Bacteria’s evolution sheds light on great oxygenation event


Cyanobacteria may have learnt the trick of photosynthesis by stealing genes from other microbes. Stephen Fleischfresser reports.


A light microscopy image of a Cyanobacterium (Oxyphotobacteria).
Fischer Laboratory/CAL TECH

All life as we know it owes its existence to the thieving ways of pondscum, new research shows.

Cyanobacteria is fundamentally responsible for generating the earth's atmospheric oxygen and kickstarting the evolution of modern biology, and it developed the ability to produce oxygen by stealing genes from another, unknown species of microbe, a new paper published in Science argues.

Rochelle Soo, Donovan Parks, and Philip Hugenholtz from the University of Queensland and Jim Hemp and Woodward Fischer from California Institute of Technology have published their findings concerning the evolutionary tree of cyanobacteria.

Despite not being related, cyanobacteria are sometimes called blue-green algae, and are thought to be one of the most ancient organisms on the planet. Fossilised cyanobacteria in the form of Stromatolites found in Western Australia have been dated as far back as 3.5 billion years ago.

Importantly cyanobacteria are photosynthetic, which means they convert sunlight into usable energy and produce dioxygen (O2) as a by-product. Photosynthesis is the process by which all life ultimately gets its energy. But just when, and how, this oxygenic photosynthesis became a feature of these archaic lifeforms (called Oxyphotobacteria) has been a topic of some speculation.

Making this even more difficult was the absence of evidence of closely related organisms (sister taxa) or evolutionary precursors. In 2013, however, a sister taxa (Malainabacteria) was discovered for the first time. Soo and colleagues are now reporting the discovery of yet another; Sericytochromatia.

Interestingly these sister taxa seem not to have been able to carry out photosynthesis of any kind, indicating that these taxa split from the known cyanobacteria before the latter evolved the ability to photosynthesise.

This leads the researchers to conclude that that the ancestors of modern cyanobacteria gained this capacity by what is called’ lateral gene transfer’ which involves the movement of genetic material between extant organisms, not from parent to offspring.

Genes for parts of the photosynthetic process must have come from some other microbe, the authors argue, and then these evolved further within the ancestors of Oxyphotobacteria. Remarkably, this indicates that oxygenic photosynthesis evolved in only one branch of the cyanobacterial family.

This is one of the first times anyone has been able to establish how the Oxyphotobacteria might have evolved. As Fischer says, “It’s a big deal that we can now say with some certainty that lateral transfer was important…”

It’s also a big deal that it is these bacteria that are responsible for the Great Oxidation Event, the period in earth’s history starting 2.4 billion years ago during which there was an explosion in abundance of molecular oxygen.

This had profound consequences, the mass extinction of anaerobic bacteria and the creation of many of the world’s minerals, amongst them.

Most importantly for us, however, was the production of the environment conducive to the evolution of the most recent and familiar of the three domains of life, the eukaryotes, to which all plants, animals and fungi belong.

So, who do we thank for all this free oxygen? And being alive?

Fischer suggests that while it might be tempting to think the genes for oxygenic photosynthesis came, via lateral transfer, from one of the six phyla of bacteria capable of non-oxygenic photosynthesis, in reality, “… it seems just as possible that whoever gave Cyanobacteria the genes for photosynthesis went extinct long ago.”

Stephen fleischfresser.jpg?ixlib=rails 2.1
Stephen Fleischfresser is a lecturer at the University of Melbourne's Trinity College and holds a PhD in the History and Philosophy of Science.
  1. http://science.sciencemag.org/content/355/6332/1436
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