Two keynote lectures will set the stage for contributed talks and posters by giving broad overviews of past, current and emerging microbial mats and microbialites research.

We are pleased to welcome our two eminents keynote speakers Drs. Benzerara and Des Marais:

Karim Benzerara

Monday 28th, 10 AM

Director of Research (CNRS) in Geobiology
Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie
Université Pierre et Marie Curie
4 place Jussieu
75252 Paris Cedex 05

Microbial biomineralization processes in microbialites

Microbialites, as defined by Burne and Moore (1987) are organosedimentary deposits that accrete as a result of a benthic microbial community trapping and binding detrital sediment and/or forming the locus of mineral precipitation. In this talk, I will focus on the latter processes, i.e. the in situ formation of minerals by microorganisms which I enclose within the term “biomineralization” whatever the involved mechanisms are. I will start by reviewing what we can learn from model experimental systems about nucleation processes and the physico-chemical conditions that influence them (e.g., De Yoreo and Vekilov, 2003; Giuffre et al, 2013). This provides a framework to analyze what may play within microbialite biofilms but also interrogates about what we presently do not know. The (local) increase by microbial activity of the supersaturation of solutions with solid phases has been one process particularly well advocated, with reason, as major for the formation of microbialites. In this context, I will briefly remind the ongoing debate about which primary metabolism may impact the most the “alkalinity” engine, i.e. the increase of alkalinity (e.g., Dupraz et al., 2009; Aloisi 2008). However, I will also argue why an approach purely based on assessing the relative importance of primary metabolisms may fail in predicting the calcification capability of a biofilm. Another issue relates to where minerals precipitate respective to cells. This has major implications for the potential preservation of traces of life in microbialites (e.g., Li et al., 2014). I will remind that while some authors think that this may be controlled by environmental conditions (e.g., Arp et al, 2001), several studies (e.g., Gerard et al., 2013; Couradeau et al., 2013) advocate for an intrinsically biological diversity in the capability to localize biomineralization. Last, while most of the studies about biomineralization in microbialites have dealt with carbonate mineral phases, other phases such as hydrated Mg-silicates have been increasingly shown to precipitate in situ, sometimes in association with the biofilms (e.g., Zeyen et al., 2015; Pace et al., 2016). Here I will discuss this emerging literature and discuss the potential biological processes at stake.

David J. Des Marais
Tuesday 29th, 10 AM

Principal investigator at NASA Space Science & Astrobiology – Exobiology branch
NASA Ames Research Center
M/S 245-1
Bldg. N245,
Moffett Field, CA  94035-0001

Microbial mats and the search for their biosignatures in deep time and space

Cyanobacterial mats provide insights into ancient benthic microbial communities and their biosignatures. Thick mats occupy hypersaline saltern ponds at Guerrero Negro, Baja California, Mexico. Mat biota maintains rapid rates of biogeochemical processes under steep and rapidly changing environmental gradients. Cycling of C, O, and S all increased identically with temperature, indicating the tight coupling of these cycles. An enormous microbial diversity exhibits a highly structured spatial distribution of populations. Combined universal clone libraries from all mat layers indicated Bacteria/Archaea/Eukarya ratios of 57:7:1. More than 10,000 unique bacterial sequences were present. The relative abundance of Archaea increased with depth – below 10 cm, solvent-extractable archaeal lipids were twice as abundant as bacterial lipids. Only 15 species of Eukarya were found among 890 clones analyzed. Degradation of the mats’ insoluble macromolecular organic fraction (IMOM) by hydropyrolysis released a complex variety of linear, branched and polycyclic alkane structures, e.g., hopanes, methylhopanes and steranes. Covalent binding of these biosignatures into IMOM aids their long-term geological preservation. Mars rover missions revealed evidence of long-lived fluvial lacustrine systems and organics in associated mudstones. NASA’s Mars 2020 rover mission will examine sediments in Jezero crater, including a delta and shoreline carbonate deposits, environments that on Earth have sustained microbial mats.

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