Microbiology of Soils Questions and Answers

Answer: C

The Commission's answer is Aerobic bacteria \(\gt\) Anaerobic bacteria \(\gt\) Fungi.

Counted as numbers of organisms per gram of soil, bacteria are by far the most numerous. In typical figures aerobic bacteria run to millions or more per gram, anaerobic bacteria to somewhat fewer, and fungi to tens or hundreds of thousands of propagules. Fungi may dominate in biomass because their hyphae are large, but not in numbers, which is what the textbook order ranks.

Options putting fungi first or second confuse biomass with numbers, and the last option puts anaerobes above aerobes, which does not hold for ordinary well-aerated soil.

Answer: A

Bacteria outnumber every other group by orders of magnitude: a single teaspoon of productive soil holds between a hundred million and a billion of them. Fungi usually exceed bacteria in total BIOMASS because their hyphae are far larger, which is why the question asks about numbers rather than weight.

Answer: B

Actinomycetes release geosmin as they die, and it is this compound the human nose detects at very low concentrations after rain or ploughing. The other gases listed do occur in soil, but hydrogen sulphide smells of rotten eggs and neither ammonia nor methane gives the characteristic earthy note.

Answer: C

Streptomyces, a genus of actinomycetes, produces more than fifty distinct antibiotics, streptomycin among them, which help suppress competing organisms in the soil. Rhizobium fixes nitrogen, Nitrosomonas oxidises ammonium and Thiobacillus oxidises sulphur; none of them is an antibiotic source.

Answer: D

Actinomycetes are prokaryotes, so despite the filamentous growth habit that once had them classed with fungi they are bacteria: they have no nucleus, their filaments are far narrower than fungal hyphae and their cell walls contain peptidoglycan rather than chitin.

Answer: A

The rhizosphere is the narrow region of soil whose chemistry and biology are altered by the root, chiefly through exudates of sugars, amino acids and organic acids. The root SURFACE is distinguished as the rhizoplane, and the litter layer and the saturated zone are unrelated to root influence.

Answer: B

R:S divides the microbial count in the rhizosphere by the count in the surrounding bulk soil, so it measures how strongly the root stimulates microbial growth. It is highest for bacteria, around twenty to one, and lower for fungi and actinomycetes. The other expansions of the letters are inventions.

Answer: C

Roots leak sugars, amino acids, organic acids and sloughed cells, and this carbon supply is what lets microbial populations near the root reach many times the density of bulk soil. Soil in that zone is not measurably warmer, and any antimicrobial exudates would reduce the population rather than raise it.

Answer: D

The rhizoplane is the root surface itself, along with the soil that clings to it, and it carries an even denser microbial film than the surrounding rhizosphere. Distinguishing the two matters when a method requires washing the root, which removes rhizosphere soil but leaves the rhizoplane community.

Answer: A

Nitrogenase is the enzyme complex that reduces atmospheric dinitrogen to ammonia, and it is found only in prokaryotes. Nitrate reductase works in the opposite direction of the cycle, urease releases ammonia from urea, and catalase merely destroys hydrogen peroxide.