M 31: Sulfur Cycling and Microbial Diversity

Distribution and Diversity of Sulfur Metabolism

• Phylogenetic Distribution: Sulfur cycling is widely distributed across the phylogenetic tree, appearing in both Domain Archaea and Domain Bacteria.

• Key Lineages Involved: Specific groups identified as performing sulfur oxidation or reduction include:     * Bacteria: Proteobacteria (including sulfur oxidizers and reducers), Nitrospira, Firmicutes, Deinococcus, Thermus, and Thermodesulfobacteria.     * Archaea: Various groups performing both oxidation and reduction.

• Nomenclature Patterns: Many organisms involved in this cycle are named for their metabolism, such as Thermodesulfobacteria and various "Desulfo-" prefixed genera.

• Drivers of Metabolic Diversity: Diversity within these groups is achieved through three primary mechanisms:     1. Gene Loss: An ancestral organism may lose specific metabolic capabilities. For instance, an ancestor capable of anoxygenic phototrophy might lose the genes for photosystem I or II, resulting in a divergent metabolic path.     2. Convergent Evolution: Distinct lineages evolve similar traits independently. This is analogous to bats (mammals) and birds (descendants of reptiles) both evolving wings. The traits perform the same function but are not encoded by homologous genes.     3. Horizontal Gene Transfer (HGT): The acquisition of genes from non-ancestral sources, which can occur between distantly related or closely related lineages. These traits are not passed down through vertical inheritance.

Principles of Sulfur Redox Reactions and Energetics

• Redox Pairs: Sulfur metabolisms involve sulfur compounds acting as either electron donors or electron acceptors.

• Oxidation (Electron Donors): Reduced sulfur compounds serve as donors. The full oxidation of hydrogen sulfide ($H_2S$) to sulfate ($SO_4^{2-}$) involves an 8-electron transfer. Some organisms perform partial oxidation to elemental sulfur ($S^0$).

• Reduction (Electron Acceptors): Oxidized sulfur compounds serve as terminal electron acceptors in anaerobic respiration.

• Metabolic Classification:     * Chemolithotrophs: Obtain energy from inorganic sulfur compounds.     * Chemoorganotrophs: Obtain electrons from organic compounds while using sulfur as an acceptor.

• Energetic Efficiency: Anaerobic respiration using sulfur is less energetically favorable than aerobic respiration ($O_2/H_2O$ pair). On the redox tower, oxygen represents the highest reduction potential.

• Alternative Acceptors: Facultative anaerobes may switch between oxygen, nitrate ($NO_3^-$), iron, manganese, or sulfate depending on environmental availability. Sulfate is less favorable than oxygen or nitrate.

• Sulfidic Environments: Environments rich in sulfide are termed "sulfidic." Examples include:     * Natural: Salt marshes (characterized by a rotten-egg smell from $H_2S$).     * Built: Sewers and wastewater treatment plants.     * Marine: Hydrothermal vents and sulfide-rich muds.

Assimilative vs. Dissimulative Sulfur Metabolism

• Assimilative Metabolism:     * Purpose: Incorporation of inorganic nutrients into cellular biomass (e.g., for biosynthesis of sulfur-containing amino acids and proteins).     * Energetics: This process consumes energy in the form of $ATP$ and reducing power.     * Outcome: The sulfur becomes a structural part of the cell.

• Dissimulative Metabolism:     * Purpose: Energy conservation through electron transport systems.     * Mechanism: Electron acceptors are reduced and the resulting products are excreted from the cell as waste.     * Outcome: The products do not become part of the cell’s biomass.

Specialized Sulfur-Cycling Bacteria and Compounds

• Sulfur Compounds in the Cycle:     * Sulfide ($S^{2-}$ or $H_2S$): The most reduced form; can only act as a donor.     * Elemental Sulfur ($S^0$): An intermediate redox state.     * Sulfate ($SO_4^{2-}$): The most oxidized state ($+6$).     * Dimethyl Sulfide (DMS) and Dimethyl Sulfoxide (DMSO): Abundant sulfur compounds in marine environments, often originating from algae where they act as osmolytes.

• Sulfur-Oxidizing Bacteria (SOB):     * Beggiatoa: A filamentous sulfur oxidizer often studied in early microbiology by Winogradsky.     * Thiovulum: Found in freshwater and marine habitats where sulfide-rich mud meets oxygenated water. They create a ‘slimy veil’ and use rotary motion to position themselves within chemical gradients.     * Thiomargarita: Notable for being the largest known bacteria (some visible to the naked eye). They are non-motile and contain a large central vacuole filled with nitrate ($NO_3^-$). They store elemental sulfur in intracellular granules and can aerobically oxidize it when external $H_2S$ is unavailable.     * Mixotrophy: Some bacteria, like Acidithiobacillus, can grow mixotrophically, using a combination of organic carbon and inorganic energy sources.     * Carboxysomes: Protein-enclosed structures within some chemolithotrophic sulfur oxidizers where the Calvin Cycle occurs for autotrophic carbon fixation.

• Sulfate-Reducing Bacteria (SRB):     * Common genera often begin with the prefix "Desulfo-".     * Carbon Sources: They can utilize a wide range of donors, including fermentation products (lactate, pyruvate, ethanol, acetate, propionate, butyrate) and hydrocarbons (gasoline, which can lead to pipe corrosion).

Hydrothermal Vent Ecosystems and Chemosynthesis

• Environmental Conditions: Found at geothermal sites (e.g., East Pacific Rise). Temperatures can reach $350\,^\circ C$ at the vent source (3.5 times the boiling point of water), while surrounding seafloor water is approximately $4\,^\circ C$.

• Chemosynthetic Foundation: All life at vents relies on microbes performing chemosynthesis (using sulfide, hydrogen, manganese, or iron as energy sources) in the absence of sunlight.

• Symbiotic Relationships:     * Riftia (Giant Tube Worms): Fast-growing invertebrates that lack a mouth, gut, or anus. They possess a trophosome, an organ filled with symbiotic bacteria. The bacteria use sulfide and $CO_2$ (transported by the worm's red gill plumes) to support the worm via the RTCA cycle.     * Alvinella (Pompeii Worms): Small worms covered in symbiotic organisms, adapted to extremely high temperatures (up to $80\,^\circ C$).

• Exploration History: Proposed in 1976/1977. Using the Alvin submersible, researchers discovered diverse life where they expected only dead shells, fundamentally changing the understanding of life's requirements (beyond sunlight).

Microbiological Laboratory Diagnostics and the Sulfur Cycle

• Triple Sugar Iron (TSI) Agar:     * Acid Production: Color change indicates fermentation and acid production.     * Salmonella: Produces acid and hydrogen sulfide ($H_2S$).     * E. coli: Produces acid and gas throughout the tube.     * Pseudomonas: Does not produce acid.

• Hektoen Enteric (HE) Agar:     * Type: Selective and differential medium.     * Selection: Favors gram-negative organisms (enteric bacteria).     * Differentiation: Based on sugar fermentation and sulfur reduction.     * Salmonella: Forms black colonies due to the reduction of thiosulfate to sulfide, which reacts with ferric ammonium citrate to form black iron sulfide.     * Shigella: Forms green colonies (no pigment) as it does not ferment the sugars or reduce sulfur.     * Clinical Use: Used to differentiate Salmonella and Shigella in fecal stool cultures for patients with persistent diarrhea.

• Enteric Bacteria Characteristics:     * Genera include Enterobacter, Salmonella, Shigella, and Proteus.     * Proteus is noted for its distinct "rotten fish" smell and swarming motility on agar plates.     * Infectivity dose: Salmonella requires a higher dose than Shigella; Shigella can cause illness with very few cells present.