Sulfur Cycle in Microorganisms

Sulfur cycle in microorganisms

Sulfur is a key constituent of certain amino acids, proteins, and other biochemicals of both eukaryotes and prokaryotes. For example, sulfur is a component of an enzyme called coenzyme A, which is vital for respiration of plant and animal cells.

Plants are not able to directly use elemental sulfur. Instead, they rely on the ability of certain types of bacteria to convert elemental sulfur to another form. Bacteria that are known as chemoautotrophic bacteria can combine sulfur with water and oxygen to produce hydrogen sulfate. Plants are able to incorporate the sulfate compound into proteins.

Bacteria can participate in the reduction of sulfur, in which the sulfur compounds act as an electron receptor, or in the oxidation of sulfur, in which an electron is removed from the sulfur compound.

Hydrogen sulfide, a gas that has the characteristic smell of rotten eggs, is toxic to air-requiring plant and animal tissue. However, the gas can be utilized by oxygen-requiring bacteria such as Thiothrix and Beggiatoa, and by the anaerobic purple sulfur bacteria. These bacteria utilize the hydrogen sulfide and carbon dioxide to produce elemental sulfur.

Sulfur can occur in many chemically reduced mineral forms, or sulfides, in association with many metals. The most common metal sulfides in the environment are iron sulfides (called pyrites when they occur as cubic crystals), but all heavy metals can occur in this mineral form. Whenever metal sulfides are exposed to an oxygen-rich environment, certain bacteria begin to oxidize the sulfide, generating sulfate as a product, and tapping energy from the process that is used to sustain their own growth and reproduction. This autotrophic process is called chemosynthesis, and the bacteria involved are named Thiobacillus thiooxidans. When a large quantity of sulfide is oxidized in this way, an enormous amount of acidity is associated with the sulfate product. Indeed, Thiobacillus prosperus has an optimum pH of between pH=1 and pH=4, and Thiobacillus ferroxidans has an optimum pH range of between pH=2 and pH=4.

Some species of the genus Thiobacillus, including Thiobacillus thiooxidans and Thiobacillus ferroxidans are able to process elemental sulfur and iron sulfate, respectively.

Within the past several decades, the existence of bacteria that utilize sulfur at hydrothermal vents deep within the ocean has been chronicled. These bacteria form the basis of the entire complex ecosystem that springs up, in the total absence of light, around the sulfur-rich emission form the vents. Some of the bacteria live in symbiosis with the socalled tubeworms that thrive in this ecosystem. The worms provide protection and an incoming supply of nutrients to the bacteria. In turn, the bacteria metabolize the sulfur to forms usable to the worms. The discovery of the bacterial basis of this undersea ecosystem greatly increased human awareness of the microbial diversity on Earth.

See also Biogeochemical cycles; Economic uses and benefits of microorganisms