Abs

Complex organic molecules ("cast-offs")

H2 producing acetogenic bacteria

methane (CH4) carbon dioxide (CO2)

methane (CH4) carbon dioxide (CO2)

Figure 6.5 Ecological relationships among various types of anaerobic bacteria. [After Ferry (1997)]

them produce ATP, supports our own aerobic metabolism.

Retreat to Anaerobic Refuges

For the bacteria that were not so metabolically fortunate, retreat from the encroaching oxygen was the only option for them. Fortunately, they did not have to go very far, because most aquatic environments offer a variety of anaerobic refuges. In very deep oceans, for example, there is little oxygen, because there is no photosynthesis, and whatever oxygen does make it to great depths is quickly consumed by aerobic bacteria. Consequently, the abyssal depths are a very reducing environment, particularly where there is some input of reducing agents, as around hydrothermal vents. In shallower waters, where light can penetrate, though, photoautotrophs produce lots of oxygen, and aerobic bacteria can live in the water and in the upper layers of the shallow sediments of coastal margins (Fig. 6.6).

Below these upper layers, it is a very different world. Aerobic bacteria in the upper layers consume oxygen, just as the protozoans do in the oxygen transport "bucket-brigade" described in Chapter 4, and with the same result: a decline of oxygen concentration as one goes deeper into the sediments. In muds, though, the diffusion rate of oxygen is very much slower than the rate in a liquid culture broth. Consequently, oxygen concentration declines steeply with depth in sediments, so steeply that an abrupt discontinuity may be identified, usually at a depth of a few millimeters to a centimeter or so. This abrupt decline of oxygen concentration constitutes a sort of metabolic "Great Wall," behind which anaerobic bacteria could hide. And so, two and a half billion years ago, there they went, to escape the clouds of toxic gases swirling just a few millimeters above them. And there they hid, at least until about 600 million years ago, when this snug little world was rudely penetrated by those newly evolved Precambrian animal burrowers thrusting themselves forcefully through the Great Wall and bringing poison with them—oxygen.

oxygen ,) redox ( + concentration potential

I redox Jpotential discontinuity, layer

Figure 6.6 Distribution of bacteria along the gradient of redox potential typical in surface muds.

H2 producing acetogenic bacteria oxic muds oxic muds

Figure 6.6 Distribution of bacteria along the gradient of redox potential typical in surface muds.

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