Today’s post features a super photosynthesizer that doesn’t require the sun. It’s truly an example of an extreme life form and pushes the very definition of photosynthesis. This is the story of GSB1.
The ocean supports a multitude of life forms, and it should be no surprise to find photosynthetic organisms there. It offers 139.4 million square miles with a ‘sunlight zone’ of 656 feet, where sufficient light penetrates to support photosynthesis. That’s a lot of potential habitat. The oxygenic photosynthetic organisms in this zone contribute significantly to global nutrient cycles for carbon and nitrogen. However, you won’t find GSB1 anywhere near there.
GSB1 was found near a hydrothermal vent called Nine North at a depth of 7875 feet in the Pacific Ocean. It was identified by automated water sampling and DNA sequencing that confirmed the presence of a green sulfur bacterium- an obligate photosynthetic organism that requires light and sulfur to live. Since sunlight only penetrates to 3280 feet, you may be wondering how GSB1 can be photosynthetic. The emissions from the hydrothermal vents are so intense that the black body radiation they emit borders between infrared to red visible light. This also means that these bacteria also live on the edge of an extreme temperature difference (662 F vs. 36 F) at the interface of the vent emissions and the cold water of the ocean bottom. However, the meager ‘photon droppings’ found there are sufficient for the survival of GSB1.
This isn’t too surprising for scientists that have been studying green sulfur bacteria for decades. These organisms are champions at harvesting light with their elaborate chlorosome antenna system. Chlorosomes are antenna structures filled with hundreds of thousands of bacteriochlorophylls (bacteriochlorophyll c to be exact) that absorb red and far-red light. These are energetically coupled to a baseplate comprised of a protein that contains a bacteriochlorophyll a and another layer of bacteriochlorophyll a protein called FMO. All of these pigments and pigment-protein complexes funnel harvested light energy into the photosynthetic reaction center of the green sulfur bacterium. This flux of energy is sufficient for survival as an autotroph.
I’ll leave it up to you as to whether GSB1 constitutes something new under the sun, but it does open up new possibilities as to where life might exist within our universe. In that sense, there may be some GSB1-like organism eking out a photosynthetic living on some distant moon or planet under some sun or source of radiation.