Promoting Life on Earth
And Beyond

Earth was dramatically different when it formed around 4.5 billion years ago. Its atmosphere consisted primarily of carbon dioxide, methane and water vapor, in contrast to the present-day atmosphere that consists primarily of nitrogen and oxygen. The earliest forms of life on Earth (having emerged over 3.7 billion years ago) were photosynthetic bacteria, such as purple non-sulfur bacteria (PNSB). These ancient phototrophs were anoxygenic; firstly, their particular form of photosynthesis (which relies on bacteriochlorophylls rather than chlorophyll as the primary photopigment) utilizes water rather than sulfide, hydrogen, organics or similar electron donors as reducing power, and therefore does not generate oxygen as a byproduct. Their reddish-purple photosynthetic pigments (which absorb green and yellow light while transmitting red and blue) preceded chlorophyll as the dominant sunlight-harvesting molecule. The Purple Earth Hypothesis proposes that the primitive Earth had a deep purple hue due to the abundance of these bacteria.

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Things changed relatively swiftly around 2.7 billion years ago with the rise of the first oxygenic bacteria. Freshly evolved from their anoxygenic ancestors, these blue-green microorganisms, the cyanobacteria. As oxygen accumulated, methane was eventually displaced, and oxygen became a major component of the atmosphere. This event, known as the Great Oxidation Event, took place between 2.4 and 2.1 billion years ago. It was essentially a mass extinction event; many anaerobic species disappeared, while a few adapted. As cyanobacteria flourished, purple bacteria receded into niche anaerobic habitats. And, as cyanobacteria evolved into green algae, and green algae evolved into plants, the color of the Earth gradually shifted from purple to green.

 

Today, purple bacteria have adapted to a variety of environments, and can be found from hot springs to the deep sea to the rhizosphere of terrestrial plants. Several PNSB genera (e.g., Rhodopseudomonas, Rhodospirillum, Rhodobacter) are counted among the plant growth promoting rhizobacteria (PGPR). PGPR are a special type of rhizobacterium that essentially improve a plant's ability to acquire nutrients while improving its immunity to diseases and pests. The beneficial effects of rhizobacteria on plants have been well documented and continue to be studied in order to develop better methods of increasing plant growth and productivity.

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The future of humankind's use of PNSB shall not for long be restricted to the confines of Earth; since the late 1980s, the European Space Agency (ESA) has been developing an integration of these microbes into circular life support systems for coming manned missions to Mars. MELiSSA (Micro-Ecological Life Support System Alternative) studies regenerative systems, seeking the highest degree of autonomy in the recovery of food, water and oxygen from mission wastes. And that's not all; PNSB such as Rhodopseudomonas and Rhodospirillum have already been investigated as potential terraforming agents for when we finally reach and colonize Mars. Perhaps we will soon be able to paint the red planet purple? That may not be a far cry--seems that Mars may have once already been covered in living purple bacteria! In fact, magnetite chains found in Martian meteorites (similar to those formed by certain Rhodospirillaceae) are suggested as evidence of previous life on Mars. It is thus entirely possible that Life on Earth arose from seed carried from Mars via meteorites.