 In this video, I'd like to give an example of how microbial communities have changed the chemistry of the surface of Earth. This was through the evolution of oxygen producing photosynthesis. This is one of the topics I study professionally. And so parts of this talk are parts of a presentation I give to various scientists on a regular basis. This work is really collaborative as part of a community with people from UC Davis, the people in bold or students or postdocs who have been in my lab who contributed directly to this work. And then I have a lot of other collaborators as well. So I want to start with some background. First about molecular oxygen. Almost all of it comes from photosynthesis, which means that it's produced by biology. There's a small amount of oxygen that's produced with reactions in the upper atmosphere from radiation from the sun or cosmic background radiation, but it's a very small component. Essentially, all the oxygen that we have in the atmosphere and breathe comes from photosynthesis. The accumulation of the oxygen in the environment depends on the balance of how much is produced through photosynthesis and how much is consumed by oxidation of reduced compounds. So obviously, organic carbon is one of those things that is reacted with oxygen and is one of the biggest sinks on a short term. There are also other sinks, for example, of reduced volcanic gases that come up through volcanoes or the weathering of reduced minerals in soil profiles. There are many things like that. The bulk of earth is reduced and it's just the interface that accumulates the oxygen. And so photosynthesis has to produce enough oxygen to overcome that consumption to actually accumulate the oxygen in the atmosphere. And there's a certain type of bacteria, the cyanobacteria, which are the ones that evolved oxygenic photosynthesis. They were the dominant primary producers of microbial mats since at least two and a half billion years ago. Plants in algae adopted the cyanobacteria as chloroplasts that allowed the algae in plants to also do photosynthesis, but early on the cyanobacteria were the ones who were producing the oxygen. Within the community, there's a lot of controversy over when oxygenic photosynthesis first evolved relative to when we have the signatures of oxygen in earth's atmosphere. Some people have proposed that the photosynthetic process evolved more than three billion years ago. Other people have proposed that it happened just at about 2.4 or 2.3 billion years ago. And I'll talk a little bit more about that a little later in this video. The third background point is about microbial mats. And I have a video on microbial mats that talks about how they change the geochemistry. And microbial mats, because of the metabolism going on by the bacteria, they're out of equilibrium with their environment. And what that means is if you have photosynthesis in the microbial mat, it can produce oxygen that will accumulate within the mat, even if the surrounding environment is anoxic and lacks that oxygen. So the molecular oxygen comes from cyanobacteria, and its distribution is often out of equilibrium with the environment, with oxygen higher where it's actually being produced. So we can look at the history of oxygen. And this is a cartoon of the overall concentrations of oxygen through time. So today we're sitting here with, this is, access is PAL, it's present atmospheric level. So we'll write it one for the current atmospheric level. And this is going back in time in billions of years. So the first animals evolved about a little over 500 million years ago. And there is thought to be an increase of oxygen during that time. It was lower for large periods of time. It could go back before about 2.3 or 2.4 billion years ago. The thought was that there was almost no oxygen in the atmosphere. And there could have been oxygen in local environments, where it was being produced by oxygenic photosynthesis. But most geologists think that a lot of that oxygen was consumed by the oxidation of reduced iron, the oxidation of methane coming from volcanoes, the weathering of rocks, and those sorts of processes, so that there might have been some oxygen in some environments between 3 billion and 2.5 billion years ago. But most of it was consumed until so much was being produced and the sinks of oxygen declined, and that allowed the amount of oxygen in the atmosphere to increase with time. So how do we actually think about tracking oxygen in some of these local environments? So we can do that by looking at fossil bubbles. So this is a really nice paper looking at thin sections and these are microbial mats that were growing in lakes on flood basalts, and there were calcite minerals that were growing on the microbial mats that helped fossilize them as they were forming. And it also fossilized these molds of bubbles and the material between them, like where these white arrows are pointing, have little, linear traces, hair-like features of organic inclusions which are interpreted as being fossil cyanobacteria. So other gases in addition to oxygen can form bubbles and one of the reasons Wilmuth and his co-authors interpreted these bubbles as actually having oxygen in them is because they are coated by little bits of iron oxides. And so their model is that in the lake water there was almost no oxygen and there was reduced iron which is soluble. There was actually an iron in water without oxygen but where photosynthesis produced enough oxygen that the bubbles were oxidizing, the iron reacted with oxygen in the bubble to form these oxides with the iron 3 plus state. What we see here are two things. One is the morphology of the bubble which is consistent with photosynthesis in the mat. Photosynthetic mats often create bubbles and also the redox signal of the iron suggesting that there was an oxidizing gas in the bubble. So we can take this model and extend it over a broader area and sort of think about it on a regional to global scale. In this particular case, the idea is that there was an ocean that was probably anoxic, did not contain very much oxygen. There might be like a few patches where there's a lot of primary productivity and oxygen production that might have had oxygen but the atmosphere was anoxic at this point. But there were lots of microbial mats in shallow seawater in lakes and rivers and in soil crusts that were actually producing oxygen. And when you produce oxygen in these microbial mats, then you can get some oxic weathering and it will change the flux of elements. For example, sulfur and nickel and molybdenum into the oceans. And nickel and molybdenum are really important trace metals for a lot of microbial processes. So the model is that before Earth accumulated oxygen in the atmosphere, there was still a lot of oxygen around in these special places and mats. And if you have those mats, you can get these what I call weathering lifts that actually produce the geochemical signatures of oxygen containing weathering. So in my model and the model that we're working with, there was oxygen production back probably at least 3 billion years, possibly older than that, but it wasn't accumulating, it was just very locally distributed. So thanks for watching.