Ecological Diversity Among Microorganisms — Phototrophs, Nitrogen Fixers, and Extremophiles | Chapter 15 from Brock Biology of Microorganisms

Chapter 15 of Brock Biology of Microorganisms offers a fascinating journey through the vast ecological diversity of microbial life. From oxygenic phototrophs to sulfur reducers, microbes have evolved to thrive in virtually every niche on Earth. This chapter emphasizes how different metabolic traits, environmental tolerances, morphologies, and evolutionary backgrounds allow microorganisms to occupy unique ecological roles—even when their metabolisms appear similar.

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Phototrophic Bacteria and Their Ecologies

Cyanobacteria

These oxygenic phototrophs were the first to produce atmospheric O₂. They are morphologically diverse—ranging from unicellular to colonial forms—and contain thylakoid membranes with chlorophyll a and phycobilins. Many cyanobacteria also fix nitrogen and can survive in the dark by metabolizing sugars. They're major contributors to global oxygen and nitrogen cycles, though some produce harmful toxins.

Purple and Green Bacteria

• Purple Sulfur Bacteria: Anoxygenic phototrophs that use H₂S as an electron donor, forming elemental sulfur and sulfate.
• Purple Nonsulfur Bacteria: Versatile organisms that can grow photoheterotrophically or via fermentation and anaerobic respiration. They often carry photosynthetic gene clusters subject to horizontal gene transfer.
• Green Sulfur Bacteria: Found in anoxic, sulfide-rich zones, these use chlorosomes to harvest light and fix CO₂ via the reverse citric acid cycle.
• Green Nonsulfur Bacteria (Chloroflexi): Use organic compounds for photoheterotrophic growth and can also grow aerobically in the dark.
• Heliobacteria: Unique Firmicutes that use FeS-type photosystems and bacteriochlorophyll g.
• Phototrophic Acidobacteria and Gemmatimonadetes: Thermophilic and aerobic photoheterotrophs that harness light energy using chlorosomes.

Microbial Roles in Nutrient Cycles

Nitrogen Fixers (Diazotrophs)

These microbes convert atmospheric nitrogen (N₂) into ammonia (NH₃) using the oxygen-sensitive nitrogenase enzyme. Some protect nitrogenase using specialized cells like heterocysts or through spatial and temporal regulation. Diazotrophs range from free-living soil bacteria (Azotobacter) to symbionts (Mesorhizobium) and anaerobes (Desulfovibrio).

Nitrifiers and Denitrifiers

• Nitrifiers: Aerobic chemolithotrophs that oxidize NH₃ to NO₂⁻ (Nitrosomonas) and NO₂⁻ to NO₃⁻ (Nitrobacter); some species like Nitrospira perform complete nitrification (comammox).
• Denitrifiers: Facultative aerobes that reduce nitrate to gaseous nitrogen compounds like N₂, contributing to nitrogen loss in ecosystems.

Sulfur Cycling Microbes

• Dissimilative Sulfate Reducers: Reduce SO₄²⁻ or S⁰ to H₂S using hydrogen or organics as electron donors (e.g., Desulfovibrio).
• Sulfur Oxidizers: Oxidize reduced sulfur compounds with O₂ or NO₃⁻ as electron acceptors. Key genera include Thiobacillus and Beggiatoa.

Iron and Methane Metabolizers

• Iron Reducers: Anaerobes like Geobacter and Shewanella reduce Fe³⁺ and use conductive pili (nanowires) for electron transfer.
• Iron Oxidizers: Include both aerobic (Gallionella) and anaerobic types. Some are phototrophic.
• Methanotrophs and Methylotrophs: Oxidize CH₄ or other C1 compounds. Aerobic species like Methylomonas and Methylosinus use methane monooxygenase; anaerobic methane oxidation occurs in consortia.

Ecologically Unique Microbial Groups

• Microbial Predators: Bdellovibrio invades gram-negative bacteria; Myxococcus forms fruiting bodies in cooperative hunting.
• Spirochetes: Helical, motile bacteria with endoflagella; include pathogens like Treponema pallidum.
• Budding and Stalked Bacteria: Caulobacter and Hyphomicrobium form prosthecae or stalks for surface attachment and budding.
• Sheathed Bacteria: Sphaerotilus grow within tubes and release swarmer cells under stress.
• Magnetotactic Bacteria: Use internal magnetosomes (magnetite or greigite) to orient along Earth’s magnetic field (magnetotaxis).

Glossary of Key Concepts

• Diazotroph: Organism that fixes atmospheric nitrogen (N₂ → NH₃)
• Chlorosome: Specialized light-harvesting structure in green phototrophs
• Endoflagellum: Periplasmic flagellum used by spirochetes for movement

Conclusion: Microbial Ecology Shapes the Biosphere

The remarkable adaptability of microorganisms makes them foundational to Earth’s ecosystems. From deep-sea vents to soil communities, microbial life not only responds to environmental conditions but actively shapes them. Chapter 15 of Brock Biology of Microorganisms showcases this incredible ecological and evolutionary diversity through a wide range of microbial strategies and lifestyles.

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