First description of the microbial communities of the Old City alkaline hydrothermal site, South-West Indian Ridge

Discovered in the 2000s along the Mid-Atlantic Ridge (30°N), the Lost City hydrothermal vent field has radically changed our understanding of the conditions in which life can establish and thrive on Earth and possibly on other planets. Unlike the “black smokers” previously found on the basalts of the ocean floor, made up of polymetallic sulphide vents from which extremely hot, acidic fluids emanate, Lost City’s hydrothermal vents, known as alkaline vents, are underlain by oceanic rocks of mantle origin, known as peridotites. The pipes are composed of carbonate and brucite minerals, which give them their white colour. The fluids emitted by these pipes are of moderate temperature (< 120°C), rich in molecular hydrogen (H2) and organic compounds (in particular methane, CH4, and formate, HCOO-). These organic compounds are unique in that they are formed abiotically, i.e. without the intervention of living organisms. They are derived from a reaction called serpentinisation, which occurs under the ocean floor when seawater flows through the peridotites and hydrates them. These water-rock interactions could have provided the sources of energy (such as H2) and carbon (such as methane) that enabled life to develop, long before the advent of photosynthesis. The discovery of the Lost City site has therefore stimulated research over the last 20 years to understand how life could have emerged in these dynamic environments where geological, chemical and biological processes are intimately linked. These various studies have given a predominant role to methanogenic archaea (microorganisms that produce methane from carbon dioxide and H2) and/or methanotrophic archaea (that consume methane) in these ecosystems, fuelling the first models of ancestral metabolisms. However, all these studies focused only on the Lost City site, the only example known to date in an oceanic context, even though predictions suggested the existence of many other sites of this type due to the abundance of rocks of mantle origin on the ocean floor.

The recent discovery of the Old City alkaline hydrothermal site in 2017, the first oceanic analogue of the Lost City site, sheds new light on these issues. The site is a hydrothermal system located at a depth of around 3,100 metres in a region of the South-West Indian Ridge that has been little affected by magmatism. Rocks of mantle origin are abundant, making it a particularly relevant site for studying the influence of serpentinisation on microbial communities.

In an article published in the ISME Journal, researchers from the Institut de Physique du Globe de Paris (University of Paris, IPGP, CNRS) in collaboration with the Geosciences Environnement Toulouse laboratory (University of Toulouse III, CNRS, IRD) present the very first description of the microbial communities of carbonate chimneys at the Old City site. The taxa dominating these communities are relatively similar to those found in the hydrothermal vents at Lost City or in the alkaline springs associated with ophiolites where serpentinisation is also active. However, this initial sampling at Old City shows that the microbial populations there appear to make significant use of carbon monoxide (CO) and formate, both of which are potentially abiotic. They would be used as sources of carbon or electrons, depending on the environmental conditions within the chimneys. Methane would be oxidised by aerobic bacteria. This variability in the sources of energy and carbon used by deep-sea ecosystems could be linked to the intensity of hydrothermal activity and therefore to the serpentinisation regime, as well as to the nature of the underlying mantle, all of which together control the yields and reaction pathways associated with abiotic organic synthesis in these environments. The geological, geochemical and hydrogeological characterisation of Old City is currently the subject of ongoing work led by Mathilde Cannat at the IPGP.

The discovery of the Old City alkaline hydrothermal site and this initial microbial ecology work opens up promising prospects for exploring the environmental parameters that shape life in environments influenced by serpentinisation. These microbial communities also provide us with information about the abiotic organic synthesis pathways that could prevail in these environments and support life, or even prebiotic chemistry, in subsurface rocks.