N nitrogen

Figure 6.9 The nitrogen rectifier in the burrow lining of a lugworm.

lining acts as a nitrogen rectifier: it allows nitrogen to escape easily in its positively charged form, ammonium, but it impedes backflow in its negatively charged form, nitrite. The retention of nitrogen outside the burrow has other beneficial consequences. The accumulating nitrite is a feedstock for nitrite-oxidizing bacteria, which produce nitrate as an end product. This nitrate can then serve as an electron acceptor for anaerobic nitrate-oxidizing bacteria, in turn stimulating their growth (Fig. 6.9).

The chromatographic burrow lining also manages some interesting interactions with sulfur. In anoxic muds, hydrogen sulfide (H2S) accumulates from the activities of sulfate-oxidizing bacteria. This is what gives anoxic muds their "rotten egg" smell. Hydrogen sulfide, like ammonia, is highly toxic to most animals. In water, it acts as a weak acid, dissociating into hydrogen ions and sulfide ions:

When hydrogen sulfide contacts oxygen, the sulfide is oxidized to sulfate (SO4-2). This reaction is spontaneous and very fast. Certain aerobic bacteria, known as sulfide oxidizers, can also oxidize sulfide to sulfate. Sulfide oxidizers, therefore, have a problem: they must compete with oxygen for the energy released by sulfide oxidation. If the electrons in sulfide can be channeled through the bacteria, these electrons can be made to do physiological work for them. If the electrons go directly to oxygen, of course, the bacteria get nothing. Consequently, sulfide oxidizers tend to inhabit the margins of the RPD layer, where there is enough oxygen to accept electrons but not so much that it outcompetes the bacteria for them.

The occasional ventilation of a lugworm burrow introduces oxygen into sediments only intermittently, and the oxygen concentrations in the muds around the burrow will tend to be low. These rise during ventilation, of course, but when ventilation stops, the oxygen is quickly consumed by aerobic bacteria and small animals residing around the burrow, and the local concentrations of oxygen will fall.

On average, the oxygen concentrations around the burrow are low enough to favor the sulfide-oxidizing bacteria. The burrow lining's low permeability to sulfide will ensure that it does not leak back into the burrow, and thus its concentration will remain high in the surrounding sediments. Thus, the growth of sulfide oxidizers will be stimulated, and their production of sulfate will provide oxidant to the anaerobic sulfate reducers, in turn stimulating their growth.

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