Imagine that you are studying the microbiology of a soil in the Mojave Desert near Death Valley that is devoid of (i.e. doesn't contain any) organic carbon and fixed nitrogen. Despite this, you notice what could be some faint greenish microbial growth in some patches of soil. Previous gene sequencing studies on samples of this soil (we will get to this later in the course!) indicate that there are two major types of bacteria present: one is a member of the phylum cyanobacteria that is photolithoautotrophic but, unlike other some other members of this phylum, cannot fix nitrogen. The other is a chemoorganoheterotrophic bacterium related to the genus Azotobacter that uses sugars such as glucose or mannitol as a sole C source and is capable of nitrogen fixation (using N gas as an N source). You hypothesize that these two microbes work together to help 2 each other grow: the cyanobacterium fixes carbon and excretes some of the resulting sugars that it produces that serve as C sources for the Azotobacter relative, which in turn fixes nitrogen and excretes some of the fixed N that can be used as an N source by the cyanobacterium. You are interested in trying to isolate each of these microbes separately from this soil sample so that you can better study them. [Note that I am making up the details of this example, but these types of microbes do exist!] Explain how you would design two different types of minimal media and growth conditions that would allow you to 1) grow and enrich for the photolithoautotrophic cyanobacterium (but not the Azotobacter ), and 2) grow and enrich for the nitrogen-fixing, chemoheterotrophic Azotobacter (but not the cyanobacterium, or other heterotrophic microbes that might be present). For each medium, explain what ingredients you would include or exclude, and why. Also, explain which growth/incubation conditions you would use and why. Medium/conditions for enrichment of the photoautotrophic cyanobacterium: Medium/conditions for growth of the nitrogen-fixing, Azotobacter relative: .

1. Imagine that you are studying the microbiology of a soil in the Mojave Desert near Death Valley
that is devoid of (i.e. doesn't contain any) organic carbon and fixed nitrogen. Despite this, you
notice what could be some faint greenish microbial growth in some patches of soil. Previous
gene sequencing studies on samples of this soil (we will get to this later in the course!) indicate
that there are two major types of bacteria present: one is a member of the phylum cyanobacteria
that is photolithoautotrophic but, unlike other some other members of this phylum, cannot fix
nitrogen. The other is a chemoorganoheterotrophic bacterium related to the genus Azotobacter
that uses sugars such as glucose or mannitol as a sole C source and is capable of nitrogen fixation
(using N 2 gas as an N source). You hypothesize that these two microbes work together to help
each other grow: the cyanobacterium fixes carbon and excretes some of the resulting sugars that
it produces that serve as C sources for the Azotobacter relative, which in turn fixes nitrogen and
excretes some of the fixed N that can be used as an N source by the cyanobacterium. You are
interested in trying to isolate each of these microbes separately from this soil sample so that you
can better study them. [Note that I am making up the details of this example, but these types of
microbes do exist!]
Explain how you would design two different types of minimal media and growth conditions that
would allow you to 1) grow and enrich for the photolithoautotrophic cyanobacterium (but not the
Azotobacter ), and 2) grow and enrich for the nitrogen-fixing, chemoheterotrophic Azotobacter
(but not the cyanobacterium, or other heterotrophic microbes that might be present).
For each medium, explain what ingredients you would include or exclude, and why. Also,
explain which growth/incubation conditions you would use and why.
Medium/conditions for enrichment of the photoautotrophic cyanobacterium:
Medium/conditions for growth of the nitrogen-fixing, Azotobacter relative: