Biogeochemical Cycles in Microbial Ecology

Sulfur Cycle

The sulfur cycle describes the movement and transformation of sulfur through various reservoirs in the environment, driven by both biological and chemical processes. Sulfur exists in multiple oxidation states and forms, influencing its role in ecosystems.

• Oxidation States and Chemical Forms of Sulfur:

  • Sulfide, sulfhydryl (H2S, cysteine, methionine): Oxidation state -2

  • Elemental sulfur (S0): Oxidation state 0

  • Sulfate (SO42-): Oxidation state +6

• Reservoirs and Physical/Chemical Forms:

  • Lithic (rock) sulfur: Mainly as pyrite (FeS2) or gypsum (CaSO4), abundant in rocks and sediments.

  • Oceanic sulfate: Large active reservoir, key in the global sulfur cycle.

  • Biological sulfur: Important for redox reactions and protein function.

  • Atmospheric sulfur: Produced by fossil fuel combustion and volcanic activity, contributes to acid rain.

• Sulfur Geochemistry: Sulfur compounds vary in solubility and oxidation state.

  • Sulfate (SO42-): Soluble anion, oxidation state +6

  • Sulfide (H2S): Soluble anion, oxidation state -2

  • Sulfur dioxide (SO2): Gas, oxidation state +4

Iron Cycle

The iron cycle involves the transformation of iron between its main oxidation states, impacting microbial metabolism and environmental chemistry.

• Oxidation States and Chemical Forms of Iron:

  • Ferrous iron (Fe2+): Reduced form, soluble in anoxic water and acidic conditions.

  • Ferric iron (Fe3+): Oxidized form, forms insoluble minerals in neutral pH, oxic environments.

• Ferric Iron (Fe3+) Reduction:

  • Electron donors: Organic acids, H2, acetate, lactate, alcohols.

  • Products: Fe2+ (soluble), CO2, H2O.

  • Example reaction:

  • Microbial examples: Geobacter spp., Shewanella spp.

• Environmental Significance: Metal respiration affects the solubility and transport of toxic metals (e.g., Mn, As, Cu, U).

• Example Applications:

  • Microbial sulfate reduction: Key process in anaerobic environments, producing hydrogen sulfide.

  • Iron oxidation: Important in acid mine drainage and formation of iron minerals.

  • Bioremediation: Microbial reduction of toxic metals (e.g., uranium) for environmental cleanup.

Cultivation Methods in Microbial Ecology

Lecture 13: Cultivation-dependent Methods

Cultivation-dependent methods are foundational in microbial ecology, enabling researchers to grow, isolate, and study microorganisms from environmental samples. These approaches are essential for understanding microbial physiology, metabolism, and ecological roles, despite their limitations in capturing the full diversity present in nature.

Methods in Microbial Ecology

• Cultivation-dependent analyses: Involve growing microbes in the laboratory to study their properties.

• Cultivation-independent genetic analyses: Use molecular techniques (e.g., DNA sequencing) to assess community composition without culturing.

• Measuring microbial activities in nature: Directly assessing metabolic processes and ecological functions in situ.

Why Study Microbial Pure Cultures?

• Unambiguously assign physiological and metabolic properties to specific species.

• Enable reproducibility and experimental control.

• Facilitate hypothesis testing and application of Koch's postulates.

• Support international standardization and creation of strains for research and industry.

Microorganisms in Lab vs Nature

• Pure culture (axenic culture): Laboratory cultures containing a single microbial species, typically derived from a colony on solid media.

• Microbial community: In nature, microbes exist as complex mixtures of species, interacting with each other and their environment.

Microbial Nutrition

Microbial nutrition involves the acquisition of essential nutrients required for growth and metabolism. Nutrients are classified as macronutrients (needed in large amounts) and micronutrients (trace elements).

• Macronutrients: C, H, O, N, P, S, K, Mg, Ca, Na, Fe

• Micronutrients: Zn, Mn, Cu, Co

• Growth factors/vitamins: Some microbes synthesize their own; others require external sources.

• Examples of Nutrient Sources:

  • Carbon: Glucose, polysaccharides, amino acids, fatty acids, CO2 (for autotrophs).

  • Nitrogen: Amino acids, peptides, NH4+, NO3-, N2 (for N2-fixers).

  • Sulfur: SO42- (must be reduced), H2S, cysteine, methionine.

  • Phosphorus: PO43-, nucleic acids, phospholipids.

Culture Media

• Defined media: Exact chemical composition is known.

• Complex media: Contains extracts (e.g., peptone, yeast extract); composition not fully known.

Culture Vessels

• Petri dishes (for colonies)

• Tubes, flasks, serum bottles (for liquid cultures)

• Droplets (for microcolonies)

Microbial Cultivation Strategies

Solid Media Techniques

• Streak plate, spread plate, and serial dilution methods for isolating pure cultures.

• Useful for enumeration and isolation of dominant organisms.

• Limitation: Some organisms do not grow on plates; low-abundance species may be missed.

Liquid Media Techniques

• Growth in tubes, flasks, or serum bottles.

• Allows cultivation of organisms that do not form colonies on solid media.

• Enrichers cultures can be established by controlling chemical and physical conditions.

• Limitation: Direct isolation of pure cultures is not always possible.

Enrichment Culture Techniques

• Developed by Martin Beijerinck to select for microbes with specific metabolic traits.

• Selection is based on medium ingredients and incubation conditions.

• Examples: Isolation of nitrogen-fixing bacteria (Azotobacter), sulfate-reducing, and sulfur-oxidizing bacteria.

• Enrichment often favors fast-growing "weeds" and may not reveal the most abundant organisms.

• Example: Enrichment for Azotobacter from soil using mannitol salts and N2 as the sole nitrogen source under aerobic, non-symbiotic conditions.

Isolation and Quantification Methods

• Serial dilution and plating: Used to isolate pure cultures and enumerate viable cells.

• Most Probable Number (MPN): Statistical method for estimating cell numbers based on growth in serial dilutions.

• Optical tweezers and flow cytometry: Advanced methods for isolating individual cells for genomic analysis.

Cultivation-independent Genomic Methods in Microbial Ecology

Lecture 14: Cultivation-independent Methods

Cultivation-independent methods are essential tools in modern microbial ecology, allowing scientists to study and identify microorganisms without the need for laboratory cultivation. These approaches utilize molecular biomarkers, such as the 16S rRNA gene, to survey microbial diversity and function in natural environments.

The Great Plate Count Anomaly

• Definition: The observation that the number of microorganisms detected by direct microscopic counts is much higher than the number that can be cultured on laboratory plates.

• Significance: Many microbes cannot be grown in standard laboratory conditions, leading to underestimation of microbial diversity.

• Example: Direct counts of bacteria in seawater show many millions of cells per milliliter, but only a few thousand colonies may grow on plates.

Identification of Microorganisms

• Many microbes cannot be grown or isolated by traditional methods.

• Scientists use molecular techniques that do not require growth or isolation.

• Use of biomarkers: (e.g., DNA, RNA, proteins) to detect and identify organisms.

• Application of 16S rRNA gene sequencing: To survey microbial communities directly from environmental samples.

Microorganisms We Can Grow and Isolate

• For culturable microbes, traditional microbiological methods can be used for identification, but molecular techniques provide higher resolution and diversity estimates.

16S rRNA Gene as a Barcode

• The 16S rRNA gene serves as a molecular barcode for bacterial identification due to its conserved and variable regions.

• Workflow for 16S rRNA gene sequencing:

  1. Bacterial colony selection

  2. DNA extraction

  3. PCR amplification of the 16S rRNA gene

  4. Purification of amplicons

  5. Check amplicon (e.g., gel electrophoresis)

  6. Cycle sequencing

  7. Purification of sequencing product

  8. Sequence data analysis

  9. Database comparison (e.g., NCBI, SILVA, RDP, EzTaxon)

• Example: Thermus thermophilus 30S ribosome structure, with the 16S rRNA highlighted, is used for taxonomic identification.

• Key Databases: NCBI, SILVA, RDP, EzTaxon

Summary Table: Cultivation-dependent vs. Cultivation-independent Methods

Additional info: 16S rRNA gene sequencing is foundational for microbial taxonomy and ecology. Metagenomics and other meta-omics approaches (meta-transcriptomics, metaproteomics, metabolomics) provide functional and taxonomic insights into microbial communities.