Hot Stuff! Life On A Hydrothermal Vent

Deep underwater, close to areas where new oceanic crust is being formed, cracks in the ocean's crust let seawater percolate into the hot rocks below. As the water moves through the rocks it picks up minerals to create a fluid rich in iron and other metals. It doesn't stay in the rocks for long, though...

Mid-Ocean ridges are peppered with hydrothermal vents, deep sea chimneys where water erupts back into the ocean, forming a plume that can reach over 400 °C! These plumes can be black or white in colour, which is why hydrothermal vents are also known as black or white smokers. The colour depends on the fluid's chemistry: if the local rock is rich in metals like iron, the plume will be black in colour and if it's rich in lighter minerals, such as calcium, the water will be much lighter too.

Deep sea vents are home to a host of curious creatures including a crab fondly known as 'The Hoff', which farms bacteria on its hairy body, and the tube worm Riftia, which are found in thickets at the base of hydrothermal vents. These worms thrive here because they have bacteria working with them to produce sugars and other substances needed for growth, but how do they do this so far from the sun? After all, there's no light for photosynthesis this far under the ocean, as it only penetrates through the first few hundred metres.

Instead, the bacteria use hydrogen sulphide to produce their sugars. Organisms that do this are known as chemoautotrophs. What light is to plants, hydrogen sulphide is to chemoautotrophs, giving them a way to take carbon from their surroundings and turn it into something useful. By combining oxygen in the water with hydrogen sulphide from the vent (oxidising it) the bacteria produce energy, sulphur and water. This energy can be used to create sugars for growth, making them the primary producers of the deep ocean. It's these same bacteria that 'The Hoff' scrapes off his hairy body for dinner!

Because they can withstand the high temperatures at hydrothermal vents, the bacteria found there are known as hyperthermophiles, which roughly translates as 'extreme heat loving organism'. It's thought that life originated in an environment similar to that at hydrothermal vents (although there are many hypotheses for how life originated) as 3.8 billion years ago, the oceans would have been much hotter than they are today. They may even have approached 100 °C after major asteroid impacts! In such an environment, early life would have been hyperthermophilic too.

Closest to the vent you find an abundance of the worm Alvinella, which colonises the sulphate chimneys and can withstand temperatures in excess of 50 °C. Instead of working with the bacteria to obtain its energy, Alvinella ingests it – on, in and outside its tubes – nowhere is safe! Riftia is generally found at the base of the chimneys where the temperature is a more mild 25 °C. Further still, where seawater is less than 15 °C, the seafloor is colonised by bivalves like these, which feed on organic particles in the water. Out here, you can barely detect the minerals erupted at hydrothermal vents.

As the vent fluids cool and mix with the surrounding seawater they form the black substance responsible for the smokers' name – iron sulphide (FeS). Locking sulphur away in this form prevents organisms from using it in their metabolism. Further from the vents, as the water cools, more and more of the free sulphide is locked away like this. This means that communities, particularly those that rely on hydrogen sulphide for their energy, are confined to a small area around the smoker.

Gollner S. et al. Diversity of Meiofauna from the 9°50′N East Pacific Rise across a Gradient of Hydrothermal Fluid Emissions. PLoS ONE 5 (2010).

Haase, K. M. et al. Fluid compositions and mineralogy of precipitates from Mid Atlantic Ridge hydrothermal vents at 4°48'S. PANGAEA (2009).