Lila gefärbte, mikrobielle Matten im Middle Island Sinkhole im Lake Huron, Juni 2019. Kleine Hügel und „Finger“ wie dieser entstehen durch Gase wie Methan und Schwefelwasserstoff verursacht, die unter der Oberfläche der Matten blubbern. | Foto: Phil Hartmeyer, NOAA Thunder Bay National Marine Sanctuary

03/08/2021 | Life on Earth today relies on the presence of oxygen. However, the process behind the step-wise rise of oxygen levels in the atmosphere, which took place over nearly two billion years, remains under debate. An international team of scientists around Judith Klatt from the Max Planck Institute for Marine Microbiology in Bremen, Germany and Arjun Chennu from the Leibniz Centre for Tropical Marine Research (ZMT), proposes an intriguing explanation: that increasing daylength, resulting from slowing Earth rotation, may have allowed microbes to release more oxygen, thereby creating the air we breathe today. The scientists now present their result in Nature Geosciences.

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.

Ein Taucher beobachtet die violetten, weißen und grünen Mikroben, die die Felsen im Middle Island Sinkhole des Lake Huron bedecken. (Foto: Phil Hartmeyer, NOAA Thunder Bay National Marine Sanctuary)

A scuba diver observes the purple, white and green microbes covering rocks in Lake Huron’s Middle Island Sinkhole. Photo: Phil Hartmeyer, NOAA Thunder Bay National Marine Sanctuary

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.

Participating Institutions:

  • Microsensor Group, Max Planck Institute for Marine Microbiology, Bremen, Germany
  • Department of Earth & Environmental Sciences, University of Michigan, Ann Arbor, MI, USA
  • Data Science and Technology, Leibniz Centre for Tropical Marine Research, Bremen, Germany
  • Annis Water Resources Institute, Grand Valley State University, Muskegon, MI, USA

Original Publication:

J. M. Klatt, A. Chennu, B. K. Arbic, B. A. Biddanda, G. J. Dick:  Possible link between Earth’s rotation rate and oxygenation. Nature Geosciences (2021), DOI: 10.1038/s41561-021-00784-3


Video about the project