Cyanobacterial calcification in modern microbialites at the submicrometer scale

[摘要] The search for microfossils in the geological record has been a long-termchallenge. Part of the problem comes from the difficulty of identifying suchmicrofossils unambiguously, since they can be morphologically confused withabiotic biomorphs. One route to improve our ability to correctly identifymicrofossils involves studying fossilization processes affecting bacteriain modern settings. We studied the initial stages of fossilization ofcyanobacterial cells in modern microbialites from Lake Alchichica (Mexico), aMg-rich hyperalkaline crater lake (pH 8.9) hosting currently growingstromatolites composed of aragonite [CaCO₃] and hydromagnesite[Mg₅(CO₃)4(OH)₂ · 4(H₂O)]. Most of the biomassassociated with the microbialites is composed of cyanobacteria. Scanningelectron microscopy analyses coupled with confocal laser scanning microscopyobservations were conducted to co-localize cyanobacterial cells andassociated minerals. These observations showed that cyanobacterial cellsaffiliated with the order Pleurocapsales become specifically encrusted withinaragonite with an apparent preservation of cell morphology. Encrustationgradients from non-encrusted to totally encrusted cells spanning distances ofa few hundred micrometers were observed. Cells exhibiting increased levels ofencrustation along this gradient were studied down to the nm scale using acombination of focused ion beam (FIB) milling, transmission electronmicroscopy (TEM) and scanning transmission x-ray microscopy (STXM) at the C,O and N K-edges. Two different types of aragonite crystals were observed: onetype was composed of needle-shaped nano-crystals growing outward from thecell body with a crystallographic orientation perpendicular to the cell wall,and another type was composed of larger crystals that progressively filledthe cell interior. Exopolymeric substances (EPS), initially co-localized withthe cells, decreased in concentration and dispersed away from the cells whilecrystal growth occurred. As encrustation developed, EPS progressivelydisappeared, but remaining EPS showed the same spectroscopic signature. Inthe most advanced stages of fossilization, only the textural organization ofthe two types of aragonite recorded the initial cell morphology and spatialdistribution.

[学科分类] 地球化学与岩石