Sourse:Scientific American November 2021
(Changes in Earth's spin mayhave influenced the gas'sbuildup in the atmosphere)
When Judith Klatt began studying thecolorful mats of primitive microbes living ina sinkhole at the bottom of Lake Huron,shethought she might learn something aboutEarth's early ecosystems. Instead Klatt,abiogeochemist at the Max Planck Institutefor Marine Microbiology in Bremen,Ger-many,wound up confronting one of geolo-gy's greatest unsolved mysteries: How,exactly, did Earth become the only planetknown to have an oxygen-rich atmosphere?
Geologic clues suggest microbes mayhave started releasing oxygen via photosyn-thesis as early as three billion years ago oreven before. But for some reason, it tookabout half a billion years for that oxygen tobuild up in the atmosphere and then a billionmore for it to reach modern levels and setthe stage for complex life.These delayshave long puzzled scientists. Some haveproposed that chemical reactions consumedmuch of the gas or that a lack of essentialnutrients limited its production.
Now,inspired by the sinkhole work,Klatt and her colleagues have identifiedanother possible explanation, which theydescribe in Nature Geoscience: early Earth'sdays were simply too short.
Soon after the solar system formed, aMars-sized object crashed into Earth andsent up a spray of debris that became the
Jennifer ldol Stocktrek Images and Getty Images moon. Drag from our natural satellite hasgradually slowed the planet's rotation eversince,increasing day length from about sixhours in Earth's youth to 24 hours today.Scientists have known about this slow-down for decades and are continuing torefine the details.But few had linked it tooxygen levels-until University of Michi-gan oceanographer Brian Arbic heard atalk about Klatt's work with a Lake Huronsinkhole.Arbic,a co-author on the newpaper,wondered whether changing daylength could have affected photosynthesisover geologic time.
Because it is fed by oxygen-poor,sulfur-rich groundwater,the sinkhole approxi-mates conditions on early Earth,hostingcommunities of microscopic bacteria thatblanket the lake bottom in purple and whitemats. Klatt and her colleagues examinedhow photosynthesizing,oxygen-producingcyanobacteria lie hidden under sulfur-eatingcompetitors at night-and how the two lit-erally swap positions at dawn and dusk.Theresearchers found that the time they take totrade places creates a lag between whenthe sun rises and when photosynthesisramps up,limiting how much oxygen themats can generate on short days.In fact,Klatt showed in the laboratory that the matsproduced no oxygen at all on artificial12-hour "days" and that oxygen productionincreased with day lengths beyond 16 hours.
Klatt initially doubted that the sinkholeresults could help explain the mystery of theoxygen.“It's a very special type of commu-nity that might not have existed in an ancientEarth,"she says.And without such competi-tion,changes in day length should not mat-ter,because microbes would receive thesame total amount of sunlight-just deliv-ered in different increments.But eventually(after thinking for what she calls an “embar-rassingly long" time),Klatt realized therewas an even more basic link that wouldapply to any kind of bacterial mat, includingthose on early Earth:even if oxygen production remained unchanged, longer dayswould allow more gas to seep into thewater-and ultimately into the atmosphere.
That is because the amount of oxygenleaving a mat is limited by how fast gas mol-ecules can diffuse out of it and by how muchis consumed by other kinds of bacteria in themat.Longer days have a drawn-out peak insunlight,letting more oxygen build up in themat,which increases diffusion.Critically,longer days also give the gas more time toescape before nightfall,when oxygen-gob-bling microbes consume the rest.Thesemechanisms could have had a strong impacton atmospheric oxygen levels over Earth'shistory,the study's modeling results suggest.
It's “a simple but elegant idea," saysTimothy Lyons, a biogeochemist at the Uni-versity of California, Riverside, who was not involved in the study. Lyons says there arestllsignifcant unknowns, such as whetherearly photosynthetic bacteria lived mostlyon the seafloor or floated free in the water,where they could release oxygen more easilyand without much dependence on diffusion.He suspects that many processes conspiredto fillthe atmosphere with oxygen-and thatday length could certainly have contributed.
Other possible mechanisms includechanging emissions of oxygen-consumingvolcanic gases,such as hydrogen andmethane, and limited supplies of phospho-rus, a necessary nutrient for photosynthe-sis. Benjamin Mills, an Earth evolutionmodeler at the University of Leeds in Eng-land, who was not involved with the study,says he is surprised that scientists hadmostly overlooked the role of day length.The challenge now,he says, is assessing"the relative importance of this processversus the other things we know about theglobal oxygen cycle."
Both Mills and Purdue University astro-biologist Stephanie Olson, who was alsonot involved in the study, were impressedby how well the new results match the history of atmospheric oxygenation,includingthe famous two-step rise and the interven-ing “boring billion"years-when oxygenlevels flat-lined, and day length also stalledat 21 hours.“It's intriguing that the patternof oxygen accumulation and the tempo ofthe slowing of Earth's rotation rate seem tohave occurred in lockstep," Olson says.
Olson is one of the few others to haveproposed a connection between daylength and oxygen levels. In a 2020 paperthat primarily focused on exoplanets,shedescribed how changes in Earth's rotationover time might have affected ocean circu-lation and thus the transport of nutrientssuch as phosphorus that fuel photosynthe-sis.Olson and her students are now explor-ing the idea with computer models. Thismechanism and Klatt's could have workedin concert,Olson says:"I see them as high-ly complementary,not competing,ideas."
The thought of a connection betweenEarth's rotation and atmospheric oxygenstill amazes Arjun Chennu,an ecologistat the Leibniz Center for Tropical MarineResearch in Bremen,who co-led the studywith Klatt.From the motion of planets tothe movement of molecules,he says,“therange of scales at which these interactionshave an effect...is wild."
Julia Rosen