organisms, sulfur is incorporated into organic molecules, such as the thiol groups in proteins. The atmosphere holds the smallest quantity of sulfur, mainly in gaseous forms like sulfur dioxide (\(\text{SO}_2\)), hydrogen sulfide (\(\text{H}_2\text{S}\)), and dimethyl sulfide (DMS).
The Microbial Engine of Sulfur Transformation
The metabolic activities of bacteria and archaea drive the conversion of sulfur between its various oxidation states. These microbes use sulfur compounds as energy sources, electron acceptors, or building blocks, facilitating their movement through ecosystems. The process begins with assimilation, where plants and microorganisms absorb inorganic sulfate (\(\text{SO}_4^{2-}\)) from the environment. They reduce this sulfate to the sulfide form (\(\text{S}^{2-}\)) to incorporate it into organic molecules like amino acids.
When organisms die, the sulfur contained within their proteins is released back into the environment through decomposition and mineralization. Decomposing microbes convert this organic sulfur back into inorganic forms, notably hydrogen sulfide (\(\text{H}_2\text{S}\)) gas. This highly toxic gas is a critical intermediate in the cycle.
In environments lacking oxygen, such as deep marine sediments, dissimilatory sulfate-reducing bacteria (SRB) take over. These anaerobic microbes use sulfate as an electron acceptor instead of oxygen to break down organic matter, producing hydrogen sulfide (\(\text{H}_2\text{S}\)) as a metabolic waste product. This process is a dominant pathway for organic matter degradation in anoxic zones and is responsible for the characteristic rotten-egg smell.
The hydrogen sulfide produced by SRBs is then recycled by sulfur-oxidizing bacteria (SOB), which thrive at the boundary between oxygen-rich and oxygen-poor zones. These microbes use reduced sulfur compounds, including \(\text{H}_2\text{S}\) and elemental sulfur (\(\text{S}^0\)), as an energy source. They oxidize the sulfide, ultimately converting it back into sulfate (\(\text{SO}_4^{2-}\)), often passing through intermediate forms. This final step completes the local biological loop, making sulfate available for assimilation.
Movement Through Air and Rock
Large-scale transport of sulfur occurs through long-term geological and fast-acting atmospheric processes. Geological cycling is the slowest part, involving the release of sulfur from the Earth’s crust through weathering of minerals like pyrite and gypsum. This action releases sulfates into the soil and water, which eventually flow into the oceans.
Volcanic activity also contributes by injecting sulfur compounds, mainly sulfur dioxide (\(\text{SO}_2\)) and hydrogen sulfide (\(\text{H}_2\text{S}\)), directly into the atmosphere. Once airborne, these compounds participate in atmospheric deposition. Sulfur dioxide can react with water vapor and oxygen to form sulfuric acid (\(\text{H}_2\text{SO}_4\)), which returns to the surface through wet deposition (rain or snow).
Sulfur can also return through dry deposition, where particles or gases settle directly onto vegetation, soil, or water surfaces. A significant natural source of atmospheric sulfur comes from the oceans, where phytoplankton produce dimethylsulfoniopropionate (DMSP), converted by bacteria into the volatile gas dimethyl sulfide (DMS). DMS enters the atmosphere, is oxidized to sulfate aerosols, and returns to the surface via deposition.
How Human Activity Affects the Sulfur Cycle
Human actions have accelerated the natural sulfur cycle, creating an imbalance in global sulfur distribution. The primary disruption comes from the large-scale burning of fossil fuels, particularly coal and oil, which contain high concentrations of sulfur. Combustion releases sulfur that was previously locked in the lithosphere for millions of years, primarily as sulfur dioxide (\(\text{SO}_2\)) gas.
The industrial release of \(\text{SO}_2\) increases the concentration of sulfur compounds in the atmosphere, often overwhelming the natural recycling capacity of ecosystems. This elevated atmospheric sulfur leads to acid deposition, commonly known as acid rain, as sulfur dioxide reacts to form sulfuric acid. This acidic precipitation lowers the \(\text{pH}\) of lakes and streams, harming aquatic life and altering soil chemistry.