Biogeochemical Cycles

One major microbial impact on Earth is the production and fate (global cycles) of the key elements of life: hydrogen, carbon, nitrogen, phosphorus, sulfur, and oxygen. These elements are cycled through the atmosphere, the biosphere, the hydrosphere, and the geosphere and in each of these "spheres" they are processed by microorganisms.

It has only been in the past decade that it was recognized that deep subsurface microbes also play a significant role in these global cycles. The geosphere is also the source of phosphorus and many other elements that are essential for life, such as sodium, magnesium, potassium, calcium, and iron. Microbes in the subsurface extract these elements from rocks and minerals and make them available to plants.

The impact of microbes is also evident in the cycling of enormous amounts of carbon dioxide and methane, two gases that trap heat in the atmosphere and affect global warming. Much of the production of these gases occurs in the subsurface biosphere. Microbes may also remove carbon dioxide from the atmosphere and store it in the subsurface, potentially playing a key role in slowing global warming.

Among the global biogeochemical cycles, man’s influence is greatest in the nitrogen cycle, where nitrogen is transformed through a series of different chemical forms, by microorganisms in the subsurface. One observation serves to focus attention on this important cycle. In 1950, the anthropogenic input to this cycle was about 40% of the natural (or non-anthropogenic) input.

In 2000, the anthropogenic input had reached 175% of the natural input (Science, 9 Nov 2001). This is significant because the extra nitrogen is throwing some ecosystems out of balance leading to ground waters that are unfit for human consumption and surface waters that no longer support diverse aquatic communities of plants, animals, and microorganisms.

This happens when nitrogen fertilizers are applied to soils and are transformed to nitrates by microbes and then leach into ground and surface waters. It is clear that understanding the connections between agriculture, groundwater, and microbes is important for the health of our world.