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Comment by lend000 | Hacker News Reader

Maybe I'm missing something obvious, but shorter days mean shorter nights. The whole premise (that longer days gave cyanobacteria more time to accumulate oxygen that they would consume at night), at least as interpreted by this pop-sci article, makes no sense to me. There may be other more complex reasons for the change in rotational speed less directly affecting oxygen in the atmosphere, but it's only a sample size of two (and they ignore the third one where atmospheric oxygen has decreased since the time of the dinosaurs), so there isn't much to go on.

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I think the premise here is that there is some sort of physical limitation that causes cyanobacteria to have a maximum "carrying capacity" of oxygen, and stretching timecycles causes that threshold to be exceeded, leading to oxygen leaks.

Analogously, think of memory management in a computing context. You have a program running that gradually allocates memory linearly over a certain time period (daytime), but then releases all that memory linearly during another time period (nighttime). Say you're dealing with 4GB of memory and you run the memory-increasing part for 6 hours, then run the memory-decreasing part for the next 6 hours, and so on. Suppose you never allocate the full available 4GB when you're dealing with a 6 hour on/off timecycle (the rate of memory allocation is too low to get to 4GB in 6 hours) - but what happens if you extend the timecycle? There's some timecycle length at which you will finally attempt to allocate more memory than the 4GB your hardware is capable of, so the host OS starts swapping or writing stuff to disk to deal with the excess.

Biological systems don't have a "host OS" that regulates their molecular byproduct management though. Extra atoms/molecules are just going to escape into the surrounding environment. Perhaps the oxygen buildup during the daytime might have worked this way with the cyanobacteria - longer days led to more oxygen being produced than could be physically retained in the immediate vicinity of the cyanobacteria (some type of saturation effect), so all the oxygen in excess of the saturation threshold effectively "escaped" and became unavailable for metabolic "re-consumption" at nighttime. Thinking about the longer nights that accompany the longer days, there's probably a period of time in these longer nights during which all the "nearby" oxygen has been fully consumed, and the cyanobacteria more or less sit idle.

Oxygen saturation in the surrounding environment seems like the missing logical piece from this popsci article.

I think the key is that chemical reaction rates vary exponentially with temperature. As temperature increases, reaction rates increase up to hard resource limits. But there is no limit on how slow reaction rates may get as temperature falls, and a slow-enough rate is effectively zero. Thus, an increasing fraction of the night has, effectively, no activity at all.

I think the idea is that if the night comes soon after the oxygen is produced, it can be reabsorbed quickly. If the day is longer the oxygen has more time to diffuse out of where it was produced, such that it's gone by the time the night comes.

Photosynthetic activity occurs at a relatively low efficiency. If the net energy demands during day and night cycles is not constant, then adding more time to the day could result in a surplus oxygen production, as the article suggests.

Mind, I've generally found the Great Rusting narrative to be fairly convincing: all the highly oxidised rocks on Earth's crust quite likely didn't arrive on the planet in that form, and picked up free oxygen as it was introduced to the atmosphere, effectively buffering the new addition. Though perhaps a slowing rotation did help reach and flip the tipping point of the Great Oxygenation Event.

Reliable witnesses from the period are hard to find.

A longer night means a longer time for temperature to decline. The longer day brings the temperature back up. But chemical reaction rates go exponentially with temperature. So, a lengthening night includes a growing period of negligible reaction rates, in species and processes optimized for daytime temperatures.

The opposite effect is expected during the daytime: as temperature increases, reaction rates go up exponentially until they saturate according to the limiting resource. This could still mean producing much more oxygen in the daytime than can be consumed during the period of torpor at night.

A reasonable hypothesis, though I somewhat doubt it.

Blue-green algae are aqueous. Diurnal temperature changes in even reasonably shallow bodies of water are fairly minimal.

Early Earth had much lower solar insolation than today (by about 25%), though an atmosphere largely comprised of greenhouse gasses (CO2, CH4). That would probably have also had a levelling effect on any short-cycle termperature variation.

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