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 every habitat on Earth, where they form complex communities and drive essential biogeochemical cycles.

Species Richness vs. Species Abundance

Definitions and Environmental Examples

• Species richness: The number of different species present in a community.

• Species abundance: The number of individuals of each species in a community.

• Undisturbed, organic-rich soils typically have high species richness and evenness, while extreme environments (e.g., hot springs, salt flats) often have low richness but high abundance of a few adapted species.

Example: In undisturbed soils, many microbial species coexist, whereas in extreme environments, a few species dominate due to specialized adaptations.

Methods for Studying Microbial Communities

Fluorescent In Situ Hybridization (FISH)

FISH is a molecular technique used to identify and visualize specific microbial groups in environmental samples.

• Uses fluorescently labeled oligonucleotide probes that bind to complementary sequences in ribosomal RNA (rRNA).

• Probes can be general (distinguishing prokaryotes from eukaryotes), specific (Bacteria vs. Archaea), or highly specific (targeting particular groups).

• Allows direct observation of microbial cells in situ.

MAR-FISH (Microautoradiography-FISH)

• Combines FISH with microautoradiography to determine the metabolic activity of specific microorganisms.

• Used to assess whether an organism can utilize a particular substrate by tracking the uptake of radiolabeled compounds.

Gene-Based Diversity Measures

• Specific genes, such as 16S rRNA, are used as molecular markers to assess microbial diversity.

• Techniques include PCR amplification, sequencing, and analysis of community DNA to identify and quantify different taxa.

Microbial Communities and Ecosystems

Guilds and Ecosystems

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

• Microbial communities are organized into guilds, which together with their environment form an ecosystem.

• Microorganisms play key roles in recycling elements such as carbon (C), sulfur (S), nitrogen (N), and iron (Fe).

Microenvironments

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

• Example: Within a soil particle, zones can range from oxic (oxygen-rich) to anoxic (oxygen-depleted) within millimeters.

Resources and Conditions Governing Microbial Growth

Key Resources and Environmental Conditions

• Resources: Carbon (organic, CO2), nitrogen (organic, inorganic), other macronutrients (S, P, K, Mg), micronutrients (Fe, Mn, Co, Cu, Zn, Ni), electron acceptors (O2, NO3-, SO42-, Fe3+), and electron donors (H2, H2S, Fe2+, NH4+, NO2-).

• Conditions: Temperature, water potential, pH, oxygen levels, light, and osmotic conditions.

Limitations to Microbial Growth

• Suboptimal resources or growth conditions

• Non-uniform distribution of nutrients

• Competition among mixed microbial populations

Soil and Plant-Associated Microbial Communities

Microbial Habitats in Soil

• Most terrestrial microbial growth occurs on soil particle surfaces, especially around plant roots in the rhizosphere.

• Soil structure creates microhabitats with varying resources and conditions.

Soil Bacterial and Archaeal Diversity

• Soil contains a vast diversity of Bacteria and Archaea, with major groups including Proteobacteria, Actinobacteria, Bacteroidetes, and others.

• Metagenomic analysis of 16S rRNA genes is commonly used to assess this diversity.

Plant-Microbe Interactions: Root Nodules

• Root nodules are specialized structures on plant roots (e.g., soybeans) formed by symbiotic nitrogen-fixing bacteria (e.g., Rhizobium).

• Nodulation enhances plant growth by providing bioavailable nitrogen.

Mycorrhizae

• Mycorrhizae are mutualistic associations between plant roots and fungi.

• The fungus transfers inorganic nutrients (e.g., phosphorus) from soil to the plant, while the plant supplies carbohydrates to the fungus.

• Two main types: Ectomycorrhizae (fungus remains outside root cells) and Endomycorrhizae (fungus penetrates root cells, forming arbuscular mycorrhizae).

Example: Ectomycorrhizae are common in forest trees; endomycorrhizae are found in over 80% of terrestrial plants.

Aquatic Microbial Communities

Freshwater Environments

• Stratified water columns in lakes create distinct physical, chemical, and microbial zones.

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

Freshwater Lake Diversity

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

• Community composition is assessed by extracting and sequencing community DNA.

Microbial Consortia: Chlorochromatium aggregatum

• Consortia are mutualistic associations of green sulfur bacteria (epibionts) and flagellated rod-shaped bacteria.

• Found in stratified, sulfidic lakes; green sulfur bacteria are obligate anaerobic phototrophs.

• These consortia can make up a large proportion of bacterial biomass in such environments.

Rivers and Streams

• Levels of organic carbon and oxygen are inversely related downstream from pollution inputs.

• Bacterial numbers and biochemical oxygen demand (BOD) increase with organic input, while oxygen decreases; recovery occurs further downstream as algae and cyanobacteria grow.

Marine Environments

• Microbial communities in oceans are stratified by depth, with phototrophs (e.g., Prochlorococcus) dominating surface waters and chemolithotrophs/chemoorganotrophs in deeper zones.

• Archaea and Bacteria have distinct distributions with depth and environmental conditions.

Deep-Sea and Hydrothermal Vent Microbiology

• 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 very high pressure).

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

Symbiosis at Hydrothermal Vents

• Mutualism: An obligate relationship where both partners benefit.

• Example: Riftia giant tube worms and sulfide-reducing bacteria; bacteria provide organic carbon via chemosynthesis, supporting the worm's nutrition.

Summary

• Microbial ecology explores the diversity, interactions, and environmental roles of microorganisms.

• Microbes form complex communities, drive nutrient cycles, and adapt to a wide range of habitats, from soils and freshwater to deep-sea vents.

• Modern molecular techniques, such as FISH and metagenomics, are essential for studying microbial diversity and function in nature.