21. Chemosynthesis: from Abysses to Mangroves

21. Chemosynthesis: from abysses to mangroves

Nadine Le Bris

CO2 fixation by the photo- productive marine environments. hydrothermal fluid and seawater. synthetic pathway is considered as Chemoautotrophic activity is Hydrothermal fluids are also rich the main source of energy for ocean supported by complex interactions in iron and dissolved manganese ecosystems. However, sunlight is between microorganisms, bacteria and sometimes in hydrogen, the rapidly absorbed by marine waters and archaea, and the environment oxidation of which is another major and, at depths below 200 meters, chemistry. To date, six metabolic mode of energy acquisition. Another photosynthesis is no longer possible. pathways that enable microbes mode of carbon fxation is used by

The organic matter produced by to transform CO2 into organic methanotrophs to produce organic phytoplankton on the surface is gra- molecules have been identifed. To compounds from the methane dually remineralized and only a small ‘fx carbon’, these microorganisms present in certain hydrothermal part of this resource reaches the obs- exploit a variety of chemical fluids, by exploiting the chemical cure regions, which comprise 98% of compounds, whose reactions release energy of its oxidation. the volume of the ocean (cf. II.20). energy. Hydrogen sulfde, abundant As a result, the seabed is considered in hydrothermal fuids, is the main The fauna that colonize these to be low in energy, especially in electron donor used. Its oxidation fuid emission zones therefore depend places where it lies far from the sur- into sulfate using dissolved oxygen on these chemoautotrotrophic face. But in reality this model is not is one of the most energetic reactions micro-organisms, and sometimes totally relevant. Today, we know that available in the mixing zone of the form close associations with them. alternative sources of energy are available in the ocean, and represent a mosaic of "chemosynthetic" habitats, in which communities of organisms proliferate through local production of organic matter. For example, deep hydrothermal vents or hydrocarbon seeps can harbor animal communities that are comparable to tropical reefs in terms of biomass.

Symbiotic activity

Deep-sea hydrothermal vents (cf. II.19), which were discovered on oceanic ridges 40 years ago, are emblematic of these environments, where intense microbial Fig. 1 – Gills of a giant tubeworm capable of extracting the chemical production fuels ecosystems compounds (CO2, hydrogen sulfide, oxygen) needed by its endo-symbiotic identifed as being among the most bacteria. © Ifremer / MESCAL expedition (UPMC-CNRS). n

90 | What is the ocean? An increasing number of symbioses between invertebrates and one or several bacteria that develop on the surface or inside their host’s organs, is being described in these environments. Beyond the emblematic giant tubeworms of the Eastern Pacifc of Galapagos, where they were first discovered (Fig. 1), diversified families of bivalves, gastropods, and hydrothermal crustaceans that form symbioses with chemoautotrophic bacteria are now known. Tese ultra-specialized species and their multiple types Fig. 2 – Aggregation of mussels forming symbioses with chemoautotrophic of association reflect the optimal bacteria associated with methane hydrates in a submarine canyon at a depth utilization of the sources of chemical of 1,600 m, North American margin. © Deepwater Canyons 2013 – Pathways energy available in environmental to the Abyss, NOAA-OER-BOEM-USGS. n conditions that vary extremely over time and in space. of organic matter degradation Characterizing the variability of

are indeed methane, CO2 and environmental conditions remains a hydrogen sulfide produced from major challenge in deep-sea environ- the conversion of seawater sulfate. ments, where limited access to sites Unsuspected habitats Such molecules may in turn serve as constrains monitoring over time. A a starting point for a chemosynthetic few sites have benefted from more The enhanced exploration of trophic chain. Tese chemosynthetic than 20 years of long-term studies. the ocean floor in recent decades symbioses are not limited to great However, the functioning and dyna- over regions of high heterogeneous depths. Chemoautotrophy is mics of chemosynthetic ecosystems tectonic and magmatic activity, beginning to be identified as an depend to a large extent on the rela- beyond mid-ocean ridge axes, important component in organic tions between organisms and, ofen along volcanic arcs and continental matter-rich marine ecosystems, ephemeral, microenvironments, margins, including in the most such as mangrove forests, seagrass whose role in the ocean remains challenging regions of the Antarctic or upwelling regions associated difficult to grasp, considering the and the Arctic oceans, have with high planktonic productivity great diversity of these systems. Te progressively revealed the ubiquitous in surface waters. The diversity of extreme variability of physicoche- nature of these communities. Tese these associations demonstrates the mical conditions (temperature, pH, findings allow chemosynthesis to adaptability of benthic species to the oxygen sulfde toxicity...) and the ins- be integrated in a wider ecological variety of energy sources available via tability of the habitat that accompa- context, which also includes different electron donors (sulfide, nies these resources are essential keys submarine canyons from which methane, hydrogen) available, to understanding the relationships sulfur-rich or methane-rich fluids regardless of the geological setting that structure communities over seep, and faults where alkaline fuids (Fig. 2). time and in space. rich in hydrogen and methane are formed by the interaction between References seawater and the rocks that form the • B. DAVID,C. OZOUF-COSTAZ and M. TROUSSELLIER (dir.) – Mondes Earth’s mantle. marins. Voyage insolite au coeur des océans, Cherche Midi / CNRS, 2014. • Vivre dans les milieux extrêmes, Dossier Biofutur, n° 336, 2012. More ephemeral habitats, • D. DESBRUYÈRES – Les trésors des abysses, Carnets de sciences, such as massive organic substrates, Éditions Quae, 2010. wood or whale carcasses, also • La biodiversité à travers des exemples. Les réseaux de la vie, harbor communities that exploit CSPNB, 2004. chemosynthesis. Te end products

Chemosynthesis: from abysses to mangroves | 91