2018 Volume 34 Issue 2 Pages 57-72
The emergence of molecular oxygen changed the Earth and the life on it irreversibly and is responsible for the current global environment and ecosystem. Oxygen was produced as a byproduct of the oxygenic photosynthesis that appeared in cyanobacteria (blue-green algae), which may date back 3.5 billion years. Since then, oxygen has been oxidizing the atmosphere and oceans. There were two great oxygenation events, GOE-I and GOE-II, that occurred around 2.5 and 8 billion years ago. After the first“ snowball Earth” event, GOE-I triggered the evolution of eukaryotes. Oxygenic photosynthesis was transferred to eukaryotes and generated eukaryotic algae possessing plastids, called primary plants, via primary endosymbiosis. Plastids spread to various eukaryotic lineages via the endosymbiosis of red and green algae (secondary endosymbiosis). These new algal lineages are known as secondary plants. GOE-II drove the evolution of multicellular organisms. Green algae evolved into land plants and contributed to the development of the terrestrial ecosystem. The secondary plants dominated the ocean at the boundary between the Paleozoic and Mesozoic eras. They helped to form today's marine ecosystem and continue to play important roles in driving the carbon cycle and circulation of water and sulfur. However, the diversity of algae is not well understood even today. Research is needed to understand the true contribution made by algae to the global ecosystem.
