Why do photosynthetic microbial mats assemble in the way they do (in terms of green bacteria, purple bacteria, cyanobacteria, and sulfate reducers)? Be sure to explain why the species assemble in the order they do (from top to bottom) and why the layers of different species may be more or less thick compared to others.

Question

What will be an ideal response?

Answer 1

If you look closely at the mat, you will see a series of colored layers atop a black layer. The green layer at the top of these mats is occupied by cyanobacteria and diatomaceous algae. The next two layers of are also photosynthetic but neither one produces oxygen. The final layer, which is black, is where the sulfate reducers are present.

So why are they arranged in this order? One reason is the amount of light energy that each group is able to utilize. Cyanobacteria take out the more high energy light in the 600 and 700 nm wavelengths. Next in the mat are the purple bacteria and take up light in the far end of the visible spectrum above 800 nanometers.
The light that is left for the Green bacteria is in the middle of those two ends. Its chlorophyll absorbs at around 700-800 nm.

However, another reason for the stratification is the availability of electron donors. You can see gradients in sulfur compounds, for example. In this case, there is decreasing sulfate with depth, do its abundance in seawater and consumption by sulfate reducers. H2S is generated by sulfate-reducing bacteria, which are chemotrophs, and so reside underneath the green sulfur bacteria. Green sulfur bacteria are below purple bacteria because they need a supply of H2S as electron donors. This is also good, but sulfides are toxic to most organisms and their movement out of the sediments could be harmful to other organisms. All of these gradients are set up by microbes and the layers optimally arranged for each of the groups involved.

If you look closely at the environmental conditions in these mats they are definitely characterized by lots of gradients. Light only penetrates to a certain depth, but it is present in all the layers where photosynthesis is occurring. Oxygen is produced by the cyanobacteria and diatoms, but diffuses up into the water and air column or it is consumed by other bacteria and does not penetrate into the lower layers. This is good for the purple and green bacteria, which only photosynthesize when oxygen is not present.