Laboratory Activity to Effectively Teach Introductory Geomicrobiology Concepts to Non-Geology Majors †

Example of workflow of the experiments conducted over the three-week period to introduce main geomicrobiology concepts. Week 1. Characterization of soil microorganisms based on Gram type, morphology and pigmentation. Unique morphotypes were streaked on different B4 media. Week 2. An example is provided of possible results: strain A is grown on both Standard B4 and Buffered B4. When strain A is grown on Standard B4 it can modify the pH by itself, inducing alkalization and fostering precipitation. Below, when the same strain is grown on a buffered acidic environment (Buffered B4), the precipitation does not occur. A similar but opposite example is shown for strain B. Students discriminated between acid/alkaline conditions and presence or absence of crystal precipitation. Week 3. Crystals and their matrices were collected from the biofilms and analyzed using the optical microscope.

Box-and-wisher plot showing the median of 25th to 75th percentile of positive responses in the pre- and posttests to assess student learning in geomicrobiology after the three-week laboratory experiment.

Assessment for student performance. The scale ranges from very good ( 5 ), to good ( 3 ), to poor ( 1 ). Tasks assigned: (A) The student is able to prepare the B4 medium as instructed * . (B) The student uses aseptic techniques to streak the isolates on B4 media plates. (C) The student can discriminate among alkaline (red) and acidic (yellow) conditions in the B4 media plates. (D) The student is able to associate acidic conditions on B4 plates with impairment of crystal formation. (E) The student discriminates among different crystal morphologies using the microscope. (F) The student is able to visualize the EPS matrix using the microscope. (G) The student properly identifies the crystals on the biofilm.

Main findings reported after the three-week laboratory experience to introduce geomicrobiology concepts. On panels A and B, two different morphotypes were grown on standard B4 plates with Phenol Red as indicator and incubated for 7 days at 40°C. Biofilms in panel A were alkaline (see red coloration, indicative of pH ≥ 8.2 with an arrow indicating the crystals). In panel B, we show a strain with a more acidic metabolism and, consequently, no crystals were formed (yellow, indicative of pH ≤ 6.4). Panels C and D demonstrate the observation of CaCO3 crystals (see arrows) with the stereomicroscope (magnification 4X). Panels E and F show examples of CaCO3 crystals observed with the optical microscope (magnification 100X). Panel G shows the matrix stained with crystal violet associated with the carbonate crystals that are progressively dissolved after 0.1 N HCl treatments.