For Educators

Cyanobacterial Motility Experiment:

Brief Description
Cyanobacteria living in microbial mats can move, both up and down, and sideways. They may orient themselves, and find the optimal depth at which to live, using chemical, light or gravity cues. In this experiment, cultures of cyanobacteria isolated from a stromatolite by Lee Prufert-Bebout were exposed to a directional light source. Over the period of a few days, their movement either away from, or towards a light was measured with a ruler and recorded.

Plot the movements of the cyanobacteria over time. Make a separate plot for movement away from the light and toward the light in order to see if there is a difference.

Plot the speeds at which the cyanobacteria moved on a chart. Make separate bars for movement towards and away.

Questions
Did the cyanobacteria respond to the light?
Did the different species respond in the same way?
Did they move at the same speed over the course of the experiment?
Which were faster or slower?

Things to Think About
Which cultures would be most likely to be found at the top of the stromatolite? Which would be more likely to be at the bottom?
What would be the best cue to use in a mat (light, gravity, etc.)?

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Nitrogen Fixation Experiment:

Brief Description
Nitrogen fixation is a process through which microorganisms can supply themselves with the nitrogen that they need by “fixing” it from the air. Atmospheric dinitrogen gas (N2) is reduced into biologically available ammonia (NH3) using an enzyme called nitrogenase. We measured nitrogen fixation rates in microbial mats over about 9 months. There were six mats used, two had nitrogen added to them, two had phosphorus added to them, and to had nothing added to them.

Activities
Plot nitrogen fixation over time in each of the treatments.

Questions
Did the mats fix nitrogen?
Did the amount of nitrogen fixation change over time?
Were there differences in treatments?

Things to Think About
How can we explain any difference in nitrogen fixation with respect to the treatments?
Why wouldn’t all organisms fix nitrogen?

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Profiles of Oxygen Concentration Experiment:

Brief Description
Many microorganisms in microbial mats produce or consume oxygen. Oxygen is produced by photosynthesis and consumed by respiration. By measuring the concentrations of oxygen in a mat, it is possible to see what the net balance of photosynthesis and respiration is. By measuring oxygen concentrations at various depths, it is possible to see what the net balance is at each of the depths. A set of measurements of oxygen with depth is called a “profile of oxygen concentration”. This spreadsheet contains oxygen profile data from microbial mats being incubated in Greenhouse One at Ames Research Center. A robotic table was used to make one profile of oxygen concentration in the center of each of 18 mats. The 18 mats are arranged into 6 flow boxes, each of which contains 3 mats. The flow boxes are subjected to different experimental treatments, as follows:

Flow Box 1 (Mats 1, 2 and 3) High salinity, Low sulfate concentration
Flow Box 2 (Mats 4, 5 and 6) Low salinity, Low sulfate concentration
Flow Box 3 (Mats 7, 8 and 9) Low salinity, Normal sulfate concentration
Flow Box 4 (Mats 10, 11 and 12) High salinity, Low sulfate concentration
Flow Box 5 (Mats 13,14 and 15) Low salinity, Low sulfate concentration
Flow Box 6 (Mats 16, 17 and 18) Low salinity, Normal sulfate concentration

Activities
Make plots of oxygen concentration versus depth for each of the mats.

Combine the data into treatments and make plots of mean oxygen concentration versus depth for each of the treatments.

Questions
Are the mats producing oxygen?
Are the mats consuming oxygen?
At what depths are production and consumption highest?
At what depths are production and consumption the lowest?
Are there differences in the profiles in the different treatments?

Things to Think About
What would explain the differences in the treatments?
Are these mats a source or a sink of oxygen?
What would the profiles of oxygen concentration look like at night?

+ View Experimental Data

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