Active hydrothermal chimneys are porous mineral structures subjected to a steep physico-chemical gradient resulting from mixing of the hot hydrothermal fluid (300 ° C and above) with cold sea water (2 ° C). The decrease in temperature is more less correlated with the anaerobic/aerobic transition. Hydrothermal chimneys microbial communities are under the effect to these changes in oxygenation, temperature conditions and variations in the nature of donors and electron acceptors, which will determine the nature of the organisms and metabolisms implemented. The diversity of microbial communities in hydrothermal vents is well documented. Although some studies dealing with microbial colonization processes were performed, many questions remain: 1) Who are the first colonizers of hydrothermal chimneys, 2) how the microbial populations succeed each to other as the the hydrothermal chimney is growing (3) are changes in the structure of these communities directly related to the mineral composition of the chimney and the hydrothermal fluid?

In order to answer these questions, a small active chimney of the Eiffel Tower site on the Lucky Strike hydrothermal field has been sampled several times since 2005 (EXOMAR), in 2006 (MoMARETO) and then in 2008 (MoMAR 2008) and preliminary study showed the existence of a correlation between the diversity of the Bacteria and the Archaea and the mineralogical composition, therefore the age of the chimney (Le Roy, 2009). Since this hydrothermal chimney was sampled again in 2013, 2014, 2015 and 2016 during the MOMARSAT cruises. This sampling has been extended since 2015 to other sites of the Lucky Strike hydrothermal vent field, notably at Y3, Cypress and Capelinhos sites where the chemistry of the fluids was shown to be different. The preliminary results have been confirmed and show that the diversity of microbial communities evolves during the formation of a hydrothermal chimney (François, 2017). The objective of the LuckyDivMic project is to carry out a more exhaustive sampling of different active chimneys at the different sites of the Lucky Strike hydrothermal field. Thus the systematic description of microbial diversity (high throughput sequencing of 16S rRNAs encoding genes, function genes and, if possible, metagenomes) in the chimneys but also in particles present in the fluid at different levels of the temperature gradient will make it possible to evaluate the evolution of this diversity, 1) in the different sites with different chemistry, 2) over time (correlated with the evolution of the hydrothermal fluid composition for a given site and 3) as a function of age (obtained thanks to the mineralogical composition) of the hydrothermal spire.