Question

You identify two bacterial species growing together in different environments: species A is an
organoheterotroph and species B is a lithoautotroph. Below are three environments (E1-E3) that both bacteria
can grow and reproduce in, different only in the one environmental change highlighted, that you use to test the
assumptions of the Molecular Clock hypothesis. You create a taxonomic tree comparing rRNA from one
environment (no parentheses) with another control environment (parentheses). Assuming the theory is correct,
choose one prediction (A or B; each can be used more than once or not at all) that you expect to observe in your
experiment compared to the control (2 pts each). Then, explain WHY you predict this outcome (3 pts each).
A. decreased time since divergence B. longer time since divergence
E1- An organic carbon-rich environment (an organic carbon-limited environment) ______. Explain why:

E2- Growth in the absence of mutagen (growth in the presence of mutagen) ______. Explain why:

E3- An environment with limiting reduced inorganic molecules (an environment with excess reduced inorganic
molecules) ______. Explain why:

Answer 1

E1 - In an organic carbon rich environment, the organoheterotrophic bacteria would grow rapidly as they would directly feed on the organic carbon compounds. The lithoautotrophs on the other hand would remain largely unaffected as they would not use the organic carbon compounds since they derive energy by reducing inorganic compounds. They would grow and divide at their normal pace. Since the organoheterotrophs would divide more rapidly taking advantage of the carbon compounds, they will have a decreased time since divergence (A) since they woud be evolving faster (More the number of divisions, more the generations produced).
In the case of the control, the organoheterotrophic bacteria lose their organic carbon rich substrate advantage and stop their growth. They may eventually die or may identify another substrate to work with. The lithoautotrophs continue to grow unaffected. Here if the organoheterotrophs start working with another substrate, they may have adecreased time since divergence as it is the only way to survive. (A)

E2 - In the absence of a mutagen in the media, there is higher probability that it would take a longer time for divergence (B). However mutations can occur randomly as well and some positive mutations can addup to provide a certain advantage to the bacteria so divergence can occur sooner as well in a media without mutagens, though the chances are rare. (A)

E3 - An environment with limiting reduced inorganic molecules can spell havoc for the lithoautotrophs but can also corner them into seeking unconventional reduced molecules in the media to metabolise from. There may be an alternative pathway that only comes into play when the preferred substrate is scarce and hence those few bacterial cells that may be able to metabolise the secondary substrate may survive. There may be a decreased time since divergence for the latter kind (A). For the organoheterotrophs, presence of limit reduced inorganic molecules should not affect their metabolism in anyway except if those molecules play a role of co-factors in cerain enzyme system in the cell. Still they would be required in trace quantities.

In the case of the control carrying excess reduced inorganic molecules, it may boost the growth of the lithoautotrophs and more divisions lead to more generations and a decreased time since divergence (A). But an excess of the reduced inorganic molecules can also drastically change the pH and the gH of the media, creating a stressed environment for the organheterotrophs. Mutations are likely to occur and hence would likely decrease time since divergence (A).