What is an ecosystem, and what does microbial ecology study?
An ecosystem is a community of organisms and their natural environment.
Microbial ecology studies how microorganisms interact with each other and with their physical environment. It aims to:
1. Understand microbial biodiversity and community interactions.
2. Measure microbial activities in nature and their effects on ecosystem functions.
What components exist within an ecosystem?
Populations: All the individuals of one microbial species in a place
Guilds: Metabolically related organisms that use similar resources
Communities: Multiple populations / guilds interacting
What did M.W. Beijerinck say?
“Everything is everywhere, the environment selects”
What does “Everything is everywhere, the environment selects” mean?
Microbes disperse globally, but local environmental conditions (e.g., pH, temperature, nutrient levels) determine which organisms grow and dominate.
Ex. If a hot spring is 70C, only heat loving microbes will survive
What physicochemical factors influence microbial growth in nature?
Microbial growth requires:
Nutrients
Proper temperature
pH
Water activity (a_w)
Oxygen
Light
Pressure
What is a niche?
A niche is the functional role of an organism within a system — including:
- Where it lives
- What nutrients/resources it uses
- When it uses them
What is a microenvironment, and why is it usually heterogeneous?
A microenvironment is the specific physical space where a microbe actually lives and metabolizes.
It is heterogeneous because conditions (nutrients, O₂, pH) change rapidly over micrometer scales, creating many micro‑niches supporting physiologically distinct groups.
What is meant by a “feast‑or‑famine” lifestyle in microbes?
Nutrients in nature enter ecosystems intermittently, so microbes often experience:
- Short “feast” periods with abundant nutrients
- Long “famine” periods with scarcity
To survive, microbes evolved storage polymers (e.g., PHB, glycogen) to store carbon/energy for starvation periods.
Why is exponential growth rare in natural environments?
Because:
- Nutrients are limited
- Nutrients are unevenly distributed
- Competition is intense
e.g.
E. coli t_gen ≈ 20 min in lab
E. coli t_gen ≈ 12 hours in the intestinal tract
Why are surfaces important for microbial growth?
Microbial growth is often optimal on surfaces because they provide:
- Nutrient accumulation
- Protection from flow/shear
- Stability for attachment
What is a biofilm?
A biofilm is a community of microorganisms embedded in EPS (extracellular polymeric substances) attached to a surface.
Biofilms show strong cooperation, nutrient sharing, resistance to antimicrobials, and structured microenvironments.
What is a microbial mat, and how does it differ from a biofilm?
A microbial mat is a thick, layered, macroscopic community (e.g., hot springs, beaches).
It includes multiple trophic layers, strong vertical gradients, and extremely high microbial numbers and activity.
What are major consequences of biofilms (positive and negative)?
Problems:
- Colonize artificial implants
- Cause dental disease
- Reduce pipeline flow
- Lower water quality
- Increase corrosion rates
Useful roles:
- Biological filtration (water/wastewater)
- Microbial ore leaching
- Food fermentation
What is competition in microbial ecology?
Competition occurs when distinct microorganisms attempt to acquire the same limiting resource.
This is governed by the competitive exclusion principle — two species competing for the same resource cannot stably coexist.
What is antagonism?
A form of competition where one microbe inhibits another by releasing:
- Specific inhibitors
- Antibiotics
- Toxic metabolites
What is syntrophy, and why does it occur?
Syntrophy is a cooperative interaction where two or more microorganisms work together to perform a transformation neither can carry out alone.
Characteristics:
- They share the same microenvironment
- Often involves complementary metabolisms
Examples:
- Nitrification: NH₃ → NO₂⁻ (nitrosifiers) → NO₃⁻ (nitrifiers)
- Sulfur coupling: SO₄²⁻ → H₂S (sulfate reducers); H₂S → S⁰ (sulfide oxidizers)
What are the major carbon reservoirs on Earth according to this lecture?
- Sediments and rocks of the Earth’s crust (largest long‑term reservoir)
- Humus (recalcitrant organic matter in soils)
- Land plants (large, active organic C reservoir)
Define humus and explain its significance in the C cycle.
Humus is a complex mixture largely derived from resistant plant material and soil microorganisms; it is resistant to decomposition and thus slows carbon turnover, acting as a long‑lived soil carbon pool
Soil that is highly fertile is usually high in humus
How is the carbon cycle closely linked to the oxygen cycle?
CO₂ fixation during oxygenic photosynthesis produces O₂.
Respiration consumes O₂ and returns CO₂ to the atmosphere.
These opposing fluxes couple the C and O cycles.
Who performs oxygenic vs anoxygenic photosynthesis in this course?
Oxygenic phototrophs: Green plants, cyanobacteria, algae.
Anoxygenic phototrophs (anaerobic): Purple and green sulfur bacteria. [BI374 lect…2-26 post | PDF]
What are the two primary processes that create new organic carbon?
- Oxygenic photosynthesis (carried out by higher plants and microbes)
- Chemosynthesis: Autotrophic CO₂ fixation powered by chemical energy (not light).
What is the relationship between photosynthetic rate and respiration for biomass increase?
Net biomass increases only if photosynthetic CO₂ fixation rate is greater than the respiration rate (i.e., a net positive photosynthesis–respiration balance).
What are the dominant carbon end‑states produced during organic matter decomposition and why is their balance critical?
- Carbon dioxide CO₂ (oxidized end‑state)
- Methane CH₄ (reduced end‑state)
A proper balance between oxidative (CO₂‑forming) and reductive (CH₄‑forming) processes is critical for ecosystem stability and greenhouse gas fluxes.
Write the methanogenesis reaction and name the key enzyme.
CO2 + 4H2 -> CH4 + 2H2O
Catalyzed by methyl reductase within methanogenic pathways
Methanogenesis happens only in anaerobic environments
