Which of these fixes nitrogen while living free in the soil rather than in a nodule?
Answer: C
Azotobacter is a free-living aerobic fixer needing no plant partner, as are Azospirillum and Clostridium. Rhizobium and Bradyrhizobium nodulate legumes and Frankia nodulates non-legume trees, so all three of those depend on a host.
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Frankia forms nitrogen-fixing nodules on
Answer: D
Frankia is an actinomycete that forms actinorhizal nodules on woody non-legumes including alder and casuarina, which is why those trees colonise poor soils so readily. Legume nodules are the work of Rhizobium, and the fern association is Anabaena inside Azolla.
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The nitrogen-fixing partner living inside the water fern Azolla is
Answer: A
Anabaena azollae is a cyanobacterium housed in cavities of the Azolla leaf, and the pairing is used as a green manure in flooded rice fields. Rhizobium and Frankia nodulate roots on land, while Nitrosomonas is a nitrifier and fixes nothing.
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Leghaemoglobin in a legume root nodule functions to
Answer: B
Nitrogenase is irreversibly inactivated by oxygen, so the nodule uses leghaemoglobin to bind oxygen tightly and deliver it to respiring bacteroids while keeping the free concentration near zero. The pink colour of a healthy cut nodule comes from this pigment.
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A healthy, actively fixing legume nodule when cut open appears
Answer: C
The pink or red interior is leghaemoglobin, so its presence is the standard field check that a nodule is actually fixing nitrogen. A white or green interior indicates a nodule that has formed but is not effective, and a blackened one is senescent.
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The specificity by which a given Rhizobium strain nodulates only certain legume species is called
Answer: D
Legumes and rhizobia are sorted into cross-inoculation groups, within which any strain can nodulate any host but across which it generally cannot, which is why inoculant must be matched to the crop. The other terms describe a counting method and two forms of microbial interference.
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In nitrification, ammonium is oxidised to nitrite by
Answer: A
Nitrification runs in two steps carried out by different genera: Nitrosomonas takes ammonium to nitrite, and only then does Nitrobacter take nitrite to nitrate. Confusing the two is easy because the names are similar, so it helps to note that nitrite comes first alphabetically and from NitroSOmonas.
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The oxidation of nitrite to nitrate in soil is carried out by
Answer: B
Nitrobacter completes nitrification by oxidising nitrite to nitrate, the form most readily taken up by plant roots and also the most easily leached. Nitrosomonas performs the first step, Rhizobium fixes nitrogen and Desulfovibrio reduces sulphate.
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Nitrifying bacteria such as Nitrosomonas and Nitrobacter obtain their energy from
Answer: C
They are chemoautotrophs: energy comes from oxidising ammonium or nitrite, and carbon comes from carbon dioxide rather than from organic matter. That is why nitrification proceeds in soils with very little organic material, and why these organisms grow so slowly.
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Nitrification proceeds most rapidly in soil that is
Answer: D
Both steps are aerobic oxidations, so they need oxygen, and the organisms prefer a pH above about 7. Waterlogging removes the oxygen and favours denitrification instead, while strong acidity slows the nitrifiers markedly.
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