Long before plants evolved, cyanobacteria developed the ability to split water (H2O) with light, producing oxygen (O2) as a waste product. The evolution of this new ability caused oxygen to begin accumulating in the earth’s atmosphere and oceans about 2.5 billion years ago. However, for some reason the earth’s oxygen transformation stalled, and it took another 2 billion years for oxygen concentrations to approach their current-day levels. The cause of the long delay in the earth’s oxygen evoluton is the subject of debate among scientists who study habitable planets, including Earth and planets outside our solar system that could potentially support life.

Little Salt Spring is a flooded cave that has water containing much less oxygen than the modern oceans, more similar to oceans during the Proterozoic Eon, between 2.5 and 0.5 billion years ago. Also like Proterozoic oceans, water in Little Salt Spring contains hydrogen sulfide (H2S), produced when microorganisms react organic matter with salts present in seawater. We proposed to study the ecosystem in Little Salt Spring in order to learn why oxygen concentrations in the cave water remain very low, even though they are in contact with the oxygen-rich atmosphere above the water surface. Our team included highly trained technical and science divers who performed experiments and made measurements underwater, so that we could avoid artifacts that might be created by bringing samples into the lab. Our team also included experts in geochemistry and microbiology who measured the chemicals dissolved in the water, and sucessfully learned how to grow the main microbial species in the laboratory (a rare achievement given than less than 1% of microorganisms can generally be grown outside their natural habitats). Our team included both experienced scientists and younger scientists and graduate students that were guided through all stages of doing science, from planning experiments to communicating results. We maintained close ties with local environmental conservation groups interested in the health of the spring, and with the University of Miami administration that controls access to the spring and surrounding land. Our results were communicated in 12 presentations at scientific conferences and universities, and 4 scientific papers published in top journals in this field.

We learned that Little Salt Spring is home to an ecosystem dominated by cyanobacteria, much as we would expect for shallow water zones of the Proterozic oceans before the evolution of algae, plants and animals. The most abundant primary producer in the ecosystem is a purple cyanobacterium that we named “hensonii” after muppets creator Jim Henson, beause of it’s tufted, furry appearance during growth. By measuring the production and consumption of oxygen and other chemicals in the purple “hensonii” mat, we learned that unlike plants and most modern cyanobacteria, it grows for most of each day without splitting water, and therefore produces little oxygen. If Proterozoic cyanobacteria behaved in a similar way, it could explain why oxgyen levels remained low in the Proterozoic Eon despite the presence of cyanobacteria with the potential to produce oxygen.

Why does the purple “hensonii” mat grow for most of the daylight hours without producing oxygen?  Below the sediment surface, in deeper oxygen-free zones of sediments, organic matter produced by photosynthesis is consumed by a specialized group of bacteria that “breathe” sulfate (SO42- , one of the salts present in seawater) because oxygen is not available. The waste product of bacterial sulfate breathing is hydrogen sulfide (H2S), which interferes with the cell machinery cyanobacteria use to split water and produce oxygen. Our measurements show that the “hensonii” cyanobacterium switches quickly and easily between oxygen-producing and non-oxygen producing kinds of photosynthesis, and that it produces oxygen only when sulfide is completely consumed by either rust particles (FeOOH) or other light-harvesting bacteria in the mat. This leads us to suggest that the presence of rust and close spatial associations between bacterial cells common in microbial mats may have been critical factors allowing oxygen production to overcome sulfide in Proterozoic oceans. In the near future we may be able to test these ideas directly by conducting more experiments at Little Salt Spring, and by examining the rock record for clues connecting rust, hydrogen sulfide, and cyanobacterial oxygen production.

Last Modified: 04/20/2017
Modified by: Jennifer L Macalady