Bac­te­r­ial His­tory Found in An­cient Mud Scrolls

by Elio

Dried roll-ups from Bar­de­nas Reales, Spain, lo­cated on a muddy slope close to a ver­nal pool. Blade width = 1.5 cm. Source

Who hasn't walked along a pre­vi­ously flooded area and seen flakes  of dried mud cakes mag­i­cally curled up in geo­met­ri­cal shapes? At times, the curls are so pro­nounced that they make com­plete scrolls. The area looks like a field of shards. No big deal, just dried mud you might say (as you reach down, tempted to pick up some pieces to play with). Au con­traire, such curlicue struc­tures, known as roll-ups, are of con­sid­er­able in­ter­est to ge­ol­o­gists. Roll-ups are sed­i­men­tary struc­tures capped with a sur­face layer of clay and or­ganic ma­te­r­ial that form when the sur­face dries out. Once formed, they are quite re­sis­tant to wet­ting and weath­er­ing, the open curls much less so.

Mi­cro­bial ar­chi­tec­ture of a roll-up. Cross-sec­tion of a roll-up from Bar­de­nas Reales, Spain. Lay­ers of cyanobac­te­ria (in­creas­ing in abun­dance to­ward the sur­face) can be seen in­ter­ca­lated within lay­ers of fine sed­i­ment (ar­rows). Bar = 0.5 mm. Source

To ex­plain why we're play­ing with mud cakes on this blog, it turns out that the roll-ups are largely bio­genic. Their sur­face in­cludes dis­tinct pop­u­la­tions of cyanobac­te­ria, which are much less com­mon in the ad­ja­cent, non-rolled soil. The cyanobac­te­ria are largely fil­a­men­tous and of the non-het­e­ro­cyst form­ing va­ri­ety. How do they con­tribute to the curl­ing? Ac­cord­ing to H. Be­raldi-Campesi and F. Gar­cia-Pichel of Ari­zona State, the au­thors of a re­cent study, they pro­vide the or­ganic ma­te­r­ial in the form of bac­te­r­ial ex­tra­cel­lu­lar poly­sac­cha­ride (EPS). Enough there and it alone can be the rea­son for the cur­va­ture. With less bio­mass present (<1% of the to­tal), the mi­cro­bial fil­a­ments in com­bi­na­tion with their EPS and some fine sed­i­ment suf­fice to do the curl­ing. The au­thors could re­pro­duce cred­i­ble roll-up for­ma­tions by us­ing a clay that swells on wa­ter­ing (smec­tite) or agar (we hope they en­joyed do­ing it).

Pret­zel-shapped sec­tion of a roll-up from the Sono­ran Desert (Chandler,AZ) dis­play­ing the di­am­e­ter (yel­low line) and length of its cross-section(blue line) used to mea­sure the de­gree of cur­va­ture. Bar = 1 cm. Source

Fos­sil roll-ups sim­i­lar to mod­ern ones are found in Pre­cam­brian rocks where they serve as in­ter­est­ing biosig­na­tures for an­cient life. Their very pres­ence sug­gests that mi­crobes ex­isted at that time and place. But can we be sure that the an­cient roll-ups are bio­genic in ori­gin? One ar­gu­ment is that be­cause mi­cro­bial roll-ups are strong enough to with­stand wet­ting and trans­port, they had a bet­ter chance of be­ing pre­served. The old­est fos­sil roll-up dates from 1.8 bil­lion years. They were thought to be of bio­genic ori­gin based on the co­he­sive strength in­ferred for the muddy lay­ers that form them. The work re­ported on mod­ern roll-ups sup­ports this in­ter­pre­ta­tion more strongly.

Fos­sil ev­i­dence from a cou­ple of bil­lion years ago is not ex­actly abun­dant. There­fore, any­thing that can add to that record con­cern­ing early life is wel­come, even if it be rolled-up cakes of mud.

 

Ref­er­ence

Be­raldi-Campesi H, Gar­cia-Pichel F. (2010). The bio­genic­ity of mod­ern ter­res­trial roll-up struc­tures and its sig­nif­i­cance for an­cient life on land. Geo­bi­ol­ogy. PMID 21040397