I was formally introduced to Purple Bacteria very recently, when I began collecting samples with a microbiologist friend of mine, Matt Melnicki, near the shores of the Salton Sea. I wanted to see if they produce any meaningful data. Well, they do, but that’s for next week’s article. Today is more sciency, where I want to share what I learned all about this amazing family of microorganisms.

Purple bacteria, yes, that is the name of them, once ruled the planet. This was a mere 2.5-3.2 billion years ago. These microbes were among the first organisms to transform sunlight into chemical energy. This was before the Great Oxidation Event, where the oxygen that we know and love suffused the Earth’s atmosphere. The atmosphere was likely made of nitrogen (N₂) and carbon dioxide (CO₂) at the time.

Purple Bacteria thrived in this environment, feeding on hydrogen sulfide and other reduced compounds that seeped from volcanic vents and anoxic mud. This was more than two billion years ago, in the deep dawn of life, when sunlight was abundant but the chemistry of the oceans was alien to anything alive today. The purple bacteria were, and still are, anaerobic, since there was no oxygen around.

“Chromatium okenii al microscopio (600×)” by Mariiantoniietta, licensed under CC BY-SA 4.0. Source: Wikimedia Commons — File:Chromatium Okenii al microscopio.jpg

Not Your Usual Photosynthesis

Purple bacteria were the first bacteria discovered to practice anoxygenic photosynthesis — photosynthesis that does not produce oxygen as a byproduct. Instead, purple bacteria utilize electron donors like hydrogen sulfide (H₂S), which produces elemental sulfur (S⁰) instead of oxygen as a byproduct of photosynthesis, giving off a Hades-like odor.

The purple bacteria use special pigments called bacteriochlorophylls that absorb light in the near infrared range (~720 nm to 1050 nm), unlike the chlorophyll of plants that absorb most strongly in the blue-violet (~430 nm) and red (~662 nm) of the visible spectrum. These unique bacteriochlorophyll pigments enable them to capture the faintest glow beneath the surface in extreme anoxic environments inhospitable to most lifeforms, just below where sunlight fades into darkness.

Interestingly, the ability to utilize bacteriochlorophylls in the infrared range has prompted researchers to use purple bacteria in biotechnological applications for treating anoxic wastewater with high pollutant loads, where their unique light-harvesting ability allows efficient energy use under low-oxygen conditions.

The Purple Earth Hypothesis

What if purple bacteria still dominated the Earth instead of land plants, algae, and cyanobacteria? Scientists still debate the earliest era of this choreography. Some now suggest that before chlorophyll-based photosynthesis arose, the planet might have glowed purple, a world dominated by bacteriochlorophyll pigments tuned to the reddish end of the spectrum. This idea, called the Purple Earth Hypothesis, imagines an ancient biosphere painted not in green abundance, but in violet or magenta survival. The Earth then would not appear blue and green as we see it today, but instead tinted by countless purple microbial mats rippling in the shallows.

The Great Oxygenation Event Changed Everything

Then, some 2.4 billion years ago, some ancient lineages of purple bacteria, most likely the common ancestors of modern purple bacteria and cyanobacteria, adapted to use water as an electron donor, releasing oxygen as a waste product. This shift changed everything when it precipitated the Great Oxygenation Event. Oxygen slowly accumulated in the atmosphere, reacting with iron in the oceans, rusting the planet red, and paving the way for more complex aerobic lifeforms like our own, forever changing the course of the history of life on Earth. But that’s a story for another day.

Still, for the purple anoxygenic photosynthetic pioneers, oxygen was poison. Their dominance of the Earth ended with the Great Oxygenation Event. After that, they were forced to retreat to sulfur springs, stagnant ponds, and the deep sediments where they still survive to this day.

Stacking Microbial Mats

But here’s where it gets even more interesting to me. These purple bacteria have survived and even thrived in their microbial mats for billions of years. As oxygen levels increased and ecosystems formed layers through the water column, photosynthetic organisms evolved to utilize the remaining wavelengths. Cyanobacteria and green algae dominated the oxygen-rich surface, absorbing blue and red light for photosynthesis and reflecting green, releasing more oxygen into their environment in the process.

Beneath them, in murky or oxygen-poor zones, purple bacteria continued absorbing in the near infrared as the water column itself also attenuates the near infrared to less than 1% of the surface, further depleting the available light in their zone. They also don’t need oxygen to survive; it’s poison to them, so they can live underneath in the anoxic zone, harvesting whatever near-infrared light is available for photosynthesis.

“Purple and Green Sulfur Bacteria and Their Biomarkers” by Alatleephillips, licensed under CC BY-SA 4.0. Source: Wikimedia Commons — File:Purple and Green Sulfur Bacteria and Their Biomarkers.png

Below that, the green sulfur bacteria survive and thrive in yet another anoxic layer. Green sulfur bacteria possess light-harvesting chlorosomes that form self-aggregated structures, resulting in red-shifted absorption bands. This allows them to utilize light energy at greater depths than other organisms that have these chlorosomes, allowing them to take what little light remains in the water column below the purple sulfur bacteria, harvesting far-red photons that are invisible to all others.

This is how “stacking” works in microbial mats: each species sits in its own spectral niche, filtering the light and passing the leftovers down to the next. Those who find a way to use what’s left survive and thrive in the following stacked mat below.

Final Thoughts

If I had a time machine for a day, I’d love to go back with an oxygen tank and check out the possibly purple Earth. What would a sunset look like? What would the oceans look like? I can only imagine.