Mineralisation of filamentous cyanobacteria in Lake Thetis stromatolites, Western Australia

Abstract

Stromatolites are cited as some of the earliest evidence for life on Earth, but problems remain in reconciling the paucity of microfossils in ancient carbonate examples with the abundance of microbes that help construct modern analogues. Here, we trace the mineralisation pathway of filamentous cyanobacteria within stromatolites from Lake Thetis, Western Australia, providing new insights into microfossil preservation in carbonate stromatolites. Lake Thetis cyanobacteria exhibit a spectrum of mineralisation processes that include early precipitation of Mg-silicates, largely controlled by the morphochemical features of the cyanobacteria, followed by aragonite formation that is inferred to be driven by heterotrophic activity. Fossilised cyanobacteria with high-quality morphological preservation are characterised by a significant volume of authigenic Mg-silicates, which have preferentially nucleated in/on extracellular organic material and on cell walls, and now replicate the region once occupied by the cyanobacterial sheath. In such specimens, aragonite is restricted to the outer sheath margin and parts of the cell interior. Cyanobacteria that display more significant degradation appear to possess a higher ratio of aragonite to Mg-silicate. In these specimens, aragonite forms micronodules in the sheath zone and is spatially associated with the inferred remains of heterotrophic bacteria. Aragonite also occurs as an advancing front from the outer margin of the sheath where it is commonly intergrown with Mg-silicates. Where there is no evidence of Mg-silicates within filaments, the fidelity of microfossil preservation is poor. In these cases, individual filaments may no longer be visible under light microscopy, and little organic material remains, but filament traces remain detectable using electron microscopy due to variations in aragonite texture. These data provide further evidence that authigenic silicate minerals play a crucial role in the fossilisation of micro-organisms; in their absence, carbonate crystal growth potentially mediated by heterotrophic microbial decay may largely obliterate morphological evidence for life within stromatolites, although mineralogical traces may still be detectable using electron microscopy.

A microbial role in the construction of Mono Lake carbonate chimneys?

“…In support of this interpretation, we note that many contained Al, which is often cited to indicate a detrital sediment component (e.g., Koning, Epping, & Van Raphorst, 2002), and alkali feldspars cannot have formed in situ under lakewater pressures. Furthermore, the very patchy isolated pattern of grain distribution and their angular shapes (Figures 6,8,9) are in stark contrast to the generally massive nature of Mg-Si precipitates found replicating entire cyanobacterial sheaths or large volumes of thrombolites (Burne et al, 2014;Caselmann, 2005;Wacey et al, 2017) in other alkaline lakes such as Lake Clifton and Lake Thetis in Western Australia.…”

“…Authigenic microbial precipitation of silicates has recently been reported in a number of alkaline lakes (e.g., Lake Satonda (Arp, Reimer, & Reitner, 2003); Lake Clifton (Burne et al, 2014;Caselmann, 2005); Mexican Crater Lakes (Zeyen et al, 2015); Great Salt Lake (Pace et al, 2016); Lake Thetis (Caselmann, 2005;Wacey, Urosevic, Saunders, & George, 2017)), where it has been suggested that local increases of pH during oxygenic photosynthesis favour the precipitation of Mg-Si phases in and around cyanobacterial sheaths and F I G U R E 9 Chemistry of Type 1 filaments and surrounding minerals. High angle annular dark field scanning TEM image showing longitudinal and transverse sections, respectively, through two Type 1 filaments.…”

“…; Wacey, Urosevic,Saunders, & George, 2017)), where it has been suggested that local increases of pH during oxygenic photosynthesis favour the precipitation of Mg-Si phases in and around cyanobacterial sheaths and F I G U R E 9 Chemistry of Type 1 filaments and surrounding minerals. High angle annular dark field scanning TEM image showing longitudinal and transverse sections, respectively, through two Type 1 filaments.…”

Tracking Organomineralization Processes from Living Microbial Mats to Fossil Microbialites

“…Although sheaths are mineralized, the mineralization of the inner part of a filament itself is very rare in the observed crusts. Merz-Preiss [22], Gradziński [41], Merz-Preiss and Riding [47] and Wacey et al [50] have reported the mineralization of cyanobacteria filament sheaths in microbial mats. Merz-Preiss and Riding [47] have described the calcification of the sheath in low-energy environments, representing low DIC and SI calcite values between 0.2 and 0.8.…”

Jurassic Non-Carbonate Microbialites from the Betic-Rifian Cordillera (Tethys Western End): Textures, Mineralogy, and Environmental Reconstruction

“…However, other types of non-carbonate rocks are also organo-sedimentary deposits, resulting from different processes of BMC. There are examples related to iron oxides and oxyhydroxides [6,7], manganese oxides [8][9][10], phosphates [11][12][13], sulfates [14][15][16][17], and silicates [18]. Some of these examples are related to extremophile microbes [16,19,20], and they hold great interest for the study of the origin of life [21][22][23] and for astrobiology [24][25][26].…”