Posts Tagged ‘cyanobacteria’
oxidised by hydrogen escape…? Chasing the origins of life…
first life? something tiny and green?
I’ve been intrigued and mystified by this phrase (among many others) in Oliver Morton’s book on photosynthesis, the chemistry of which I’m really struggling with. I first read the book a decade or so ago, and in this second reading I’m really beginning to understand how little I’ve understood about the process so central to life on Earth. So, as I too often do, I’ll start with AI (never lies):
Oxidation by hydrogen escape is a planetary process where light hydrogen atoms permanently drift into space from a planet’s upper atmosphere. Because hydrogen is lost, leftover oxygen atoms and other heavy molecules chemically bind to crust and mantle rocks. This leaves the planet’s surface and interior permanently oxidized over geologic time…
I don’t fully understand this. Light hydrogen atoms are also known as protium, which is the most common and lightest isotope of hydrogen (AI again). It’s just a proton in a nucleus. So this drifting off of light hydrogen in an atmosphere, leaving oxygen, etc behind, suggests that the atmosphere must have contained water vapour, inter alia?
So there are further explanations. Ultraviolet sunlight breaks apart water vapour in the upper atmosphere, as well as methane CH4), The hydrogen, being light and fast-moving, escapes the atmosphere, while the heavier elements, such as oxygen, drop down to the planetary surface, and, for example react with iron in the crust.
So what’s with this oxidising thing? It’s a bonding in particular with iron and manganese and appears to have a connection with the Great Oxidation Event (GOE). According to Science Direct, ‘oxygenic photosynthesis appears to be necessary for an oxygen-rich atmosphere like Earth’s’. Are there other kinds of photosynthesis? Apparently so – anoxygenic photosynthesis is used by some bacteria, with hydrogen sulphide being the likely electron donor, but let’s not get side-tracked…
Atmospheric oxygen today is at around 21% concentration, and this is known as PAL, the present atmospheric level by volume. The GOE has been calculated as a 200 million-year transformation occurring about 2.4 billion years ago, bringing about a relatively rapid rise in atmospheric O2…
So, our atmosphere is layered: nearest the ground and rising about 12 kilometres is the troposphere, and above that is the stratosphere (up to 50 kms), then the mesosphere (85), the thermosphere (600) and finally the exosphere, where it thins out into space and where satellites are mostly situated. But to complicate matters we also have the homosphere, and at its upper boundary the turbopause. This ‘sphere’ is so called due to its more or less uniformity. To quote AI it’s ‘the lower layer of the atmosphere where constant mixing by wind and turbulence keeps the chemical composition uniform’.
So when did our atmosphere stabilise, assuming it ever has? Again, I find this is divided into periods, the first one being ‘primordial outgassing’, some 4.5 to 4 billion years go, ‘As early Earth cooled, volcanic outgassing created a thick, toxic precursor atmosphere mostly made of water vapour, carbon dioxide, and nitrogen’. And the Smithsonian Environmental Research Centre goes on, re the Archaean Eon, during which:
methane droplets in the air shrouded the young Earth in a global haze. There was no oxygen gas on Earth. Oxygen was only in compounds such as water. Complex chemical reactions in the young oceans transformed carbon-containing molecules into simple, living cells that did not need oxygen to live. Instead they made energy out of sulphur and other elements.
It also points out that in this early phase of our planet’s life the sun was only around three quarters as bright as it is now – it has gotten brighter over time. The Earth was prevented from freezing over by the greenhouse gases, mostly CO2 and CH4 (methane). But over time, cyanobacteria began to flourish in the oceans, and they were photosynthesising, making free oxygen gas from CO2, H2O and the sun’s light, and so creating something like the atmosphere we know today.
So what exactly are cyanobacteria and why did they come to exist, and then flourish? As to when – likely not before 3 billion years ago, at least not in great numbers. As to where – in the oceans, which were more plentiful, a ‘water world’, according to you know what. And as to how, that’s of course more complex and uncertain. AI gives this not-so-satisfactory explanation:
- Ancestral Microbes: Early Earth had an anoxic (oxygen-free) atmosphere filled with methane and carbon dioxide.
- New Biochemistry: Single-celled anaerobic bacteria in the oceans evolved specialised cell membrane structures.
- Splitting Water: These organisms harnessed sunlight to split water molecules (H2O) and carbon dioxide into sugars for energy.
- Oxygen Waste: The process released oxygen gas (O2) as a waste product, which built up over time and triggered the [GOE]
So, it’s all about ‘ancestral microbes’ and ‘anaerobic [meaning ‘sans oxygen’] bacteria’. So, the ‘life from non-life’ problem presumably remains unsolved. Understandable, I suppose.
References
https://www.sciencedirect.com/science/article/pii/S0009254113003513
https://en.wikipedia.org/wiki/Anoxygenic_photosynthesis
https://arxiv.org/html/2512.09844v1