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How interest in the deep sea is resurfacing

New technologies helping to discover deep sea life

Anaïs Marechal, science journalist
On June 8th, 2022 |
4 mins reading time
New technologies helping to discover deep sea life
Lénaick Menot
Lénaick Menot
Researcher in the Deep Environment Laboratory at Ifremer
Key takeaways
  • The abyssal plains lie some 5,000 kilometres from the surface and are extreme environments where no light penetrates and therefore difficult to explore.
  • However, new technologies are making more things possible. Since 2020, France has had one of the four AUVs in the world capable of diving to a depth of 6,000 metres.
  • Seabed observatories also exist to provide a maximum of information to researchers.
  • There are both autonomous and cabled ones.
  • The aim today is to learn more about the underwater fauna and flora. In the samples taken from the abyssal plains, 90% of the species found are unknown.

The abyssal plains, which lie at a depth of about 5,000 metres, are extreme envi­ron­ments where no light pen­e­trates. The water tem­per­a­ture is about 2°C and the pres­sure is high. Con­trary to all expec­ta­tions, a great diver­si­ty of species can be found there. How­ev­er, many of them have only a small num­ber of indi­vid­u­als because food is scarce: only 1% of the organ­ic mat­ter pro­duced on the sur­face reach­es the deep sea. Fish, crus­taceans, sea cucum­bers, starfish, sea urchins and marine worms live there. How can this great diver­si­ty be explained? This remains a mys­tery today.

Some envi­ron­ments are real oases in the deep sea: canyons, seamounts, and hydrother­mal springs. Around the lat­ter, con­di­tions are very dif­fer­ent. The water is hot (sev­er­al hun­dred degrees) and acidic, very poor in oxy­gen and rich in methane and hydro­gen sul­phide (H2S). These com­pounds are oxi­dised by bac­te­ria which pro­duce ener­gy and organ­ic mat­ter con­sumed by the fau­na. The fau­na is very abun­dant in terms of species and num­ber of indi­vid­u­als. Giant mus­sels (Bathy­modi­o­lus sp.), giant tube worms (Rif­tia pachyp­ti­la), hairy gas­tropods (Alvini­con­cha sp.) and swarms of shrimps (Rim­i­caris sp.) can be found. 

How long have these hostile environments been explored?

It goes back to the first manned sub­ma­rine dives. Ifre­mer launched the Cyana in 1969, and the hydrother­mal springs were dis­cov­ered in 1977. But it was the Nau­tile in 1984 that made the great­est advances thanks to its abil­i­ty to descend to 6 000 metres. Manned sub­marines are still essen­tial to observe unknown sites with the naked eye.

New unmanned craft com­plete the arse­nal. ROVs, the first of which date back to the 2000s, are remote-con­trolled under­wa­ter drones linked by a cable to the ship. Oth­er ful­ly autonomous drones, the AUVs, are now being used. Since 2020, France has had one of the four AUVs in the world capa­ble of div­ing to a depth of 6,000 metres 1. These drones can film and map large areas of the sea. Ifre­mer’s next step is to add a lar­val col­lec­tion sys­tem to them, as the lar­val life cycle is one of the miss­ing ele­ments in our under­stand­ing of the life cycle of deep-sea organisms.

Deep-sea bio­di­ver­si­ty: a range of eco­nom­ic resources

Deep-sea organ­isms, par­tic­u­lar­ly those from hydrother­mal springs, are adapt­ed to hos­tile liv­ing con­di­tions: high pres­sure, high tem­per­a­ture, acidic envi­ron­ment, pres­ence of tox­ic mol­e­cules (H2S), etc. Their com­po­nents are of inter­est to many indus­tries using biotech­nolo­gies, such as chem­istry, phar­ma­ceu­ti­cals, and food pro­cess­ing. Their com­po­nents are of inter­est to many indus­tries using biotech­nolo­gies, such as chem­i­cals, phar­ma­ceu­ti­cals, and food pro­cess­ing. For exam­ple, some annelids (worms) con­tain mol­e­cules with inter­est­ing antibi­ot­ic prop­er­ties 2. Anoth­er poten­tial spin-off is that some bac­te­ria have the poten­tial to pro­duce hydro­gen by fer­men­ta­tion, a major asset in the ener­gy transition.

Do new instruments help to advance our knowledge?

Yes, for the past ten years or so we have been tak­ing advan­tage of the numer­ous data col­lect­ed by the seabed obser­va­to­ries. These are real obser­va­tion sta­tions per­ma­nent­ly installed on the ocean floor. There are two types: autonomous obser­va­to­ries, which are bat­tery-pow­ered and require annu­al main­te­nance to recov­er the data; and cabled obser­va­to­ries, which are very expen­sive and trans­mit their data in real time. Obser­va­to­ries installed near hydrother­mal springs – such as on the east­ern Pacif­ic rim or in the Azores 3 – con­tin­u­ous­ly film the fau­na and mea­sure var­i­ous envi­ron­men­tal parameters. 

They rep­re­sent a real advance: unlike cam­paigns at sea, they offer con­tin­u­ous mea­sure­ments that pro­vide infor­ma­tion on the dynam­ics of ecosys­tems. Against all expec­ta­tions, they have revealed the great sta­bil­i­ty of the Azores hydrother­mal mus­sel field over a decade. The influ­ence of the tide on deep-sea ecosys­tems was also described thanks to these observations.

© NOAA Office of Ocean Explo­ration and Research

And what about environmental DNA – a technique for identifying species from DNA traces left in the water – which seems to be increasingly used in aquatic environments?

Envi­ron­men­tal DNA was first used in the deep sea a few years ago, as part of the Pourquoi pas les abysses? (“why not the abysses?”) project led by Ifre­mer 4. The aim of this project is to car­ry out an inven­to­ry of deep-sea bio­di­ver­si­ty. The num­ber of species report­ed by this tech­nique is phe­nom­e­nal com­pared to obser­va­tions alone. But the deep sea is a cold envi­ron­ment in which DNA can be pre­served for a long time: did these organ­isms real­ly live there, or are these sam­ples the trace of a frag­ment that drift­ed there? We don’t know.

The oth­er lim­i­ta­tion of envi­ron­men­tal DNA is that the DNA detect­ed must be linked to a species whose mor­phol­o­gy has already been iden­ti­fied. How­ev­er, very few species are known: the objec­tive of the inter­na­tion­al Bar­code of Life project 5 is to increase the size of this data­base. Today, 90% of the species in each sam­ple from the abyssal plains are unknown.

If the biodiversity of the deep sea is so poorly known, how can the impact of mining be assessed?

They are indeed poor­ly known. The uncer­tain­ties con­cern first and fore­most the extent of the impact. The plume of sed­i­ment gen­er­at­ed by min­ing will dis­perse in the water col­umn and then fall to the bot­tom. This dis­per­sion is dif­fi­cult to assess. More­over, as men­tioned above, the abyssal plains are inhab­it­ed by many rare species – with very few indi­vid­u­als – and whose lar­val cycle is not well known. It is impos­si­ble to know their recoloni­sa­tion capac­i­ty, nor their role in each ecosys­tem. The risk of extinc­tion can­not be ruled out.

A first pilot exploita­tion test has been car­ried out by the Bel­gian com­pa­ny Glob­al Sea Min­er­al Resources in the Clar­i­on-Clip­per­ton zone, prized for its poly­metal­lic nod­ules. The envi­ron­men­tal impact is being assessed as part of the MiningIm­pact research project 6. But we are still far from being able to assess the long-term impact of min­ing over sev­er­al thou­sand square kilometres.

It must be recog­nised that the fau­na we are talk­ing about rep­re­sents a very small bio­mass. Its dis­tur­bance would not real­ly have any con­se­quences on the major bio­geo­chem­i­cal cycles, such as that of car­bon. The car­bon arriv­ing at the bot­tom would still be degrad­ed by bac­te­ria, which we know will not be durably impact­ed by exploita­tion. The ques­tion that must be asked is: does bio­di­ver­si­ty have an intrin­sic value?

1https://​wwz​.ifre​mer​.fr/​E​s​p​a​c​e​-​P​r​e​s​s​e​/​C​o​m​m​u​n​i​q​u​e​s​-​d​e​-​p​r​e​s​s​e​/​O​d​y​s​s​e​e​-​d​e​s​-​g​r​a​n​d​s​-​f​o​n​d​s​-​u​n​-​n​o​u​v​e​a​u​-​r​o​b​o​t​-​s​o​u​s​-​m​a​r​i​n​-​p​o​u​r​-​l​e​s​-​s​c​i​e​n​c​e​s​-​o​c​e​a​n​iques, accessed on 30 May 2022.
2See for exam­ple the work of Aurélie Tasiem­s­ki at the Lille Cen­tre for Infec­tion and Immu­ni­ty.
4https://wwz.ifremer.fr/Actualites-et-Agenda/Toutes-les-actualites/L‑ADN-environnemental-au-secours-de-la-biodiversite-des-fonds-marins-ScienceDurable, accessed on 30 May 2022.