Soil is inhabited by many different kinds of tiny microorganisms, also called microbes, which play an essential role in the cycling of matter and energy through an ecosystem. The majority of soil microbes are single-celled bacteria that break down organic matter for the energy they need to live. In the process, these decomposers recycle nutrients (including carbon and nitrogen) back into the environment by converting matter into forms that can be readily used by plants. Soil microbes were once thought to be dormant at extremely cold temperatures. However, scientists have found evidence of microbial activity at temperatures as low as –10°C (14°F).

In Alaska, the majority of the land is affected by permafrost—ground that is continuously frozen for two or more years. The top layer of permafrost, known as the active layer, thaws and freezes seasonally and ranges from a few centimeters to over one meter thick. Soil microbes live in the water within this active layer; even when the bulk of this layer is frozen, thin films of liquid water can exist between the soil and ice particles. As a result, soil microbes can remain active throughout the year, even during the winter.

Microbial activity produces gases such as carbon dioxide and methane during the process of decomposition. (Generally, carbon dioxide is produced in drier conditions and methane in wetter conditions.) The finding that soil microbes metabolize year-round raises the possibility that they may significantly contribute to the levels of greenhouse gas levels in the atmosphere. Furthermore, as the active layer of the permafrost increases in thickness with warming temperatures, microbial activity would also increase.

Soil, microbes, shrubs, and snow may be linked together as part of positive feedback loops that accelerate the climate change that is already in progress. Increasing soil temperature increases the rate of microbial activity, which produces more greenhouse gases contributing to increased warming. Increasing microbial activity also produces more nutrients, which then promotes increased shrub growth. Shrubs hold snow, which acts like an insulating blanket to keep the ground warm; increased shrub abundance leads to deeper and less dense snow cover, which increases its insulation properties. This leads to warmer ground temperature and increased microbial activity, and the cycle continues.

Melting permafrost, and the subsequent increase of carbon dioxide and methane emissions, may be one of the most important issues for climate-change researchers to consider. In addition to the current production of greenhouse gases, enormous amounts of methane and carbon dioxide have been stored in the frozen soil. As temperatures increase and land thaws, the release of these stored gases into the atmosphere could result in runaway global warming. In particular, methane, which is more potent than carbon dioxide as a greenhouse gas, may be a key factor to take into account in predicting climate change.

Therefore, microbes are an important component of both the nitrogen and carbon cycles. The following video explains how microbes that live in the permafrost in the tundra biome are contributing carbon dioxide (CO₂) and methane (CH₄) to the atmosphere.


Source: Teachers' Domain, Soil Microbes and Global Warming, published February 26, 2009, retrieved on October 20, 2010, http://www.teachersdomain.org/resource/ean08.sci.ess.earthsys.microbe/