Climate Change: Let Us Not Forget the Microbes

Microbes, or microorganisms too small to be visible to the naked eye, ruled the earth for billions of years.  Larger living structures, macroorganisms, including humans, would evolve from these smaller objects while embedded in a microbe soup.  We and these microbes are thus intimately related and inseparable.  All life on earth is dependent upon a microbe support system.  We humans are making rapid changes to Earth’s ecosystems and trying to predict what the effects of that activity might be on our future.  We spend most of our efforts studying what we can see, while forgetting to learn about changes to this unseen world of microbes.  There is a group, scientistswarning, devoted to educating the public about the various threats we face from our human activities.  They recently released a report on this subject: Scientists’ warning to humanity: microorganisms and climate change. 

“Human activities and their effects on the climate and environment cause unprecedented animal and plant extinctions, cause loss in biodiversity and endanger animal and plant life on Earth. Losses of species, communities and habitats are comparatively well researched, documented and publicized. By contrast, microorganisms are generally not discussed in the context of climate change (particularly the effect of climate change on microorganisms)…simply put, the microbial world constitutes the life support system of the biosphere. Although human effects on microorganisms are less obvious and certainly less characterized, a major concern is that changes in microbial biodiversity and activities will affect the resilience of all other organisms and hence their ability to respond to climate change.”

Before humans arrived on the scene, a carbon cycle existed that determined the sources and sinks of the various greenhouse gases.  These sources and sinks were two very large numbers whose difference could vary significantly.  The climate would respond to whatever balance was attained.  Great variations in climate have been observed over geological times as that balance has shifted.  Since the industrial revolution, we have been disturbing this balance by pumping excess carbon dioxide and methane into the atmosphere.  The climate has responded by getting warmer.  This is easily determined.  What is much less understood is the role microbes are playing—and will play—as more greenhouse gases are pumped into the air.  Microbes, as “the life support system of the biosphere,” control these sources and sinks and thus determine how the Earth responds to climate change.  We understand these microbial processes so little that we cannot predict with certainty exactly how the planet, as a whole, will respond.  What we know is that, thus far, rising temperatures appear to support ever higher temperatures by favoring greenhouse gas sources over sinks.  Rising levels of carbon dioxide can, independently of temperature, affect planetary response as well.

“Although microorganisms are crucial in regulating climate change, they are rarely the focus of climate change studies and are not considered in policy development. Their immense diversity and varied responses to environmental change make determining their role in the ecosystem challenging. In this Consensus Statement, we illustrate the links between microorganisms, macroscopic organisms and climate change, and put humanity on notice that the microscopic majority can no longer be the unseen elephant in the room. Unless we appreciate the importance of microbial processes, we fundamentally limit our understanding of Earth’s biosphere and response to climate change and thus jeopardize efforts to create an environmentally sustainable future.”

Both the oceans and land masses produce complicated microbial behavior contributing to sources and sinks of greenhouse gases.  The ocean, because of its fluid nature and large fraction of the planet’s surface, produces more immediate responses to human (anthropomorphic) changes.  It has been estimated that 90% of marine biomass is microbial, and that the oceans provide about 50% of the planet’s carbon dioxide fixation through photosynthesis.  But increased levels of carbon dioxide in the oceans change the acidity level and thus change many critical processes.  Human caused pollution and the effects of warming are also affecting ocean environments in complex ways that are discussed in the report.

Here, the focus will be on terrestrial processes which are more easily observed.  Controlled experiments are even possible.  Plants and the soil that support them provide most of the Earth’s biomass and are the critical components.  Plants absorb carbon dioxide and release oxygen via photosynthesis.  Soil provides the opposite effect by having microorganisms break down organic materials and release carbon dioxide and/or methane.  Approximately 50% of plant mass will eventually end up as a component of the soil and provide plenty of material for consumption.  In addition, soil has provided long-term storage for organic material that has not been available for consumption in the form of permafrost and peatlands.

“Peat (decomposed plant litter) covers ~3% of the land surface and, due to plant productivity exceeding decomposition, intact peatlands function as a global carbon sink and contain ~30% of global soil carbon. In permafrost, the accumulation of carbon in organic matter (remnants of plants, animals and microorganisms) far exceeds the respiratory losses, creating the largest terrestrial carbon sink. Climate warming of 1.5–2 °C (relative to the global mean surface temperature in 1850–1900) is predicted to reduce permafrost by 28–53% (compared with levels in 1960–1990), thereby making large carbon reservoirs available for microbial respiration and greenhouse gas emissions.”

If we are to be relieved of this seemingly inevitable feedback mechanism for producing ever more greenhouse gas emissions, it will depend on the behavior of microorganisms as temperature and greenhouse gas contributions increase.

“Soils store ~2,000 billion tonnes of organic carbon, which is more than the combined pool of carbon in the atmosphere and vegetation…Soil microorganisms regulate the amount of organic carbon stored in soil and released back to the atmosphere, and indirectly influence carbon storage in plants and soils through provision of macronutrients that regulate productivity…”

Thus far, it appears we will get no relief: greenhouse gas production will elevate temperatures and produce even more greenhouse gases.  Drew Pendergrass provided an article titled Ground Control for Harper’s Magazine that provided some additional insight into the issue of soil response to elevated temperature.

“Natural processes in the soil contribute more than six times as much carbon dioxide to the atmosphere each year than does the burning of fossil fuels. Microbes in the dirt release the gas as they consume dead plants and animals. Before the Industrial Revolution, those emissions were offset by the uptake of carbon by plants, but as global ecosystems react to a sudden shot of carbon dioxide, the balance has been disturbed. Even a slight change in how soils behave would have drastic impacts on the environment.”

Harvard University was granted a section of forest west of Boston.  Since 1907 they have chosen to use it as a research area to study the resident ecosystem.  Of particular interest is the emission of carbon dioxide and methane as the temperature of the soil heats up.  In order to study this phenomenon, they created patches of earth with heating coils in them that would keep the soil five degrees Celsius warmer than control patches so they could compare emissions between the two types.  That experiment is still ongoing. 

The obvious assumption was that higher temperatures would increase microbial activity and lead to greater carbon dioxide emission.  That was what was initially observed.

“Microbes flourish when they are warmer and have more freedom to move around, so Melillo and his colleagues expected the microbes in the heated soil to consume more nutrients and emit more carbon dioxide as a byproduct. They were right. ‘The initial response was, in some ways, exactly what you would expect,’ said David Foster, a silver-haired ecologist who has been the director of Harvard Forest since 1990. ‘If we heat up soils, we’re going to release a ton of carbon dioxide. And that’s going to have this very strong positive feedback’.”

However, things would change after about ten years and it appeared that there was a limit to the amount the microbes could produce.

“It took ten years for the story to change. In 2001, scientists noticed that the soil in the warmed plots had stopped releasing excess carbon dioxide—its greenhouse gas emissions looked just like those of the control plots. When the scientists looked more closely, they found that the microbes, which had been eating quickly in the warmer environment, were now dying off in large numbers because they were out of balance with the rest of the ecosystem. Eventually the pantry emptied, and the microbes starved.”

This potentially good news was merely a pause in the process.  The initial microbial population was suitable for devouring the low hanging fruit, the easily digested carbon compounds.  There remained much more carbon in less easily attacked forms; the microbes merely had to evolve the ability to consume those—something they seem to be quite capable of doing.  By 2008 the heated plots were emitting carbon dioxide at nearly the same rate as when the experiment with elevated temperatures began.  Kristen DeAngelis, a microbiology professor provided the explanation.

“From her experience as a microbiologist, DeAngelis knew that microbes often gain new abilities under stress. She found that the microbes in the warmed plots were better at digesting cellulose, hemicellulose, and lignin—tough, mealy molecules known as “recalcitrant carbon”—than the same species in the control plots. Soils around the world are full of recalcitrant carbon, but scientists had not anticipated that microbes would be able to consume it. The fact that warmed microbes were willing and able to chow down on these molecules was bad news for the climate.”

Pondering these results, Pendergrass inserted these thoughts.

“Recent research suggests that the Amazon rainforest will soon transform from a carbon sink into a carbon source, because of warming and other human-driven change. Farther north, the permafrost is melting. The Arctic tundra contains gobs of easy-to-digest carbon that has been frozen for centuries; once it melts, the microbes there could start a feeding frenzy. Scientists expect that the carbon emitted by thawing land up north will be far worse than anything Melillo observed in his warming experiment.”

Recall that soil emissions of carbon dioxide are already six times higher than that from burning fossil fuels.  What will there be to balance even higher rates?  The fate of the oceans is less certain.  Perhaps they will provide some assistance as our reckless behavior continues.  Unfortunately, like us, terrestrial microbes are only concerned with their immediate needs.

We seem unable or unwilling to change our habits.  The discussion presented here suggests that there could be a point of no return when climate change will continue without our contribution.  What do we do then?  How will it end?