Breakout Sessions

This workshop session will be organized in four main topics. Each participant will choose a workshop session. The objective is to establish a debate and a discussion moderated by the different participants of the workshop by exchanging their experience and their approach in the different thematics. In order to construct the session, a 2-slides PPT presentation is requested in order to determine the main topics developed during the session.

The day will end with a join session and a presentation of the main results and questions addressed in each workshop session.

Workshop 1

Experimental Modelling in Laboratory: how far can we go ?

Two major approaches for characterizing mineral formation during the activity of microorganisms. The first is the study of naturally-occurring organominerals and the combined environmental parameters associated with the biomineralizing environment. The main difficulty of this approach is the deconvolution and interpretation of complex interconnected processes that vary in time and space (e.g., with pronounced 3-D chemical and physical gradients). The second approach, “experimental modeling”, relies on laboratory studies conducted under controlled chemical, physical and biological conditions, often with pure cultures. These studies are typically carried out under unnatural conditions lacking multiple microbial species with diverse metabolisms interacting simultaneously. Such a natural complexity is hard to reproduce in laboratory, mainly due to the difficulty of mimicking physicochemical gradients and/or creating the microniches where mineral precipitations occur under local (µm-scale) supersaturation. However, the experimental approach may reveal the potential role of individual metabolisms on mineral formation. In addition, controlled laboratory experiments permit screening of a wide range of environmental conditions. Ultimately this aids in (i) assessing various extreme scenarios of microbial organomineralization and (ii) better quantifying the role of each individual factor controlling mineral precipitation. In this workshop topic, we will discuss and generalize the different aspects of laboratory modeling of microbial precipitation in the order to optimize and develop this approach.

Workshop 2

Biomineralization vs. organomineralization and the role of intrinsic and extrinsic factors in shaping the microbialite systems

The study of environmental conditions that control microbialite formation is critical for understanding the evolution of Earth. Both extrinsic factors (e.g., water chemistry, climate, tectonics, geological context, sediment supply, various physicochemical parameters (T°, pCO2, pH, [Ca2+], [Mg2+]) and intrinsic factors (e.g., microbial metabolisms and their relative contribution to element cycling, microbial interactions, competition, community stability and succession, production and consumption of extracellular organic compounds) engage in microbialite formation. Field observations and experimental results are needed for a robust interpretation of various processes involved in the formation of microbialites. In light of new insights, it may be useful to revisit the terminology used in mineral precipitation. What are the major pathways of mineral precipitation and how do we define these? Which terms describe the processes best and what has recently changed, or needs to be modified, if anything? What do we need to focus on in future investigations to sharpen these definitions?

Workshop 3

Microbialite fabrics from formation to diagenesis and preservation (or lack thereof) in the fossil record

The understanding of the development, evolution and preservation of microbialites in space and time is crucial to unravel the controlling factors involved. This topic focuses on the characterization and description of the main microbial fabrics and associated mineral phases, discussing their nomenclature and definitions. We will discuss mineralisation mechanisms of ancient microbialites that can be inferred from comparison with their modern counterparts. Furthermore, we will explore the contribution of micro- to basin scale processes in the formation and development of microbial deposits. Reviewing the sequence of precipitation mechanisms (paragenesis) could help interpreting the transition between microbial mats and microbialites and then their transformation and preservation during early to late diagenesis. The evolution of petrophysical parameters in space and time will give insights in our understanding of reservoir properties, e.g. for South Atlantic petroleum systems or future storage of CO2) . Finally, all of this advances will led us to question what is needed (physically, chemically and biologically) to preserve microbialite through the fossil record.

Workshop 4

Into deep time: looking for microbialite biosignatures in the fossil record

Fossils microbialites in the form of stromatolitic limestones or silicified fossils microbial mats are arguably some of the oldest record of an active biosphere with field observations dating them back to the Paleoarchean. The metabolic chemical reactions associated with these primitive ecosystems are thus likely to have shaped the chemical evolution of the surficial environment with the most obvious effect being the protracted oxygenation of the Earth’s ocean and atmosphere. However, numerous uncertainties remain on the exact nature of these earliest biosphere metabolisms and their trajectories through the geological times. Among topics of interest the evolution of photosynthesis from anoxygenic to oxygenic forms as well as the build-up of heterotrophs using increasingly available electron acceptors (nitrate, sulfate, iron oxide…) needs to be further documented in order to build an integrated understanding of the deepest biosphere evolution. We will therefore encourage discussion around biosignatures preserved in the microbialites using state of the art techniques in geochemistry such as, but non-limited to, stable isotopes, organic matter characterization, chemical mappings using the geological record as well as laboratory experiments and modern analogues.
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