Our MIO colleague Céline Rommevaux (MEB team) took part in the MoMARSAT 21 campaign aboard the N.O. L'ATALANTE and the submersible NAUTILE from 15 May to 21 June 2021 (Toulon-Brest). This campaign successfully maintained the EMSO-Açores underwater observatory, deployed since 2010 at the Lucky Strike hydrothermal site on the Mid-Atlantic Ridge south of the Azores. During this campaign, during a Nautile dive, C. Rommevaux reinstalled the CISICS microbial colonizer/fluid sampler at the Tour Eiffel hot fluid outlet. This coloniser, which is connected to the observatory node, is used to transmit temperature data inside the coloniser (between 50 and 100°C since its deployment) and to take 8 samples of fluids and their associated microbial communities during the year of deployment. This campaign also enabled samples to be taken of mats rich in iron oxides, which have been monitored over time since 2016 as a function of environmental variations in Fe and O2.