Different microbialites have different shapes.  Why?  How do we even answer this question? One approach is to look at the building blocks of microbialites – the legos that stack together to make the structures.  Some microbialites have little tufts on them that make the surface rough.  These tufts are composed of the mineral calcite, which is intermixed with long, hair-like bacteria that come in two colors, pink and green.  The calcite makes the microbialite a rock, and the bacteria shape the tufts.  They are all lined up, pointing upward into the water like spiked hair.

Close-up depiction of smooth and rough microbialite surfaces

These tufts are one of the building blocks for the microbialites.  How many there are, how they are arranged, and how fast they grow help shape the microbialites.  Lots of closely spaced tufts growing quickly create rough surfaces on the microbialites.  If the tufts grow slowly and there is lots of calcite, they only form small bumps.  If they are absent, the microbialites are smooth.  These smooth microbialites might need a different building block to form, one we haven’t identified yet.

Smooth Microbialite Surface

The bacteria that form the tufts are photosynthetic – they use light to grow.  The tops of microbialites get more light than the sides do.  We can measure this, and it’s true in Pavilion Lake.  If tufts grow faster with more light and there is more light at the tops of the microbialites, we can predict that the tops might grow more quickly than the sides.  We haven’t measured this because the microbialites grow very slowly, and growth rates are very hard to measure.  However, the shapes of the microbialites suggest that they mostly grow upward.  Many of them have ridges on their sides pointing up or “chimneys” on top.  Thus, our prediction is consistent with the data we have so far.

Closeup image of Microbialite

Do we now understand why different microbialites have different shapes?  Nope!  We still have a lot to learn, and many of the questions surrounding microbialite formation remain unanswered. The tuft building blocks that we’ve been analyzing should all behave about the same on near-by microbialites.  But the near-by microbialites aren’t all the same shape! Maybe some of the differences are due to differences in growth rate, but I think there are probably more building blocks that we haven’t described yet.  One thing to think about before I finish: To make a lego boat, you need different blocks than you do to make a lego submersible.  To make a dome-shaped microbialite, you might need different blocks than you do to make an “artichoke-shaped” microbialite.  This kind of thinking might help us solve one of the big mysteries of Pavilion Lake!

-Dawn