Virtually all oxygen on Earth was and is produced by photosynthesis, which
was invented by tiny organisms, the cyanobacteria, when our planet was still
a rather uninhabitable place. Cyanobacteria evolved more than 2.4 billion
years ago, but Earth only slowly transformed to the oxygen-rich planet we
know today. “We do not fully understand why it took so long and what factors
controlled Earth’s oxygenation,“ said geomicrobiologist Judith Klatt. “But
when studying mats of cyanobacteria in the Middle Island Sinkhole in Lake
Huron in Michigan, which live under conditions resembling early Earth, I had
an idea.”
Cyanobacteria are late risers
Klatt worked together with a team of researchers around Greg Dick from the
University of Michigan. The water in the Middle Island Sinkhole, where
groundwater seeps out of the lake bottom, is very low in oxygen. “Life on
the lake bottom is mainly microbial, and serves as a working analog for the
conditions that prevailed on our planet for billions of years”, says Bopi
Biddanda, a collaborating microbial ecologist from the Grand Valley State
University. The microbes there are mainly purple oxygen-producing
cyanobacteria that compete with white sulfur-oxidizing bacteria. The former
generate energy with sunlight, the latter with the help of sulfur.
In order to survive, these bacteria perform a tiny dance each day: From dusk
till dawn, the sulfur-eating bacteria lie on top of the cyanobacteria,
blocking their access to sunlight. When the sun comes out in the morning,
the sulfur-eaters move downwards and the cyanobacteria rise to the surface
of the mat. “Now they can start to photosynthesize and produce oxygen,”
explained Klatt. “However, it takes a few hours before they really get
going, there is a long lag in the morning. The cyanobacteria are rather late
risers than morning persons, it seems.” As a result, their time for
photosynthesis is limited to only a few hours each day. When Brian Arbic, a
physical oceanographer at the University of Michigan, heard about this diel
microbial dance, he raised an intriguing question: “Could this mean that
changing daylength would have impacted photosynthesis over Earth’s history?”
Daylength on Earth has not always been 24 hours. “When the Earth-Moon system
formed, days were much shorter, possibly even as short as six hours,” Arbic
explained. Then the rotation of our planet slowed due to the tug of the
moon’s gravity and tidal friction, and days grew longer. Some researchers
also suggest that Earth’s rotational deceleration was interrupted for about
one billion years, coinciding with a long period of low global oxygen
levels. After that interruption, when Earth’s rotation started to slow down
again about 600 million years ago, another major transition in global oxygen
concentrations occurred.
After noting the stunning similarity between the pattern of Earth’s
oxygenation and rotation rate over geological timescales, Klatt was
fascinated by the thought that there might be a link between the two – a
link that went beyond the “late riser” photosynthesis lag observed in the
Middle Island sinkhole. “I realized that daylength and oxygen release from
microbial mats are related by a very basic and fundamental concept: During
short days, there is less time for gradients to develop and thus less oxygen
can escape the mats,” Klatt hypothesized.
From bacterial mats to global oxygen
Klatt teamed up with Arjun Chennu, who then also worked at the Max Planck
Institute for Marine Microbiology and now leads his own group at the Leibniz
Centre for Tropical Marine Research (ZMT) in Bremen. Based on an open-source
software developed by Chennu for this study, they investigated how sunlight
dynamics link to oxygen release from the mats. “Intuition suggests
that two 12-hour days should be similar to one 24-hour day. The sunlight
rises and falls twice as fast, and the oxygen production follows in
lockstep. But the release of oxygen from bacterial mats does not, because it
is limited by the speed of molecular diffusion. This subtle uncoupling of
oxygen release from sunlight is at the heart of the mechanism,” said Chennu.
To understand how the processes occurring within a day can impact long-term
oxygenation, Klatt and her colleagues incorporated their results into global
models of oxygen levels. The analysis suggests that the increased oxygen
release due to daylength change could have boosted oxygen levels globally.
It is a link between the activity of tiny organisms and global processes.
”We tie together laws of physics operating at vastly different scales, from
molecular diffusion to planetary mechanics. We show that there is a
fundamental link between daylength and how much oxygen can be released by
ground-dwelling microbes,” said Chennu. “It’s pretty exciting. This way we
link the dance of the molecules in the microbial mat to the dance of our
planet and it’s Moon.”
Overall, the two major oxygenation events (jumps in oxygen concentration) in
Earth’s history – the Great Oxidation Event more than two billion years ago
and the later Neoproterozoic Oxygenation Event – might be linked to
increasing daylength. Hence, increasing daylength could have boosted benthic
net productivity sufficiently to impact atmospheric oxygen levels. “Juggling
with this wide range of temporal and spatial scales was mind-boggling – and
lots of fun,” Klatt concludes.
Reference:
Klatt, J.M., Chennu, A., Arbic, B.K. et al. Possible link between Earth’s
rotation rate and oxygenation. Nat. Geosci. (2021).
https://doi.org/10.1038/s41561-021-00784-3