Q: Did all the oxygen in Earth’s atmosphere come from photosynthesising plants? If not, where did it come from?

WITHIN the Earth’s crust, oxygen combines with all the most common atoms to form water, rock, organic compounds and almost everything else around us. Spontaneous free oxygen is about as likely as finding round rocks perched on steep slopes. Such rocks would imply that something had pushed them uphill more strongly than they could roll downhill.

Similarly, any free oxygen about us has been torn from its compounds with more than its bonding force. And that is a lot of force that only a few things are able to produce. Ionising radiation, such as X-rays, can do it, but there is little of that about. Visible light does it laboriously, step-by-step through photosynthesis, the only process that could release the level of oxygen that we see about us. That amount is calculated to be in region of 10¹⁵ tonnes.

How much oxygen plants actually produce is another matter entirely. The chloroplasts used by plants to photosynthesise are thought to have originated as symbiotic cyanobacteria. So, in effect, all our oxygen came from photosynthesising bacteria. Hence, practically all the atmospheric oxygen is of biological origin – and is not from plants but cyanobacteria.

These single-cell organisms, which were present on Earth more than 3.5 billion years ago and pre-date plants, were initially responsible for all oxygen production and are still responsible for more than 60 per cent of current oxygen production.

Cyanobacteria come in many varieties and are sometimes called blue-green algae, although they are not really algae. A species of cyanobacteria present in the ocean, Prochlorococcus marinus, is both the smallest photosynthetic organism known and the most abundant of any photosynthetic species on the planet. It was only discovered in 1988.

Previously Questions of Science: Free the atoms

Science-in-motion: a series of short articles following topics in science.

. Biodiversity

This measures how much variety there is between all the different species of life on Earth, from single-celled bacteria through insects to blue whales, the largest known animal ever to have existed. Biodiversity also describes the genetic diversity within a single species, or even the diversity of ecosystems like wetlands and forests.

Around 1.75 million species of living organisms have been identified on Earth so far, mostly small ones like bacteria and insects, and estimates suggest the true number could be as high as 100 million. But in recent centuries, there has been a rapid increase in the rate of species extinctions due to human activities such as habitat destruction for farming.

Between 1500 and 2009, international organisations documented more than 800 species becoming extinct, including the Javan tiger that died out completely in the 1980s, but the vast majority of disappearances probably go unnoticed. Conservationists grade the vulnerability of species according to a continuum scale that runs from ‘extinct’ to ‘least concern’.

Biodiversity is of huge significance. The development of new medicines, for example, is inhibited during rainforest destruction, of which many tiny organisms are either destroyed or not even previously discovered.