When the first burrowing animals broke through the metabolic Great Wall into the anaerobic world that had long been hidden beneath it, they inadvertently tapped into one of the Earth's most potent sources of energy. Spanning the oxygen-depleted zone is a redox potential difference of about a volt. Any burrowing animal that could exploit this redox potential would be on easy street.
In a sediment, the redox potential has a different connotation than it does in a half-reaction. Sediments typically will contain a variety of electron donors and acceptors, and a sediment's redox potential is a kind of weighted average of these chemicals. For example, sediments with a lot of oxygen present will have very high redox potentials, but sediments with less oxygen will have redox potentials dominated by weaker oxidizing agents. Thus, the metabolic Great Wall, which appears where there is a decline in a sediment's oxygen concentration, exists in parallel with a gradient in redox potential (Fig. 6.6). In the top centimeter or so, where oxygen can readily move in from the water above to replace that consumed by aerobic bacteria, redox potentials are high, about +600 mV, and they decline gradually with depth to about +400 mV. At the metabolic Great Wall, redox potential drops sharply, forming a redox potential discontinuity (RPD) layer (Fig. 6.6). Below the RPD layer, redox potentials are typically about -200 mV or lower.
In an undisturbed sediment, this steep redox potential gradient is exploited by a rich microbial community (Fig. 6.6). For example, aerobic bacteria and other organisms that are obligately aerobic (like protozoans or small invertebrates) inhabit the top few millimeters of sediment, where oxygen is abundant and redox potentials are high. In this region glucose is typically metabolized to CO2 and H2O. Going deeper, as redox potential declines, the aerobic organisms disappear and fermenters take over. As the mud becomes more oxygen poor, anaerobic respiration comes to dominate. Around the RPD layer, because the gradients here are so steep, the potential to do work is high, and conditions here support a diverse bacterial ecosystem. At and above the RPD layer, where oxygen is low or only intermittently available, nitrate is the favored electron acceptor, and nitrate reducers dominate. Below the RPD layer, where the likelihood of encountering oxygen is slight, the most common electron acceptor is sulfate. Still deeper, at redox potentials where even sulfate reduction cannot work, the acetogens and their clients, the methanogens, take over. These various bacteria sort themselves out neatly along the profile of redox potential, each group that functions well at one redox potential dependent upon the groups higher in the gradient for feedstock and for keeping the dreaded oxygen gas away.
The animals that first breached the RPD layer 600 million years ago may have been driven there to escape predators. Once the metabolic Great Wall was breached, though, burrowers discovered entirely new ways to feed themselves. Many animals that now habitually live in muds in fact use their burrows as feeding burrows, and these seem to fall into two principal categories. In one type, constructed by so-called substrate feeders, the animal ingests nutrient-rich mud, leaving behind a tunnel as it eats its way through the mud. Alternatively, suspension feeders use the burrow as a conduit for water laden with planktonic organisms that can be captured and eaten.
Feeding burrows are usually simple in construction. The three simplest forms are named after the letters they resemble;I-burrows, J-burrows and U-burrows (Fig. 6.7). The I-burrow obviously points straight down, and the inhabitant (usually a polychaete worm) lies in it head down. The J-burrow is a natural extension of the I-burrow, with a straight vertical shaft terminated at the bottom by a horizontal cul-de-sac. Again, the inhabitant lies head down in the J. The common U-burrow, a further natural extension of the J-burrow, has two openings to the surface, connected, of course, by a U-shaped tube. The worm resides in the
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