Ancient SA bacteria hold secret to early life


Gavin Rishworth

Nelson Mandela Metropolitan University

It all started 3.5-billion years ago. Tiny, microscopic bacteria hustled an existence on the coastal seafloor in a world devoid of other life. Over time, these ancient organisms multiplied and diversified, developing into extensive colonies that coated and stabilised the bottom of the ocean. By tapping into chalk-like inorganic calcium carbonate reserves – which were abundant when the Earth was young – within the water, they built extensive reef-like habitats.

This formed the basis for life on our planet.

Within these sludgy reefs, microscopic algae evolved, and produced the oxygen that made the cascade of evolution possible. These tiny organisms radically changed the marine world, and built the cornerstone upon which other forms of life, and ultimately humans, could evolve and thrive.

These primordial reefs – known as “stromatolites”, which means “layered rocks” because of the organised way in which the bacteria lay-down their calcium-carbonate building-blocks – still exist today. By looking at ancient stromatolite fossils, which are the reefs left behind by the bacteria, we can decode the evolution of life on Earth.

We have more ancient stromatolites than living ones: there are fewer than a dozen living, marine stromatolite systems around the world, one of which is in South Africa. Through studying these living systems, we can understand the role their ancient counterparts played during the evolution of life.

These stromatolites are found in South Africa along a 200 km stretch between Port Elizabeth and Storms River on the south-eastern coast of the country. They form in tidal pools where groundwater mixes with salty marine water. Most scientists agree that the first life to appear on land crawled from the sea. The unique location of these stromatolites – where land meets ocean – may have sparked and encouraged this revolutionary transition of life.

Modern stromatolites are scarce because the ocean’s chemistry has changed – about 1.5-billion years ago when ancient stromatolite reefs were most common and abundant, the Earth only had a single supercontinent, Columbia or Nuna, which was beginning to break apart. The oceans then, unlike today, were rich in calcium carbonate, which is necessary for stromatolite growth. Also, more than a billion years ago, stromatolites-creating bacteria did not have much competition. Today, animals and algae, which ironically exist due to stromatolites, can outcompete the bacteria or graze upon and destroy the growing stromatolite structure.

This makes the modern stromatolites along South Africa’s coastline surprising. In short, we don’t know precisely why there are here. Our research group at the Nelson Mandela Metropolitan University (NMMU), led by Prof Renzo Perissinotto, is looking for answers.

South Africa’s stromatolite area is rather unique: calcium-carbonate-rich groundwater sustains the pools, creating an environment more like that which they would have enjoyed 1.5-billion years ago. While this may be perfect for the bacteria that form stromatolites, it is not necessarily ideal for most other life. The regular interaction between fresh and salty water orchestrates harsh conditions: the temperatures swing between extremes; sometimes the water is salty, sometimes it isn’t; and there are even times when there is very little oxygen in the water.

Those invertebrates living in these habitats, such as crustaceans, snails and worms, do not destroy the stromatolite layering – this is rather peculiar because that is exactly what has happened both to most other modern stromatolites and throughout the fossil record.

It seems as though the stromatolites provide a refuge for creatures and organisms from the harsh conditions within this environment. This was a crucial element of their ancient role: the algae that stromatolite reefs harboured allowed animals, several millennia after the algae first started releasing oxygen, to evolve a mobile lifestyle,. Food was plentiful away from the stromatolites but at the same time oxygen was scarce. Animals which could move freely between the stromatolites – a source of vital oxygen – and the distant food were far more successful than organisms which were not mobile.

Stromatolites are a fundamental piece in the puzzle of life’s evolution, and their modern descendants spoil us with the remarkable opportunity to test our theories of what might have happened billions of years ago.

But like most other ocean organisms, modern stromatolites are feeling the pressure of climate change and humans. We have already seen this in similar habitats along Australia’s western coastline. The changing, poorer water quality threatens the existence of their stromatolites, to the extent that the Australian government has begun actively monitoring water quality standards.

South African researchers at NMMU are investigating the influence of human disturbances on our stromatolites, such as water pollution in the form of rising nutrient levels or changes in groundwater due to water removal for residential or commercial use. These stromatolite environments enhance South Africa’s unique biodiversity heritage and their global intellectual importance means that we need to protect them, as we would any other threatened marine species of national or global importance.

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