Microbial Biogeochemical Cycles Practice Questions

Correct answer: Nitrogenase

Nitrogenase is the highly conserved enzyme complex found in diazotrophs that catalyzes nitrogen fixation. It is sensitive to oxygen and requires significant ATP to break the triple bond of N2.

Correct answer: Oxidation of elemental sulfur to sulfuric acid

Sulfolobus species are thermoacidophilic archaea that oxidize elemental sulfur or hydrogen sulfide into sulfuric acid. This contributes to the extreme acidity of their geothermal habitats.

Correct answer: Anaerobic oxidation of ammonium with nitrite

Anammox (ANaerobic AMMonium OXidation) is performed by specialized bacteria like Planctomycetes. They bypass the traditional denitrification pathway by converting NH4+ and NO2- directly into N2 gas in anoxic environments.

Correct answer: Methanogens

Methanogens are strictly anaerobic archaea that produce methane as a metabolic byproduct of energy generation. They typically use substrates like CO2/H2 or acetate in wetlands, digestive tracts, and landfills.

Correct answer: High carbon-to-nitrogen (C:N) ratios

DNRA competes with denitrification for nitrate. It is favored in carbon-rich, nitrogen-limited environments where maximizing the use of nitrate as an electron sink is energetically beneficial for the microbes.

Correct answer: Nitrosomonas

Nitrosomonas is a genus of chemolithoautotrophic bacteria that oxidizes ammonia into nitrite. This is the rate-limiting first step of the two-step nitrification process.

Correct answer: Reduction of ferric iron (Fe3+) to ferrous iron (Fe2+)

Geobacter species use insoluble ferric iron (Fe3+) as a terminal electron acceptor during anaerobic respiration. This reduces the iron to the more soluble ferrous form (Fe2+), significantly impacting mineral solubility in sediments.

Correct answer: Nitrous oxide (N2O)

Nitrous oxide is a potent greenhouse gas produced during the reduction of NO to N2. Environmental stressors like low pH or sudden oxygen shifts can inhibit nitrous oxide reductase, causing N2O to leak into the atmosphere.

Correct answer: A significant gas phase

Unlike nitrogen, carbon, and sulfur, phosphorus does not have a significant gaseous component in its cycle. It moves primarily through lithosphere and hydrosphere via weathering, leaching, and biological uptake.

Correct answer: Methane monooxygenase

Methane monooxygenase is the key enzyme that allows methanotrophs to utilize methane as their sole carbon and energy source. It exists in both soluble (sMMO) and particulate (pMMO) forms.

Correct answer: Carbon fixation

Carbon fixation, primarily via the Calvin Cycle in photoautotrophs and chemoautotrophs, converts inorganic CO2 into organic compounds. This process serves as the foundation for the global food web.

Correct answer: Specific members of the genus Nitrospira

Comammox (COMplete AMMonia OXidation) was discovered in certain Nitrospira bacteria. Unlike traditional nitrifiers that split the task, these organisms possess the enzymatic machinery to oxidize NH3 all the way to NO3-.

Correct answer: To bind ferric iron (Fe3+) in iron-poor conditions

Siderophores are high-affinity iron-chelating compounds secreted by microorganisms. They bind insoluble Fe3+ and transport it back into the cell, which is crucial for survival in aerobic environments where iron bioavailability is low.

Correct answer: Dissimilatory sulfate reduction

Dissimilatory sulfate reduction uses sulfate as a terminal electron acceptor for anaerobic respiration, releasing H2S into the environment. This is distinct from assimilatory reduction, where sulfur is incorporated into cellular amino acids.

Correct answer: Methylation of Hg(II) to CH3Hg+

Microbial methylation of inorganic mercury creates methylmercury, a potent neurotoxin. Because it is lipid-soluble, it biomagnifies through the aquatic food web, reaching dangerous levels in predatory fish.

Correct answer: Ureolytic activity

Ureolytic bacteria like Sporosarcina pasteurii hydrolyze urea, which increases the local pH and carbonate concentration. This creates an environment where calcium ions precipitate as calcium carbonate (calcite).

Correct answer: Oxygenic photosynthesis

Oxygenic photosynthesis, performed by cyanobacteria, algae, and plants, uses water as an electron donor and releases O2 as a byproduct. This process transformed Earth's atmosphere billions of years ago.

Correct answer: Cooperative degradation by two mutually dependent microbes

Syntrophy (meaning 'eating together') is crucial in anaerobic degradation. For example, fermenters produce H2 that must be consumed by methanogens to keep the reaction thermodynamically favorable for the fermenter.

Correct answer: Methyl groups are removed from organic compounds

Demethylation is the removal of methyl groups (-CH3) from compounds like dimethylsulfoniopropionate (DMSP) or methylmercury. This process plays a role in cycling one-carbon compounds and detoxifying certain environments.

Correct answer: Manganese-oxidizing bacteria and fungi

Manganese-oxidizing microbes convert soluble Mn2+ into insoluble Mn4+ oxides (like MnO2). These manganese oxides are highly reactive and can act as powerful oxidants for other organic and inorganic substances in the sediment.