Hot Vents and Cold Seeps

06 February 2017
Posted by Robinson Fulweiler.

Far beneath the surface of the sea – beneath the rolling swell, the breaching whale, the schooling fish – you’ll find an equally engaging and more exotic world. Our ocean floor is dotted with deep-sea ecosystems of amazing diversity and unusual ways of life...

Most likely the best known of the deep-sea ecosystems are hydrothermal vents. To understand them we need to first understand a bit of geology. Our planet is covered in a thin veneer of crust – often compared to the shell of an egg. Earth’s shell however is not wholly intact. Instead it is composed of pieces known as tectonic plates. Like puzzle pieces these plates – some very large and many smaller ones – encapsulate our planet. They slide and duck and topple over each other resulting in the familiar heavings of the earth we associate with earthquakes and volcanoes. There are gentler plate movements too that we don’t readily perceive. But everyday there is movement driven by the slow, hot, cycling of magma or molten rock beneath the surface of the Earth.

Along the ocean floor, in areas of high tectonic activity such as where tectonic plates separate or diverge, cracks and crevices are formed. Into these fissures rushes the cold ocean water where it is heated to as high as 400°C (750°F). These high temperatures spur chemical reactions that change the composition of the seawater: oxygen is removed, magnesium and sulphates enter, and the water becomes acidic. In time, more elements, such as iron, copper, and cobalt, enter the heated water.  Next this water rises out of the cracks to meet the surrounding cold ocean once more. And here additional chemical reactions cause the sulphur and other chemicals to precipitate, or come out of solution, constructing metal-rich towers on the seafloor. But it’s not just neat physical features that form. These fluids support life.

Most of us are familiar with photosynthesis – where plants, powered by sunlight, convert carbon dioxide to organic carbon: think plant leaves, roots and so on. Ultimately, most of life on Earth depends on this process – even you – whether you are a vegetarian or a carnivore. Deep in the ocean, however, there is no light. Instead, microbes harvest the energy from chemicals, such as hydrogen sulphide, to make organic carbon. This process is known as chemosynthesis, and it supports the rich and beautiful life found at hydrothermal vents. A full accounting of this life is beyond this short essay, but it is worth your time to explore them. Among my favourites are the giant tube worms (Riftia pachyptila) that rise more than 2 metres above the seafloor on white columns with caps of bright red plumage colored by haemoglobin (similar to the haemoglobin in our own blood); Yeti crabs  (Kiwa hirsute) – blind crabs with long arms covered in what appears to be thick white fur but instead are dense colonies of filamentous bacteria; and the eyeless vent shrimp (Rimicaris hybisae) which swarm over hydrothermal vents with little care for the hot fluids. Of course, we must also include the microbes that live on or in the guts, gills, and shells of these more charismatic fauna because without them there would be no hydrothermal vent ecosystem.

Unknown to us until as recently as 40 years ago, hydrothermal vents were first discovered in the Pacific Ocean by scientists diving in the submersible Alvin. Now we know that they are found throughout the world’s oceans. And they are not alone. The ocean also holds other ecosystems rich in life and supported by chemosynthesis.

Chief among them are cold seeps. Cold seeps can be found in brine pools – salt enriched ponds at the bottom of the sea – like those discovered in the Gulf of Mexico. Or along the edge of continents where tectonic plates are meeting or converging, providing a mechanism to squeeze fluids and gases out of the ocean floor. These fluids are rich in hydrogen sulphide, methane and other hydrocarbons, thereby providing the chemical energy needed to drive chemosynthesis. Like the hydrothermal vents, cold seeps support giant tube worms (Lamellibrachia luymesi) – over 3 m long that can live for over two centuries – as well as long lived mussels whose gills host the bacteria that ultimately provide all their nutrition.

Cold seeps can be spotted by looking for plumes of bubbles, rising from the ocean bottom – just like champagne bubbles hurrying to the surface of your glass. Or – and this admittedly is my favourite tell, by the dense white microbial mats, that can carpet cold seeps. These mats can be the first harbinger of life surviving off the hydrogen sulfide leaking from the sediment.

Together hydrothermal vents and cold seeps provide us with a glimpse into another world – an alien world. In doing so, they provide the opportunity to learn about life in extreme conditions like those on other planets. They give us the chance to learn about novel ways to make organic matter and provide hints to how life evolved on our planet. So next time you are dreaming of the sea and mesmerized by the sunlit surface waters, remember too the dark ocean floor teeming with life well below our easy grasp.


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