Hot mud - cold seeps

A polychaete worm

A polychaete worm Psamathe fauveli from a chemosynthetic ecosystem in the Southern Ocean

4 July 2014 by James Bell

We're still learning about the amazing life around hot springs on the deep seabed - but other kinds of hydrothermal vent are even more mysterious. James Bell describes recent work on the unique animals that live there.

You may have seen underwater video of towering pillars of deep-sea rock, billowing out super-hot black water, covered in strange white crabs or bright red tubeworms in programmes like The Blue Planet. These are hydrothermal vents, often considered among the greatest scientific discoveries in the natural world.

But impressive as these hydrothermal chimney ecosystems are, they are not alone in the deep sea. We know far less about what happens when hot water from deep within the Earth flows through muddy seabed in so-called 'diffuse-flow' vents. Here, it mixes with cold sea water beneath the seabed, which means it doesn't form the iconic chimneys, because the water cools down too slowly - but also that these ecosystems host completely different animals.

The important distinction between chimneys and diffuse-flow vents is how different they are from the rest of the deep-sea floor. Away from mid-ocean ridges and trenches, most of the deep seabed is an endless plain of thick, soft mud. Most seabed animals are adapted to this environment, endlessly hoovering up food particles that sink down from the surface.

In contrast, hydrothermal chimneys are hard and jagged, so it's difficult for animals adapted to the mud to live there; likewise, animals adapted to the chimneys can't live elsewhere. Diffuse-flow vents and cold seeps are much more like normal deep-sea conditions. They still host specialised animals, like giant deep-sea clams, but it's much easier for other deep-sea creatures to take advantage of the extra food being produced. This has a couple of interesting effects. Firstly, it promotes diversity and abundance of life; secondly, it means the animals are much more active here than in normal deep-sea sediments - and that has implications for our entire climate.

In most places on Earth, all the energy needed for life comes from the Sun, which plants use to create sugar through photosynthesis. But at deep-sea vents, the water is laden with special compounds, which bacteria can use to fix energy with no need for sunlight - a process known as chemosynthesis. Energy is supplied by unlocking stored energy in the chemical bonds of these compounds, such as hydrogen sulphide, or methane.

Temperatures at diffuse-flow vents are around 40-50°C above normal, lower than at chimneys (which can reach 400°C), but not all chemosynthetic environments are hot. Chemosynthesis can also occur at completely normal deep-sea temperatures, in regions where hydrocarbons like methane exist beneath the seabed - so-called 'cold seeps'. Like diffuse-flow vents, these seeps are physically not so different from the normal deep-sea floor, and they are interesting for the same reasons.

Turning carbon into rock

So what does this mean for the climate?

One of the most important ways that excess carbon dioxide is removed from the atmosphere is by plants in the surface oceans, some of which sinks to the sea floor. When it gets there, the animals that live in the seabed eat it, and in doing so mix up the sediment and increase how quickly the carbon gets locked down in the mud where it will eventually become rocks like chalk or limestone.

Yeti crabs on a hydrothermal chimney

Yeti crabs on a hydrothermal chimney in the Southern Ocean, south of the Falkland Islands. In some places these crabs are packed to more than 600m2

Diffuse-flow vents and cold seeps attract lots of animals because there is more food there. This means the sediment potentially gets mixed much faster, so carbon could be removed from the oceans more quickly. Until the sediment is buried and on its way to forming rock, very turbulent waters called benthic storms can stir it up again and send carbon back towards the surface. The Earth's crust is the ultimate long-term repository of carbon and to understand our climate, we need to know how carbon gets there from the atmosphere.

We have an idea of how this might work, but no real sense of how important these ecosystems are globally. Among the big questions we are trying to answer are how fast carbon is buried, and how much of it came originally from the atmosphere - some of this will have come from burning fossil fuels and some will have been produced by the local bacteria. If we can measure how quickly carbon from the atmosphere is buried, we will get a better impression of how the oceans might change in the future.

So, how do we answer these questions? First, you need a ship, and NERC sent one out to the Southern Ocean with a team of UK biologists and geochemists in 2011 to collect samples from some of these ecosystems.

The next stage is to look at the animals in these samples and work out exactly what they are. Often, the difference between two species is very small and, quite possibly unknown to science altogether, but this is a vital step towards understanding the diversity of animals present. We inevitably identify new species; this involves sequencing their DNA and describing their physical form in detail. This is important work for conservation - we can't protect species from extinction until we have discovered them and understand their distribution.

Most of the species I've found are small, segmented worms called polychaetes, not unlike the earthworms in your garden, but there are also a lot of mussels and crustaceans. The animals I am interested in are very small, typically not more than a few millimetres long, but there are so many of them that they can make a big difference to the seabed.

Once we have identified and counted all the specimens, we can begin constructing a food web. To do this, we use the elements carbon, nitrogen and sulphur, which naturally take various forms with different atomic weights, called isotopes. The isotopes of an element present in an animal's body form a unique signature based on its diet, letting us pinpoint its position in the food web. Using the data we get from measuring their isotopic signatures, we can model how they interact, both with each other and with their physical environment, for instance how carbon moves through the ecosystem.

Modelling carbon cycling in soft-sediment chemosynthetic ecosystems is an entirely new facet of marine research. Scientists worldwide are only beginning to get this kind of perspective on the deep sea and we still don't know what we are going to find. Part of what makes the project so interesting is that it involves a wide spectrum of ecological research - from the possibility of discovering brand new species, through finding out how these animals interact with their environment to discovering what this all means for big-picture issues like conservation and climate change. What's inspiring is the chance to explore an aspect of the world for the first time and find out things that no one knew before.

James Bell is a PhD student at the University of Leeds and Natural History Museum.