a bonobo humanity?

green stuff

So, since reading about photosynthesis and its emergence with cyanobacteria – perhaps – I’ve been fascinated and confused by the early ages of this planet and the beginnings of life. Here’s how Oliver Morton defines these bacteria in the glossary to Eating the Sun:

Cyanobacteria: the only bacteria which practice oxygenic photosynthesis. The chloroplasts in algae and plants are derived from cyanobacteria.

Is there another kind of photosynthesis? Clever me looked it up and, yes, anoxic photosynthesis is a thing. It’s a ‘light-driven metabolic process used by certain bacteria to make energy and organic compounds without producing oxygen as a byproduct’, according to AI.

Chloroplasts, to be clear to myself, are plant and algal organelles which convert sunlight into chemical energy. This photosynthesis process uses chlorophyll, a sunlight-absorbing green pigment, to turn CO2 and H2O into glucose and O2. It absorbs red and blue wavelengths and reflects green, hence the colour of all plants (I love that), and I notice it’s popular in dietary supplements – bien sûr!

So how did these cyanobacteria evolve, and how are they going these days? Apparently they evolved in the ocean – remembering how very watery our planet once was – some 3 billion years ago or more. The general view apparently is that those ocean waters were rather hot, with barely any oxygen. Anaerobic microbes existed in these waters (here be magic?) – for whom oxygen was toxic, although there were some ‘aerotolerant anaerobes’ who didn’t mind oxygen but had no use for it.

So cyanobacteria, aka blue-green algae, are thought to have evolved in freshwater (low salt) environments, in the ‘photic zone’, that’s to say the upper, sunlit layer. We only have clear evidence of their existence from 2.1 billion years ago, and disputed evidence from 2.7 billion, and maybe they go back to 3.5 billion, but their effect on the planet, via the ‘Great Oxygenation Event’ (GOE), wasn’t felt until after about 2.4 billion. This rise in oxygen is likely to have adversely affected the biosphere of the time (made up of archaea, not bacteria), which fed largely on methane.

Much of our evidence of early life forms is based on stromatolites and oncolites. Stromatolites (prime examples are found at Shark Bay, Western Australia), are ‘layered biochemical accretionary structures formed in shallow water [the photic zone] by the trapping, binding, and cementation of sedimentary grains by biofilms (microbial mats) of microorganisms, especially cyanobacteria’ (Wikipedia), and they are the oldest known fossil remains. Oncolites are small near-spherical structures forming around a spherical nucleus, like a shell fragment, and are untethered, unlike stromatolites. They’re an indication of warming waters in the photic zone, and are also found in contemporary freshwater. Oh, and they’re also formed by cyanobacterial growth.

So here’s another Wikipedia quote which I’ll then try to make my own sense of:

Oxygenic photosynthesis only evolved once (in prokaryotic cyanobacteria), and all photosynthetic eukaryotes (including all plants and algae) have acquired this ability from endosymbiosis with cyanobacteria or their endosymbiont hosts. In other words, all the oxygen that makes the atmosphere breathable for aerobic organisms originally comes from cyanobacteria or their plastid descendants

Aerobic organisms, that’s us, amongst quite a few others. So, to unpack. I sort of know the prokaryote to eukaryote story, but let me go a bit deeper, or a bit less shallow. It’s where  endosymbiosis comes in – a symbiosis, I think, where it’s the end o’ me as an independent organism, but I live on as an essential element in something bigger and grander, usually. Mitochondria are often the go-to example, former bacteria harnessed by eukaryotes to break down nutrients and convert them to energy in the form of ATP.

So what is ATP and why is it the energy molecule? See what I did there? I forced myself further into the murk. Adenosine triphosphate, the energy provider of all living cells, is made up of a nitrogenous purine base called adenine – formula C5H5 N5 (those three elements are always in there somewhere), and fundamental to DNA and RNA and no doubt much else that sparks with life.

This reminds me that there are purines and pyrimidines, but getting back to energy, AI never lies (and I have to say I’m quite frightened of this AI shite, because, as we all know, it’s controlled by billionaires who don’t have the interests of people like myself in mind) tells me this:

ATP does not technically create energy; instead, it acts as a chemical battery that stores and delivers energy. It releases this energy when a water molecule breaks the weak bond holding its third phosphate group off, turning ATP into ADP (adenosine diphosphate) and releasing usable power for the cell.

So why does this bond get broken? And I note that there’s also adenosine monophosphate (AMP). And the adenosine part consists of adenine, a nitrogenous base, and ribose (sugar). And all these complicated changes and breaking-downs are part of the process called metabolism, the conversion of the energy food provides into molecules that keep us going, such as proteins, but of course many others.

I’ve never spent much more than an hour or two in a lab, so I find it hard to think through these metabolic developments. Let me return to the ubiquitous cyanobacteria. I may be repeating myself, but mea culpa – they have a vast variety of forms (all with much the same colour), and some are much given to symbiosis with various unicellular and multicellular organisms. So, with their photoautotrophic, oxygen-producing abilities they’ve been the sine qua non of complex, oxygen-based life on this planet. I think I’ll just stop here.

References

Oliver Morton, Eating the Sun: the everyday miracle of how plants power the planet, 2007

https://en.wikipedia.org/wiki/Cyanobacteria