Lake Thetis domal microbialites

Abstract

Lake Thetis is a small saline lake with an increased alkalinity and abundant domal microbialites. These microbialites exhibit a rough stromatolitic structure. The recent growth results mainly from calcifying Entophysalis-biofilms which are forming a more or less laminated crust. Within the deeper parts of the Entophysalis-biofilm the outer basophilic polysaccharide envelopes contain abundant heterotrophic bacteria. Calcification events exactly start at these points. The older, subfossil portions of the microbialites are characterized by plumosely arranged Scytonema-filaments wh ich are enclosed by fibrous aragonite. Within small cryptic primary and secondary cavities clearly laminated organomicrites are lining the cavity walls. The formation of this type of "microstromatolites" is related to organic films, which contain no active microbes. These organic films are composed of degraded organic material (polysaccharides, proteins etc.) acting as matrices and templates for nucleation and growth of organomicrites and fibrous aragonite crystals.