PRINCIPLES OF MODERN MICROBIOLOGY
Mark Wheelis
ANSWERS TO STUDYQUESTIONS
Chapter 16
Biogeochemistry and Microbial Ecology
2.Oxidations in the sulfur cycle include the conversion of sulfide to elemental sulfur, and elemental sulfur to sulfate (serving as electron donor in chemoautotrophic respiration or in anoxygenic photosynthesis). Reductions include conversion of sulfate to sulfide (as electron donor in an anaerobic respiration, and assimilatory nitrate reduction). Release of sulfide from decomposition of macromolecules, and the assimilation of sulfide, involve no valence change.
4.The extinction of nitrogen fixing procaryotes would eventually lead to the extinction of all life on earth, as soluble nitrogenous compounds were gradually converted to N2 by denitrification. The lack of nitrogen fixation would prevent the return of nitrogen from its gaseous reservoir to forms usable by living organisms.
6.The initial steps in the anaerobic carbon cycles of fresh and marine sediments are the same: macromolecules are hydrolyzed, and the hydrolytic products are fermented to a variety of organic acids. In marine systems, these organic acids are then used as substrates for an anaerobic respiration by sulfate reducers, producing CO2 that is released from the sediments.
In freshwater systems there is inadequate sulfate to support much sulfate reduction, so the organic acids resulting from fermentation are converted to acetate, mainly by syntrophic associations. The acetate is then converted to methane and CO2 by methanogens, and the methane is converted to CO2 by anaerobic and aerobic methanotrophs.
8.In the marine sediment community we can recognize at least the following anaerobic guilds: polymer decomposers; fermenters; sulfate reducers; denitrifiers; and predators. There will also be a narrow aerobic zone, with the following guilds (aerobic and facultatively aerobic): polymer decomposers; aerobic chemoheterotrophs; and predators.
10.Microbial habitats are very diverse, but there are two distinct types (and many intermediates): habitats in which there is a continual flow of nutrients at low concentration into the habitat, and ones with episodic availability of nutrients.