The Origin of Life and the Role of Cyanobacteria

The planet that we live in, our Earth, is the only known planet where life exists. But the conditions were not always as friendly as they are right now. In fact, the conditions of the primitive Earth were very hostile.

Formation of earth and important events

Timeline

• 4.5 billion years ago – Formation of the Earth.
• 3.9 billion years ago – Formation of the Great Oceans.
• 3.8 billion years ago – Origin of life on Earth.
• 3.2 billion years ago – Evolution of autotrophic life forms.
• 2.7 billion years ago – Evolution of Cyanobacteria.
• 2.4 billion years ago – Great Oxidation Event.

Early Earth and atmosphere

Our Earth was formed approximately 4.5 billion years ago. The young Earth was very hot, with temperatures of more than 100°C. There were extensive volcanic eruptions releasing a huge amount of volcanic ash and gases like water vapour, carbon dioxide, sulphur dioxide, hydrogen sulphide etc. The particles of volcanic ash collided with each other, resulting in electrical discharges that caused volcanic lightning. The gases released from these eruptions formed the early atmosphere, which was slightly reducing in nature due to the presence of reducing gases like ammonia, methane, dihydrogen etc. Oxygen was either trapped in the form of oxides of nitrogen, sulphur and carbon, or in the form of water vapour. As time passed, the Earth started to cool down and the condensing water vapour poured down in the form of heavy rains. After several hundred million years of rainfall, around 3.9 billion years ago, the Great Oceans were formed. These conditions were perfect for the origin of life on Earth, but not for the evolution and existence of the diverse forms of life that exist today. The origin of life can be best explained with the help of the Oparin–Haldane Theory of Origin of Life, which will be covered in a separate article.

Origin of the first life

The first life evolved around 3.7–3.8 billion years ago, due to the conditions of the primitive Earth. Volcanic lightning provided the energy for chemical reactions between gases, resulting in the formation of simple organic molecules. These organic molecules reached the oceans along with the rain, creating the ‘prebiotic soup’. Continuous chemical reactions between these simple organic molecules led to the development of different types of complex organic compounds like carbohydrates, fats, nucleic acids and proteins. Aggregation of these biomolecules resulted in the development of protobionts (such as coacervates and microspheres), protocells and finally the first living organisms, the prokaryotes. As these organisms originated in the prebiotic soup in a reducing environment, it is assumed that they were chemoheterotrophs (obtained their energy from chemicals) and anaerobic in nature.

Around 500 million years later (3.2 billion years ago), organisms evolved that were able to prepare their own food by the autotrophic mode of nutrition. The first to evolve were chemoautotrophs (which used chemical energy to synthesize organic molecules from inorganic molecules), followed by photoautotrophs.

Anoxygenic vs oxygenic photosynthesis

The photoautotrophs were the first to harness solar energy for preparing their food, but the early photoautotrophs relied on H₂S to obtain the electrons and protons required for photosynthesis. This mode of photosynthesis does not involve the release of O₂ and is thus called anoxygenic photosynthesis. These organisms were similar to modern-day purple and green sulphur bacteria. The simplified chemical reaction of anoxygenic photosynthesis can be written as:

CO₂ + 2H₂S + Light energy → [CH₂O] + 2S + H₂O

But things started to change after the evolution of Cyanobacteria (blue-green algae), as the mode of photosynthesis performed by them was slightly different. Instead of H₂S as the source of electrons and protons, these organisms broke down water molecules. This resulted in the release of free O₂ molecules into the atmosphere for the first time. Within a period of a few hundred million years, the blue-green algae totally transformed the composition of the atmosphere, changing it from a reducing atmosphere to an oxidising atmosphere. This is referred to as the Great Oxidation Event (GOE). The simplified chemical reaction of oxygenic photosynthesis can be written as:

CO₂ + 2H₂O + Light energy → [CH₂O] + O₂ + H₂O

The Great Oxidation Event

The presence of molecular oxygen in the atmosphere also resulted in the development of the ozone layer in the upper atmosphere, preventing the majority of the harmful solar radiation from reaching the Earth and creating much more hospitable conditions. However, this change did not produce a favourable outcome for the majority of the existing life forms, as they could not adapt to the oxidising atmosphere, and this led to their extinction (hence the Great Oxidation Event is also known as the Oxygen Catastrophe). The organisms that survived laid the foundation for the evolution of much more complex life forms like eukaryotes.

Thus, it can be concluded that Cyanobacteria were the pioneer organisms responsible for modifying the atmosphere into its current oxidising form, making it much more stable than the conditions during the origin of life on Earth.

Cyanobacteria- ultrastructure

• Either unicellular, colonial, or with a long filamentous body called a trichome.
• Lack flagella; movement is provided by the mucilage layer surrounding the body.
• The body of filamentous BGA consists of two main types of cells: vegetative cells and heterocysts.
• The cell wall is multi-layered and made up of peptidoglycan, a polymer with long chains of modified sugars like N-acetyl glucosamine and N-acetyl muramic acid, linked together by tetrapeptide side branches.
• The cytoplasm can be divided into the pigment-containing peripheral part, the chromoplasm, and the colourless central part, the centroplasm.
• The chromoplasm contains membranous structures called chromatophores, with a wide range of pigments like chlorophylls, carotenoids and phycobilins for absorbing light of different wavelengths.
• The centroplasm contains structures like ribosomes, gas vacuoles, proteins, enzymes and aggregates of other such molecules (inclusion bodies).
• The genetic material is present in the cytoplasm itself, as BGA, being prokaryotes, lack a well-defined nucleus.
• Heterocysts are specialised thick-walled cells where fixation of atmospheric nitrogen takes place.
• Special conditions must be maintained in heterocysts to perform nitrogen fixation:
  • Anaerobic conditions
  • Production of the nitrogenase enzyme
  • Absence of oxygen-evolving photosystem II
• Reproduction: only the asexual mode of reproduction, either by binary fission (unicellular forms), by fragmentation (filamentous forms) or with the help of specialised large vegetative cells with abundant food reserves called akinetes.

Important examples of Cyanobacteria

• Unicellular – Microcystis aeruginosa
• Filamentous – Nostoc, Anabaena, Oscillatoria (lacks heterocysts)

* All the data are obtained from various books and scientific research papers available on NCBI.