 So in this video I'm going to spend a little bit of time talking about how microbial mats have different metabolisms in different places depending on the gradients and how they influence the cycling of oxygen, a little bit of carbon, a little bit of sulfur. We're going to leave the nitrogen out of it. Oxygen cycling is really important, but there are quite a few reactions. So a microbial mat is a community of organisms that are living on a surface, and usually there's light coming from above that's supporting photosynthesis. And the organisms at the top are usually cyanobacteria plus algae, and these are performing the oxygenic photosynthesis. So they're producing oxygen and organic carbon at the top. The oxygen then gets consumed by heterotrophic bacteria. And those heterotrophs are often living within the cyanobacterial mat, but because they don't actually require the light, they can live deeper down. So these organisms consume oxygen at the top, so the brown ones consume oxygen. Since oxygen is being produced right in the mat here, there's often a peak of oxygen concentration in the mat, and because it's not being produced down below, it becomes anoxic with depth. The reason it becomes anoxic is because they're bacteria consuming that oxygen. In the modern oceans, there's also lots of sulfate ions in the water, and sulfate reduction is one of those cases where the reaction of the organic matter produces quite a bit of energy, and most sulfate reduction can't happen in the presence of oxygen, but the microbial mats make these microenvironments within them that are anoxic, and so you do end up with some sulfate reduction associated with the cyanobacteria and also deeper down in the mats here. So the blue ones are sulfate reducers. So they take sulfate plus organic matter and create sulfide and carbon dioxide. And the sulfide dissolves in the water, so you end up with sulfide in the mats here and it decreases upward. So there are two reasons it decreases. First of all, some organisms can react oxygen and sulfide to create energy, and then the sulfide can also react with the oxygen to go back to forming sulfur or sulfate itself. And so this upper part of the mat has these two processes going on, and as you get further down in the mat, the sulfate declines as something that can oxidize organic matter. The oxygen also declines, and so there's less and less chemical reactions to form, but fermentation can take place. So the fermentation basically, there can be fermentation sort of throughout the mats, but they're usually not abundant, but when you get further down into the mat where there's not much to oxidize things, down here you end up with fermentation. And the fermentation basically takes the organic compounds and produces carbon dioxide if it's a little bit more reducing or methane or both, and then you get, still get some organic molecules that come out, but they tend to be smaller and less energetic through time. So erase some of the words, and I'm going to make a graph. So this is a concentration, and we'll do it from the water down into the mat. And then modern earth, if we look first at the oxygen, we have quite a bit of oxygen. I'll draw it in green. So there's some amount in the water column. It's being produced right at the surface of the mat so that the concentration usually goes up, and then it's getting consumed by the respiring organisms, and it goes to zero at some point with depth. So we have our typical oxygen profile here. We can look at sulfate. So sulfate has some concentration in the water column, it will vary depending on what it is. It's not typically being produced very much in the mat unless there's sulfide reacting with oxygen, and it usually just declines with depth. It persists to deeper levels than the oxygen itself does because most of it's consumed in anoxic conditions. And then we have sulfide, which in the presence of oxygen is close to zero. And where there's oxygen present, it's very low. There might be just a teeny bit from some sulfate reduction, but then as the sulfate reduction increases, the sulfide in the water increases until the sulfur is consumed. So this would be a combination of H2S and HS minus depends on the details of the pH. And then if we actually, if we have some fermentation that's producing methane, that methane will react with the oxygen, and there'll be some concentration of methane at depth, and it gets consumed when there's oxygen present and is basically zero in the water column. So one of the reasons we get stinky sediments, especially if there's sulfur around, is because the sulfide produces that rotten egg smell, which we, as humans, have evolved to think the smell's bad in most cases, because breathing in too much of it is poisonous to us. If you don't have very much sulfate in the water, the, there's not sulfide and the smell is different and you get many more of the fermentation products so you can get things like ammonia, and those, those have a distinctly different smell. So this particular chemical gradients are related very much to what's in the water column, as well as the microbial processes that are happening. Now if the water is, has a lot of turbidity in it and the light goes down, or for example at night, the oxygen production shuts off, and the mat at night will go anoxic. And then the, the sulfide can diffuse up and there's no oxygen for it to react with. That sulfur can extend up to the surface of the mat and then the sulfate will can, can still decline because you can still get the sulfate reduction. So either with night changes in the day and the amount of light, or if the sea level water goes up or becomes more turbid and cuts off the oxygen, that's when the chemistry of the mats will change a lot in response to that. If we think about the presence of iron, iron is not soluble in the water in the oxidized form, but if you have sediment grains in with your mat, it would be hematite or girtite, when they're down in this anoxic zone, they can be reduced. So you end up with iron three plus going to iron two plus with some organic molecules to support it and the iron two plus is soluble and you can get the iron two plus coming up to the surface as an iron. So if you have iron oxides and microbial mats might dissolve those and it will affect the iron cycle as well. Thanks for watching.