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How a single-celled organism almost wiped out life on Earth - Anusuya Willis


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There’s an organism that changed the world. It caused the first mass extinction in Earth’s history … and also paved the way for complex life. How? Anusuya Willis explains how cyanobacteria, simple organisms that don’t even have nuclei or any other organelles, wrote a pivotal chapter in the story of life on Earth.

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Cyanobacteria are small aquatic single celled prokaryotic organisms. Prokaryote means that they have no intracellular membrane-bound organelles. All bacteria are prokaryotes. Between 2.5 – 3.5 billion years ago, cyanobacteria evolved oxygenic photosynthesis. Photosynthesis occurs in structures called ‘thylakoid membranes’ where the energy from sunlight is used to split a water atom into oxygen and hydrogen molecules and releases an electron. The energy from the electron is used to combine carbon dioxide with hydrogen and oxygen to make sugar.  There is one left over oxygen molecule in this process. Here’s the equation for photosynthesis: 6CO2 + 6H2O + sunlight à C6H12O6 + O2.

When oxygen enters cells it forms superoxide (O2-). This is a reactive oxygen species and will react with molecules in the cell destroying them. Aerobic organisms' cells have the enzymes superoxide dismutase and catalase, which neutralize superoxide and protect the cells. Anaerobic organisms lack these enzymes and so cannot protect themselves from superoxide, which is why oxygen is toxic to them. Before the evolution of oxygenic photosynthesis, all organisms on Earth were anaerobic. Thus when the composition of the atmosphere changed during the “great Oxygen Event’, anaerobic organisms were nearly all destroyed. Anaerobic organisms still exist on Earth today, and live in areas without oxygen such as deep in the ocean.

Cyanobacteria are commonly called ‘blue-green algae’ because of their blue color, which originates from phycocyanin, a secondary pigment in the chloroplasts. Different pigments capture different wavelengths of light, or different colors, allowing photosynthetic organisms to use more of the available light. These are called accessory pigments. Chlorophyll-a is the main pigment found in all cyanobacteria, algae and plants, chlorophyll-b is found in algae and plants, and chlorophyll-c is found in some algae. Accessory pigments are carotenoids (orange), fucoxanthin (brown), phycobilins (phycocyanin (blue) and phycoerythrin (red).  These are found in different plants and algae, and are also the pigments that make autumn leaves red and yellow

On the Earth today Cyanobacteria are found in all aquatic areas, marine, freshwater, and moist soil. They are commonly known as ‘blue-green algae’ and can cause massive blooms. Blooms are when a population of cyanobacteria grows quickly to become the dominant organisms in the water. This can be a big problem in freshwater systems as many of the cyanobacteria species produce compounds that are toxic to humans and other animals. These toxic compounds can pollute drinking-water storage and can be expensive to treat. 

The frequency and severity of cyanobacteria blooms appears to be increasing globally. Blooms are promoted by water pollution, in particular excess fertilizers that run off the land in aquatic ecosystems. Additionally, increasing temperatures, with climate change, are also expected to promoted cyanobacteria blooms, as they often have greater tolerance to higher temperatures than other phytoplankton.

Interested in learning a bit more about photosynthesis? This TED Ed lesson will help: The simple story of photosynthesis and food - Amanda Ooten.

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