The methane cycle

Photo by Michael Vi

Here on Earth we have a number of different cycles. Most of us are familiar with the water cycle, for example, or the oxygen cycle. Methane is a cycle that is really important not only on Earth today, but also in Earth’s past. Let’s start with methane. What is it? Methane is a molecule that we use as a fuel. So whenever you light your stove or barbecue you are burning, in part, methane. It’s an interesting molecule because it is so robust. It’s a carbon and four hydrogens that are covalently bonded, which makes it incredibly stable. In other words, if you were to bottle up methane in a glass ampule and leave it on the bench, it would be stable for a very long time; it doesn’t react very quickly.

Methane as fuel

If you can break one of those bonds and cleave one of the carbon and hydrogen bonds, then, interestingly, you open up the whole molecule for a continued breakdown, and it’s a great source of energy. There are plenty of microbes that eat methane. They eat it for a living. They use oxygen and methane to harness energy, which is cool because these microbes are harnessing energy and putting it to use in the same way we do when we cook our food. They don’t ignite it, of course, but the principle is the same. It’s called an oxidation, and they oxidise the methane, harness the energy, grow and divide and so on. Now, there is a surprising amount of methane on Earth, the majority of which is produced, believe it or not, by microbes in the deep sea.

These so-called methanogens are microorganisms that use carbon dioxide and hydrogen, or sometimes some other molecules, and they harness the energy from these reactions. Their waste product is methane; that’s what they give off. There are gigatonnes of methane tied up in the deep sea and, because of the stability and cool temperatures of the deep sea, a lot of that methane is tied up as so-called methane ice or hydrates. These are remarkable structures that are a kind of water ice, and if you could imagine shrinking yourself down to sub-microscopic range you could see this ice lattice: ice crystals with methane trapped in between the cages of water called methane hydrate. There are gigatonnes of it.

Methane in the atmosphere

In Earth’s past there have been times when that methane, we think, was released into the atmosphere, and methane is a potent greenhouse gas. It is more potent than carbon dioxide by a long shot. It doesn’t persist in the atmosphere quite as long, but it works really well. Methane, some scientists believe, has played a role in causing rapid moments of climate change by being released en masse and warming our planet up very quickly. This could happen through volcanic eruptions or asteroid impacts, destabilising some of the methane in the deep sea. Any number of hypotheses are floating around. We know methane is a potent greenhouse gas and we know that if we release it into the atmosphere it’s problematic. In fact, cows produce a lot of methane, believe it or not – not the cows, but the microbes they harbour – and they’re a pretty big source of methane for the atmosphere. The landfills and wetlands that we sometimes create are, too.

Right now we’re at a point where our planet is warming because of carbon dioxide, and there are some concerns about how that warming may actually exacerbate the methane cycle. One example is the melting of the permafrost; in recent years it’s been published and reported on that all of this frozen ground is melting and releasing lots of methane into the atmosphere. It is even more terrifying to think about the warming of the deep ocean, where we have much more methane tied up. If we get to the point where the heat that we are seeing in our atmosphere and upper ocean warms up enough of the deep sea, we are going to be releasing gigatonnes of methane into the ocean and we don’t know what’s going to happen.

Discoveries and methane seeps

Photo by unterwegs

Several years ago, I had the privilege of leading a research expedition off the coast of southern California, the intention of which was to look for underwater hot springs. Now, we’ve never found them off of California, but there were some hints that they may be found right off of Los Angeles, perhaps a little ironically, just offshore of the beaches that I grew up on as a kid.

So there we were in 2015 on our first expedition, and then later in 2018, looking for these underwater hot springs. We didn’t find them, but what we did find in their place were expansive fields of so-called methane seeps, areas of the sea floor where microbial methanogenesis, or microbial methane production, was fuelling a whole ecosystem, where the methane being produced is being eaten by other microbes and their activity, in turn, is fuelling the growth of other microbes and so on, all based on methane.

The Yellow Brick Road

One of the largest features we found was a place we nicknamed the Yellow Brick Road, a reference to The Wizard of Oz, because this seep was about one and a half kilometres long, if I remember correctly, and about 200 or 300 meters wide, contiguous – so that is many football fields in size – and it was all covered in a yellow microbial carpet. Literally, it looked like a carpet, and it’s fuelled by methane. Now, that discovery was very interesting because we found all sorts of other animals associated with them; we’re just beginning to understand why they’re there.

Members of the lab and myself started focusing on the microbes that are eating the methane, because on this carpet we found things that looked like maybe a beehive turned upside down. They are about half a metre to a metre in height, sticking out of the sea floor, looking like little chimneys. When we collected those and measured the amount of methane being consumed in them, we found that they were consuming methane at remarkable rates – rates we had never measured anywhere on the planet before.

Currently we’re trying to understand: how are those microbes that eat methane eating so quickly? Why there? Why didn’t we find these rates of methane consumption to be as high elsewhere? We had no idea. Equally important, we continued our exploration off the coast of southern California and found many other environments that were leaking methane, or at least producing methane and supporting communities. This area of southern California has been so well travelled and mapped for decades, and right there, underneath our feet – or boats or surfboards – are these expansive environments fuelled by methane, where so much methane is being made. It’s mind boggling. As for our own activities and our warming planet, we have no idea how the warming of the ocean is affecting the methane release from those seeps. So we have a challenge.

The mysterious deep

As we study methane cycling in the deep sea, we do put some thought into what are the possible scenarios as we look to the future. What could possibly happen at these seeps, especially as it relates to our human activities and the role we’ve played in warming the atmosphere in the upper ocean? One scenario is we’re releasing a lot of methane from the deep sea into the atmosphere, and that could be hugely problematic. So there’s a lot of effort being carried out by many other research groups in this regard. My group and I are thinking a lot about how the changes in the methane cycle affect the processes in the deep sea.

Despite the fact that humankind literally evolved on the shores of the ocean, there’s a lot we do not know about the deep sea because we cannot easily see into it. When you go out on a clear night you can see the moon hundreds of thousands of kilometres away with great ease. You could buy a telescope and look at craters, but water does not afford us that opportunity. It is electromagnetically opaque, meaning I cannot shine a flashlight down and see the bottom, the deepest part of the ocean, even though it’s only 11 kilometres. It still has so many mysteries, and some of those mysteries are extraordinary. For example, 75% of the world’s volcanic activity takes place underwater. That means for every volcanic eruption you hear about on land, there have been three in the deep sea, and they’re extraordinary. Humans have only seen one. This was in 2009 on an expedition in the South Pacific. In fact, it was an expedition that happened right after I left. I missed it, but my colleagues went and drove their robot submarine, and they saw a volcano exploding underwater in real time.

The deep sea is like a basement

Photo by Kichigin

It is critical that we as humans stop thinking of ourselves as being separate from the natural world. We’re not, and that has many implications. So many people say humans should have no impact on the natural world. Well, I’m sorry, that is impossible. For as long as we are drawing breath on this planet, we will have an impact. Other people say the natural world is fine. It’s huge. It’ll take care of itself. That is also impossible. The deep sea is 80% of our planet’s living space, and it is not just big, ice-cold, dark water where nothing happens.

Sometimes I talk about the deep sea as being like the basement of a home or an apartment building. You go up to your apartment, and you sit in your lovely living room, and you go to your kitchen and turn the hot water or the heater on, and everything works and it’s lovely. However, in the basement is the furnace and the electrical panel and all those other components that keep you happy, healthy and comfortable. They’re hidden out of sight – but that does not make them unimportant. The sooner we realise that we are part of this world and it is a part of us, the sooner I hope we can achieve a balance in how we interact with the world around us, including the deep sea.

What’s in the deep sea should not be a big mystery. It’s up to us to have the foresight to do the work needed to understand these relationships so that we can promote a better planet, not just for the animals of the deep sea, but for ourselves and our children.

Discover more about

methane and what it might mean for Earth

Wankel, S. D., Huang, Y., Gupta, M., et al. (2013). Characterizing the Distribution of Methane Sources and Cycling in the Deep Sea via in Situ Stable Isotope Analysis. Environmental Science & Technology, 47(3), 1478–1486.

Girguis, P. R., Cozen, A. E., & DeLong, E. F. (2005). Growth and Population Dynamics of Anaerobic Methane-Oxidizing Archaea and Sulfate-Reducing Bacteria in a Continuous-Flow Bioreactor. Applied and Environmental Microbiology, 71(7), 3725–3733.

Hallam, S. J., Girguis, P. R., Preston, C. M., et al. (2003). Identification of Methyl Coenzyme M Reductase A (mcrA) Genes Associated with Methane-Oxidizing Archaea. Applied and environmental microbiology, 69(9), 5483–5491.

The methane cycle and the challenge of global warming