Bacterial diversity and carbonate precipitation in the giant microbialites from the highly alkaline Lake Van, Turkey

Abstract

Lake Van harbors the largest known microbialites on Earth. The surface of these huge carbonate pinnacles is covered by coccoid cyanobacteria whereas their central axis is occupied by a channel through which neutral, relatively Ca-enriched, groundwater flows into highly alkaline (pH approximately 9.7) Ca-poor lake water. Previous microscopy observations showed the presence of aragonite globules composed by rounded nanostructures of uncertain origin that resemble similar bodies found in some meteorites. Here, we have carried out fine-scale mineralogical and microbial diversity analyses from surface and internal microbialite samples. Electron transmission microscopy revealed that the nanostructures correspond to rounded aragonite nanoprecipitates. A progressive mineralization of cells by the deposition of nanoprecipitates on their surface was observed from external towards internal microbialite areas. Molecular diversity studies based on 16S rDNA amplification revealed the presence of bacterial lineages affiliated to the Alpha-, Beta- and Gammaproteobacteria, the Cyanobacteria, the Cytophaga-Flexibacter-Bacteroides (CFB) group, the Actinobacteria and the Firmicutes. Cyanobacteria and CFB members were only detected in surface layers. The most abundant and diverse lineages were the Firmicutes (low GC Gram positives). To the exclusion of cyanobacteria, the closest cultivated members to the Lake Van phylotypes were most frequently alkaliphilic and/or heterotrophic bacteria able to degrade complex organics. These heterotrophic bacteria may play a crucial role in the formation of Lake Van microbialites by locally promoting carbonate precipitation.

Mass occurrence of benthic coccoid cyanobacteria and their role in the production of Neoarchean carbonates of South Africa

“…However, on closer examination, this objection can be discarded. The singular coccoid cells occur exclusively in the etched molds of colonial imprints, between the walls of the cobweb-like structure described above, which is also typical for the occurrence in the Lake Van microbialites (Kempe et al, 1991;López-Garcia et al, 2005;Kempe and Kazmierczak, 2007) and Phanerozoic silicified cyanobacterial mats (Kremer, 2006). Similar occurrence of coccoidal cells surrounded by mucilage, in modern environments, has been shown by Horodyski (1980).…”

Specific carbonate–microbe interactions in the modern microbialites of Lake Alchichica (Mexico)

“…Despite considerable progress in our understanding of the microbial diversity associated with different microbialites and their mineralogy, the question of whether or not some microorganisms have an important active role in promoting mineral precipitation is unsettled (for example, Reid et al, 2000;Ló pez-García et al, 2005). Our study of Alchichica microbialites allows answering partially to this question.…”

Hydrochemistry and microbialites of the alkaline crater lake Alchichica, Mexico

“…The usage of these biostructures for paleoenvironmental interpretations is therefore still limited (e.g., Ginsburg 1991; Grotzinger and Knoll 1999;Sprachta et al 2001;Dupraz et al 2004). Direct comparisons of fossil and modern microbialites on a microstructural and ultrastructural level (e.g., Reid et al 2003;Kühl et al 2003;López-García et al 2005;Benzerara et al 2006;Bisset et al 2008) are particularly rare, despite wide acknowledgment of their fundamental importance for deciphering the genesis of microbial carbonate deposits and the evolution of the early biosphere.…”