Microbes, it is safe to say, are sadly neglected in most discussions of climate science. Under our noses, but mostly invisible to the naked eye, these tiny organisms are easily overlooked. And yet there is nothing we human beings can do from one minute to the next that is not based on co-operative arrangements with our microbial partners. Our respiration, digestion, immune function, wellbeing and overall survival depend on microbes. All our food is produced in alliance with complex communities of the organisms, as are many other life-sustaining materials. In fact, 90% of the cells in our bodies are microbes, and 99% of the genes in our bodies reside in these microbes. Most of these microbes live in our digestive tract although they are also found on the skin and in all our bodily cavities.

Crucially, microbes have the capacity to alter the environment in profound and lasting ways. Historically they are the most effective geoengineers and biogeochemists. Life as we now know it could not have evolved without the dramatic rise in oxygen levels – the Great Oxidation Event – some 2.3 billion years ago. Although the causes of this relatively sudden increase in atmospheric oxygen remain shrouded in controversy, it is widely agreed that cyanobacteria in the oceans were the first organisms to produce oxygen: there is little doubt that these microbes played a major role.

Today, at any rate, microbes are the drivers of the oxygen cycle and an essential part of the nitrogen cycle. Before the development of the human chemical industry, microbes were the only agents capable of fixing atmospheric nitrogen for the chemical needs of higher plants and animals.

Changing times

Microbes could have various positive and negative feedback responses to temperature change, but the magnitudes of these are inadequately understood. This lack of knowledge is probably the reason why microbial activity is missing from most climate change models. Regardless of the reasons for this neglect, climate change models that fail to factor in microbial activities are manifestly inadequate.

Microbial decomposition of organic matter is crucial to the terrestrial carbon cycle. The same goes for the oceanic carbon cycle; simply put, microbes dominate. Some 93% of the Earth’s carbon dioxide is stored in the oceans, and oceans cycle about 90 billion tonnes of carbon dioxide per year, dwarfing the 6 billion tonnes or so generated by human activities. The mechanics of this oceanic carbon cycle are dominated by micro-, nano-, and picoplankton, including bacteria and archaea.1

Also crucial, but even less well understood, is the impact of marine viruses. There are estimated to be 4 × 1030 viruses in ocean waters, but such large numbers are unimaginable for most of us. We can try to picture the scale differently: if stretched end to end, marine viruses would span 10 million light years. That’s still pretty unimaginable. How about this: viral carbon is equivalent to the carbon in 75 million blue whales.

So those are the big numbers, but what is the impact of these viruses? Viruses may lyse, or break open, up to 50% of oceanic bacteria per day. In this way they significantly affect geochemistry on a global scale by altering the storage and respiration of organic and inorganic material.3 These effects may have both positive and negative impacts on the carbon cycle and global warming. Far too little is known about these processes, although progress is being made.

Take another example of microbial interaction: vast areas of Arctic and alpine tundra are warming due to climate change. As it heats up, the tundra is producing increasing quantities of methane. The main source of methane in permafrost soils is methanogenic (methane-producing) archaea, while the only known terrestrial sink is methanotrophic (methane-consuming) bacteria. The balance between these methane-generating and methane-absorbing processes is poorly understood, but since methane is a far more effective greenhouse gas than carbon dioxide, our understanding needs to improve, and quickly.

Good bug, bad bug

But the possible microbial impact on climate change isn’t all bad news: there are microbial interventions that could counteract climate change.

Some areas of the ocean are poor in phytoplankton. If these areas could be enriched, newly fertilised phytoplankton could absorb huge quantities of carbon dioxide. One possibility that has been widely discussed is the seeding of the ocean with iron, which can produce huge blooms of algae. John Martin, the oceanographer who did most to promote this idea, famously remarked: "Give me half a tanker of iron, and I'll give you the next Ice Age."

The proposal is, however, highly controversial. It’s very difficult to predict the effect of such an intervention on the wider ocean ecology – it could certainly be highly deleterious. What’s more, the effect could be limited by rapid explosions of zooplankton populations that would feed on the phytoplankton and quickly reduce their populations to earlier levels. The important point, however, is that microbial populations can respond dramatically to environmental changes, which emphasises not only the possibilities for bioengineering, but also the potential of natural feedback loops, positive or negative, as climate changes.

There are also less contested microbial technologies which could have an indirect impact on climate change, especially in the search for alternative energy sources. There are projects to grow algae for biofuel, for example, with a typical yield 30 times more than an equivalent crop such as soya. Another possibility is that cars and other vehicles could be powered by fuel cells using hydrogen generated by fermenting plant waste to produce electricity. And some have even suggested that a bioreactor fitted to the exhaust system of a car might be able to trap most of the greenhouse gas emissions, which could then be converted into additional fuel by genetically engineered algal communities.

Until now, microbes have been missing from the public debate on climate change. Why? Does bringing in microbes make the message too complicated and unwieldy? Certainly climate change is a hard sell as it is. But how much of the public message about climate change is moral as opposed to scientific? If humans feel responsible for climate change then they may take on the responsibility for behavioural change. Would balancing out the anthropogenic emphasis by including microbes in the story take away "blame" for climate change? And to whose benefit?

1: Stewart, R. (2003). Oceanography in the 21st century. Presentation given at the National Marine Educators Association Annual Meeting, Wilmington, North Carolina, 21-24 July 2003. Available at http://oceanworld.tamu.edu/NMEA_Talk/NMEA_Talk_2003.html

2: Suttle, C. A. (2005). "Viruses in the sea". Nature, 437, 356-361.

3: Suttle, C. A. (2007). "Marine viruses – major players in the global ecosystem". Nature Reviews Microbiology, 5: 801-812.

4: Martin, J.H., Gordon, R.M., Fitzwater, S. and Broenkow, W.W. (1989). Vertex – Phytoplankton Iron Studies in the Gulf of Alaska. "Deep-Sea Research Part a-Oceanographic Research Papers", 36(5): 649-680.

5: Thanks to Maureen O’Malley for much of the background research, and Claire Packman for the original drafting.