ZMT - Leibniz Zentrum für Marine Tropenforschung

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.”

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.