Bacteria’s evolution sheds light on great oxygenation event – Cosmos

A light microscopy image of a Cyanobacterium (Oxyphotobacteria).

Fischer Laboratory/Caltech

One of the most momentous events in the history of the planet is now better understood thanks to the newly uncovered evolutionary history of pond scum the erroneously named blue-green algae.

Research published in Science provides fresh insight into the when and how of the Great Oxidation Event the period in Earths history during which the levels of atmospheric oxygen rose dramatically. Paradoxically, this event caused one of the largest-known extinction events and simultaneously paved the way for life as we know it.

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

Cyanobacteria are sometimes called blue-green algae, despite not being algae at all, 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. But just when, and how, this oxygenic photosynthesis became a feature of these archaic life forms (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 the ancestors of modern cyanobacteria gained this capacity by lateral gene transfer the transfer of genetic material between extant organisms, in contrast to vertical gene transfer, which is the process of parents conferring genetic material 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 the first time anyone has been able to establish how the oxyphotobacteria might have evolved. As Fischer says: Its a big deal that we can now say with some certainty that lateral transfer was important.

It is also a big deal that it is these bacteria responsible for the Great Oxidation Event.

This explosion in abundance of molecular oxygen in the Earths atmosphere had profound consequences. The first was the extinction via oxygen-toxicity of many types of anaerobic bacteria. The second 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.

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 extant bacteria capable of non-oxygen producing photosynthesis, it seems just as possible that whoever gave Cyanobacteria the genes for photosynthesis went extinct long ago.

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Bacteria's evolution sheds light on great oxygenation event - Cosmos