Questions: Microbial Ecology Overview

Question 1

What term describes the ecological relationship in which one microbial species's metabolic waste products serve as the energy substrate for a second species, creating a mutually dependent metabolic partnership?

• A. Commensalism
• B. Syntrophy
• C. Allelopathy
• D. Amensalism

Answer: (B)

Syntrophy (literally 'eating together') describes obligate metabolic interdependence between species: one organism produces a compound (such as hydrogen or acetate) that a second organism must consume to keep the thermodynamics of the first reaction favorable. Neither partner can function optimally without the other. Commensalism (A) involves one organism benefiting while the other is unaffected; allelopathy (C) is chemical inhibition of one organism by another; amensalism (D) is a one-sided harmful interaction.

Question 2

Metagenomics has confirmed that the majority of environmental bacteria can be cultivated and studied in laboratory culture conditions.

• T. True
• F. False

Answer: False

Metagenomics — sequencing DNA extracted directly from environmental samples — has revealed that more than 99% of environmental bacteria cannot be grown in standard laboratory culture. This 'great plate count anomaly' means that culture-based microbiology has an extreme observer bias: nearly everything we thought we knew about environmental microbial diversity before metagenomics was based on the rare organisms that happen to thrive on laboratory media.

Question 3

Why does the existence of a large unculturable microbial majority matter for understanding global biogeochemical cycles such as the nitrogen and carbon cycles?

Model answer: Unculturable microbes carry out metabolic reactions — nitrogen fixation, methane oxidation, sulfate reduction, and others — that drive major flows of elements through ecosystems. If we only study culturable organisms, we miss most of the functional diversity and substantially underestimate the scope and rate of these transformations. Metagenomics reveals entirely new metabolic pathways and previously unknown guilds of organisms responsible for important ecosystem functions.

Biogeochemical cycles are largely microbially mediated. For example, nitrification, denitrification, and anaerobic ammonium oxidation (anammox) are all carried out by specific microbial guilds. Many of the organisms responsible for anammox — now known to be globally significant in nitrogen cycling — were completely unknown before culture-independent sequencing methods were applied to marine and wastewater environments. The unculturable majority is not ecologically inert; it constitutes the metabolic backbone of most ecosystems.