When new reports about global warming come out, they typically include a picture of the land and sky, with arrows marking the movement of carbon dioxide around the planet. Some arrows rise up from cities and farmland, while other arrows plunge down to forests and oceans. This sort of diagram does a great job of illustrating the big picture. Thanks to human activity, carbon dioxide is rising into the atmosphere faster than the planet can draw it down. But the giant scale of this picture hides some of the most important players in the global warming story, which are as crucial to the future of the planet as factories and forests: the planet’s vast swarms of microbes.
A single bacterium, measuring a few millionths of an inch across, may not seem like much compared to a coal-fired power plant. But taken together, microbes are a force to be reckoned with. Some scientists estimate that our planet is home to about 5 trillion trillion bacteria They pack the oceans and the soils; they live just about everywhere they can find even a trace of liquid water. Together, microbes lock up — and release — a huge amount of carbon. The world’s soils — the product of bacteria and fungi breaking down plant matter — contain more than 2.5 trillion tons of carbon. “If you look at all the trees and the grass and the flowers and add all that up, there’s four times as much carbon in the soil,” says Steven Allison, a biologist at the University of California at Irvine.
自从35亿年前，微生物一直在吸收和释放温室气体
更令人印象深刻的是微生物在生物圈周围抽出的大量碳。在海洋的表面，光合细菌溶解在水中，并将其变成有机分子。海洋也充满了以有机物为食的细菌，并释放二氧化碳作为废物。同时，将植物损坏的微生物每年释放550亿吨二氧化碳。艾莉森说：“这是人类通过化石燃料燃烧和森林砍伐的八倍。”
Microbes have been absorbing and releasing greenhouse gases ever since they first evolved in the ocean more than 3.5 billion years ago and spread on land about 2 billion years ago. And in the process they’ve influenced the Earth’s climate. But the influence doesn’t just flow one way, from microbes to the climate. As the climate changes, it can change the planet’s microbial menagerie.
Scientists are only just starting to figure out some of the rules that govern this feedback. They know just enough to recognize that as we raise the planet’s temperature, we will alter the planet’s microbes. And as we change the world’s microbes, we will also change their impact on the climate. But scientists have only a rough sense of what those changes will turn out to be. It’s possible that we will cause bacteria to churn out greenhouse gases far faster than they do today, adding to our own global warming.
Or they may be stimulated to draw carbon dioxide out of the atmosphere, softening the blow of climate change. Or all of the above.
To see the starkest evidence of the feedback between global warming and microbes, you need only travel to the Arctic. The tundra is a tough place for microbes to make a living. They are frozen for much of the year, and the stress of the cold makes them grow poorly even in warm months. Making life even harder for them is the lack of oxygen in the boggy ground. When tundra plants die, microbes can only break down a fraction of their biomass. The rest becomes entombed in permafrost.
自从冰川在上一个冰河时代结束时撤退以来，北美和西伯利亚的苔原一直在储存碳。但是，随着北极近几十年来变暖，碳的流动已经逆转：苔原已经开始释放出比捕获的碳更多的碳。科学家认为，随着北极温度的上升减轻了微生物的压力。苔原也变得更干燥，因此氧气可以渗透到地面上，从而使微生物繁殖。分解微生物现在可以以死亡的植物物质为食，并将碳泵入大气中。
“We believe that as the polar ocean warms, the ecosystem will change to favor microbes,” says one researcher.
科学家想知道，如果我们继续向大气中拍摄更多二氧化碳，这些微生物会做什么。例如，微生物可能会加速气候变化的速度。
随着北极的变暖，微生物将二氧化碳释放到空气中。二氧化碳升高温度，使苔原变暖更干燥。刺激微生物生长更快，并释放更多的二氧化碳。由于世界上的多年冻土锁定了如此多的碳，这种失控的反馈的可能性令人震惊。
极地海洋中的微生物可能以类似的方式行事，尽管出于不同的原因。马萨诸塞州伍兹霍尔的海洋生物实验室生态系统中心主任休·多克洛（Hugh Ducklow）发现，海洋微生物只吃一小部分有机物。其余的被小动物吞噬或沉入深海。然而，随着极地海洋的温暖，小鸭怀疑海洋微生物将变得更加活跃，并吞噬更多的有机物。Ducklow说：“我们相信，随着极地海洋的温暖，生态系统将改变以偏爱微生物。”
As the microbe population grows, it will release more carbon dioxide into the water. The oceans will thus be able to absorb less carbon dioxide from the atmosphere. As a result, the carbon dioxide levels in the atmosphere will climb, warming the planet. “The effects of climate change on the oceans are a bit like what’s happening in the tundra,” says Ducklow, “but the mechanisms and timescales are very different.”
It’s possible that there are rare species of microbes that will thrive in warmer temperatures.
然而，在土地和海上，微生物受许多不同因素的影响。在海洋中，全球变暖还将改变海水的pH值和风向模式，这两者都可以减慢微生物的生长或加速。同样，在苔原中，温度较高的温度使树木和灌木向北前进，它们在地面上施放的阴影可能会以无法预测的方式影响微生物。
为了解决这种复杂性，一些科学家正在开发复杂的计算机模型。艾莉森（Allison）和他的同事们最近建立了一个生活在温带森林土壤中的微生物模型。该模型跟踪细菌在土壤中喂入碳的生长，以及它们释放到大气中的二氧化碳。然后，科学家提高了虚拟森林5摄氏度的温度，并观察到了它的变化。
The bacteria experienced stress in the warmer climate, and as a result their biochemical reactions ran less efficiently. Instead of converting much of the carbon in plants into their own biomass, the microbes released more of it as carbon dioxide.
因此，您可能会认为更热的温度会导致空气中更多的二氧化碳。但是，由于细菌效率低下，它们的生长减慢了。因此，释放二氧化碳的细菌较少。
这两种效果 - 效率低下和增长缓慢 - 互相取消。艾莉森说：“当我们加热时，我们根本看不到微生物的碳。”
艾莉森（Allison）强调，他的模型只是理解微生物世界所带来的复杂变化的首次攻击。一方面，它仅适用于温带森林。他说：“我们的模型根本不适用于苔原。”他指出，加热苔原会减轻细菌的压力，因为它们很冷，因此开始。他希望他的模型将显示苔原释放更多的碳。
这样的模型假设细菌与气候变化保持不变。但是，微生物社区在几天，几个月和几年中都改变了很多。例如，有可能有稀有的微生物在温暖的温度下会蓬勃发展。全球变暖也可能会推动微生物发展。加利福尼亚大学欧文分校生物科学学院院长阿尔伯特·贝内特（Albert Bennett）进行了实验E. coli同时稳步提高温度。仅几百代后，细菌在温度更高的温度下进化而成。我们可能会在行星尺度上运行相同的实验。
艾莉森说：“我们不明白他们是否可以适应，或者可以适应的速度。”“现在是我们作为科学家的工作，可以弄清楚哪种情况将是这种情况。”