Four decades have passed since vibrant clusters of giant, metre-long tubeworms, discovered at hot springs on the ocean floor by Corliss et al.1, were reported in Science. Until then, the ocean floor was considered to be more like a desert than an oasis.
Corliss and colleagues didn’t discover underwater hot springs by accident; rather, they were trying to discover whether the hypothesis that such sites existed was correct. Theories on the movements of tectonic plates had set the course for this discovery with the idea that the mountain ranges that girdle the globe on the ocean floor, called spreading centres, are volcanic sites at the boundaries of tectonic plates. A key clue to the existence of underwater hot springs was the unexpectedly low conductive heat flux in the ocean’s crust2. Convective heat flow through hot springs could solve the riddle of this missing heat. Warm-water anomalies documented above a spreading centre called Galapagos Ridge guided Corliss et al. to the site at which they discovered underwater hot springs (also called hydrothermal vents).
Finding these hot springs was in itself an incredible breakthrough. But what really turned deep-sea science upside down were the unexpected oases of life bathed by those warm waters. During the discovery dive in the submersible vehicle Alvin, geologist Jack Corliss called up to the crew on the surface ship from his position 2.5 kilometres below to ask, “Isn’t the deep ocean supposed to be like a desert?” “Yes,” was the reply. “Well, there’s all these animals down here”, he responded (see go.nature.com/2tdoubx).
This brief interchange marked what is arguably the greatest discovery in biological oceanography so far, and it was made by a team of geologists and geochemists. The authors noted presciently in their paper that these “fragile communities provide a unique opportunity for a wide range of zoological, bacteriological, ecological, and biochemical studies”. What has come of those studies?
It didn’t take biologists long to discover just how exquisitely giant tubeworms are adapted to their environment. In that profound darkness, generating cellular energy by photosynthesis is not an option. And because organic material produced at the ocean’s surface loses much of its nutritional value by the time it reaches the deep sea bed, it doesn’t provide a suitable energy source to sustain dense populations of large organisms. Instead, hot-spring inhabitants living in warm water enriched in hydrogen sulfide and other chemically reduced inorganic compounds (such as methane) benefit from symbiotic or free-living bacteria that generate energy through chemosynthesis — chemical oxidation of those reduced compounds3.
Soon after the initial discoveries at the Galapagos site, a different type of hot spring called a black smoker — which emits metal-rich hydrothermal fluids — was found at another ocean-floor site4. Hot-spring ecosystems (Fig. 1) have now been found on sea-floor spreading centres throughout the world. They exist as 1,000 or more submarine oases, strung like minute pearls along the spreading centres. Although numerous, they are a rare habitat in terms of their total area — together, they might all fit on the island of Manhattan, with room to spare5. They are ephemeral habitats, too, lasting for years to decades, or possibly centuries, depending on the geological setting6. This raises the question of how the invertebrate populations are maintained, and the nature of the biogeographic barriers between populations at hot springs. The life cycles of nearly all invertebrates living in underwater hot springs includes a larval stage that disperses in the water column. Larval ecology, population connectivity, and oceanographic barriers and transport routes are key topics of current research.
Different types of species are found at hot springs on different spreading centres7. Some spreading centres in the Southern Hemisphere and the Arctic remain to be explored, raising the possibility that previously unknown types of invertebrate–bacterial relationship and adaptation will be found there.
Surprising species and astonishing biological adaptations continue to come to light. Pompeii worms (Alvinella pompejana) live at temperatures as high as 42 °C. These are among the most extreme temperatures endured by any multicellular animal on Earth8. The worms challenge us to understand how the proteins in the animals’ bodies are protected from melting. Microorganisms termed Archaea can grow at 121 °C, which is the hottest life known on Earth9. ‘Blind’ shrimp (Rimicaris exoculata) sport highly derived ‘eyes’ that are inferred to detect gradients of dim light emitted by the 350 °C fluids of black smokers, which might help the shrimp to avoid being ‘cooked’ by the heat10. Yeti crabs (Kiwa tyleri) have hairy claws and legs that might aid them in farming bacteria for nourishment11. Scaly-foot snails (Chrysomallon squamiferum) creep on ‘feet’ protected by metal scales of a type not found in other living or fossil molluscs, and offer an inspiration for the design of material for armour12.
The importance of microbial chemosynthesis at hot springs also presses us to rethink our ideas about the extremes to which life can adapt, the origin of life on this planet, and even the potential for life elsewhere in the Universe. NASA’s missions to Mars in the 1970s were searching for evidence of life based on energy from sunlight; now, planetary missions also consider the potential for life fuelled by chemical energy. Astrobiologists study submarine hot springs as a way of glimpsing conditions that might reflect those of primordial Earth13, and consider oceanic hot springs as possible analogues of alien submarine environments on oceanic worlds beyond our planet14.
Together with scholarly incentives to explore hot springs come engineering incentives to design and build ever-more-capable vehicles to enable precise and reliable access to the sea bed15. Remotely operated tethered vehicles came first, and autonomous underwater vehicles soon followed, pre-programmed to glide over the sea-bed like drones, carrying instruments that map the sea floor and sense the properties of the water. The development of cables that transmit video data allows such live feeds from the sea bed to be beamed around the world on freely available websites (see, for example, go.nature.com/2xrxsuh and go.nature.com/2vhrmcs).
The latest generation of deep-sea vehicles under development is turning a sharp corner from use for discovery and scientific research towards having a commercial role. Gigantic grinders, cutters and collectors are being designed, built and tested for open-pit mining of sea-bed sulfide deposits formed by hydrothermal activity16. One Canadian company has secured a lease to mine copper-, gold- and silver-rich hot springs in the Bismarck Sea, although so far there is no commercial mining of sea-bed sulfide deposits.
Many nations have placed the hot-spring ecosystems in their territories under protection, but the fate of such ecosystems in areas beyond national boundaries lies in the hands of the International Seabed Authority, which is currently revising its mining code. Attention might be shifting from the mining of active hot springs, which risks destroying their associated species, to exploiting sulfides at locations without visible signs of hydrothermal-fluid flux or vent-dependent organisms5, but such an outcome is not yet guaranteed. Actions in the near future will determine whether the frontier of discovery at hot springs opened by Corliss and colleagues 40 years ago moves from exploration to exploitation.
Nature 567, 182-184 (2019)
Competing Financial Interests
The author has been an unpaid advisory expert for the International Seabed Authority. She has received funding previously from the Pew Charitable Trust and from Nautilus Minerals, and currently receives funding from the Global Ocean Biodiversity Initiative.