Microbial Ecology

Introduction to Microbial Ecology

Microbial ecology is the study of microorganisms in their natural environments, focusing on their diversity, interactions, and roles in ecosystem processes. Microbes are found in virtually all environments on Earth, where they drive essential biogeochemical cycles and form complex communities.

Species Richness vs. Species Abundance

Definitions and Environmental Examples

• Species richness refers to the number of different species present in a community.

• Species abundance describes the number of individuals of each species in a community.

• Undisturbed, organic-rich soils typically have high species richness and evenness, while extreme environments often have low richness but high abundance of a few adapted species.

Example: Organic-rich soils support diverse microbial communities, whereas extreme environments (e.g., hot springs, hypersaline lakes) are dominated by a few specialized taxa.

Methods for Studying Microbial Communities

Fluorescent In Situ Hybridization (FISH)

• FISH uses fluorescently labeled oligonucleotide probes that hybridize to specific sequences in ribosomal RNA (rRNA).

• Phylogenetic FISH stains can be general (distinguishing prokaryotes from eukaryotes), specific (Bacteria vs. Archaea), or highly specific (certain groups within Bacteria or Archaea).

• Allows visualization and identification of microbes in environmental samples without cultivation.

MAR-FISH (Microautoradiography-FISH)

• Combines FISH with microautoradiography to determine the metabolic activity of individual cells by tracking substrate uptake.

• Useful for linking phylogeny with function in complex communities.

Gene-Based Diversity Measurements

• Specific genes, such as 16S rRNA, are amplified from environmental DNA to assess microbial diversity.

• Techniques include PCR amplification, denaturing gradient gel electrophoresis (DGGE), and sequencing of amplified fragments.

Example: Metagenomic analysis of 16S rRNA genes reveals the composition of microbial communities in soil or water samples.

Microbial Communities and Ecosystems

Guilds and Ecosystems

• A guild is a group of metabolically related organisms that exploit the same resources in a similar way.

• A microbial ecosystem consists of microbial communities and their natural environment, where they interact and recycle elements such as carbon (C), sulfur (S), nitrogen (N), and iron (Fe).

Example: In sediments, different guilds perform processes like fermentation, sulfate reduction, and methanogenesis, forming a layered ecosystem.

Microenvironments

Definition and Importance

• The microenvironment is the specific location where a microorganism lives, often characterized by steep gradients in oxygen, nutrients, and other factors.

• Microenvironments can be oxic (oxygen-rich) or anoxic (oxygen-poor), even within millimeters of a soil particle.

Resources and Conditions Governing Microbial Growth

Key Resources and Environmental Conditions

Limitations to Microbial Growth in Nature

Why Soil Particles Are Not Covered in Microbial Growth

• Suboptimal resources or growth conditions

• Non-uniform distribution of nutrients

• Competition among mixed microbial populations

Microbial Habitats

Soil and Rhizosphere

• Most terrestrial microbial growth occurs on the surface of soil particles and around plant roots in the rhizosphere.

• Microcolonies form in microenvironments within soil aggregates.

Soil Bacterial and Archaeal Diversity

Major Groups in Soil

• Soil contains diverse bacterial phyla such as Proteobacteria, Actinobacteria, Bacteroidetes, and Firmicutes, as well as Archaea.

• Metagenomic analysis is used to assess the diversity and relative abundance of these groups.

Plant-Microbe Interactions

Soybean Root Nodules

• Root nodules are specialized structures formed by symbiotic nitrogen-fixing bacteria (e.g., Rhizobium) in legumes such as soybean.

• Nodulation enhances plant growth by providing biologically available nitrogen.

Mycorrhizae

• Mutualistic associations between plant roots and fungi.

• Fungi transfer inorganic nutrients (e.g., phosphorus) from soil to plant; plants supply carbohydrates to fungi.

• Two main types: Ectomycorrhizae (fungi remain outside roots) and Endomycorrhizae (fungal hyphae penetrate root cells, forming arbuscular mycorrhizae).

Example: Mycorrhizal fungi increase plant nutrient absorption and promote growth, as seen in experimental comparisons.

Aquatic Microbial Ecology

Freshwater Environments

• Stratified lakes have distinct physical, chemical, and microbial layers (e.g., oxic epilimnion, anoxic hypolimnion).

• Oxic zones support oxygenic phototrophs and chemoorganotrophs; anoxic zones support fermentative and anaerobic respiring microbes.

Freshwater Lake Diversity

• Major groups include Proteobacteria, Actinobacteria, Bacteroidetes, and Archaea.

Microbial Consortia: Chlorochromatium Aggregatum

• Consortia of green sulfur bacteria (epibionts) and flagellated rod-shaped bacteria are found in stratified, sulfidic lakes.

• Green sulfur bacteria are obligate anaerobic phototrophs, often dominating the bacterial biomass in these environments.

Rivers and Streams

Organic Carbon and Oxygen Dynamics

• Levels of organic carbon and oxygen are inversely related downstream from pollution inputs (e.g., sewage).

• Bacterial activity increases with organic input, consuming oxygen and altering nutrient dynamics.

Marine Microbial Ecology

Distribution of Archaea and Bacteria

• Surface waters are dominated by phototrophs such as Prochlorococcus, the most abundant photosynthetic microbe.

• Deeper waters contain more chemolithotrophs and chemoorganotrophs, with Archaea becoming more prevalent at depth.

Deep Sea and Hydrothermal Vents

• Deep-sea environments are characterized by high pressure, low temperature, and low nutrient availability.

• Microbes are classified by pressure tolerance: piezotolerant (tolerate high pressure), piezophiles (grow best at high pressure), and extreme piezophiles (require high pressure for growth).

• Hydrothermal vents support autotrophic chemolithotrophs that use reduced inorganic compounds as energy sources.

Chemolithotrophic Metabolisms Near Hydrothermal Vents

Symbiosis at Hydrothermal Vents

• Mutualism is an obligate relationship where both partners benefit.

• Example: The giant tube worm (Riftia) harbors sulfide-reducing bacteria that provide organic carbon via chemosynthesis.

Key Equations

• Nitrification by Nitrosococcus oceani: $NH_4^+ + 1.5 O_2 ightarrow NO_2^- + H_2O$

• Nitrite oxidation by Nitrobacter winogradskyi: $NO_2^- + 0.5 O_2 ightarrow NO_3^-$

Summary

Microbial ecology explores the diversity, distribution, and interactions of microorganisms in various environments. Microbes play critical roles in nutrient cycling, plant health, and ecosystem functioning, adapting to a wide range of habitats from soils and freshwater to deep-sea hydrothermal vents.

Additional info: Some explanations and context were expanded for clarity and completeness based on standard microbiology textbooks.