After clearing some scattered tiles, we realized that plenty were still in place. All of those had sand and small pebbles strongly cemented to their undersides, which may be the first step in hypolith colonization. The cement is known to be exopolysaccharides, a kind of glue that the bacteria secrete to make a biofilm. Alacia and I carefully photographed the undersides of 22 relatively undisturbed tiles and put them back. A sample photo is shown below:
Alacia scraped samples off of a dozen of them. Back in the metagenomics lab in South Africa, Don Cowan's group will analyze their DNA to see if the microbial community sticking to them looks different from that on a fully developed hypolith. In other words, we are studying microbial succession.
There was no visible green film of cells sticking to any of the tiles. If our hunch is correct, there will be visible green in a few more years time.
There was a difference between the percentages of the surface with cemented soil on the marble tiles (68%) and the glass tiles (42%). The bacteria prefer the marble, which transmits much less light, but they can colonize the glass tiles too. Half of the tiles were inoculated with local cells in 2010 before being deployed in the field, and there was no evidence that it made any difference two years later.
After that, we replaced the scattered tiles in a new row. We did not try to excavate the volcano-shaped dirt mound in the center of our array for fear that the rodent wouldn't like it and make subsequent changes. When we were all done it looked like this:
Now we wait for a few more years! In the meantime, there are other arrays, in North America and Asia, to check on. The one in the Arctic can't possibly get disturbed by burrowing rodents, because there is permafrost there. I wonder if polar bears like to scatter tiles...